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  Wed, 24 Sep 2008 14:48:00 +0200
Precision Digital AC Power Controller
SCRs and Triacs are extensively used in modern electronic power controllers—in which power is controlled by means of phase angle variation of the conduction period. Controlling the phase angle can be made simple and easy if we set different firing times corresponding to different firing angles. The design given here is a synchronised programmable timer which achieves this objective.
The following equation for a sinewave shows how firing time and the phase angle are related to each other: 
q = 2pft or qµt 
Here, q is the angle described by a sinewave in time t (seconds), while f is the frequency of sinewave in Hz. Time period T (in seconds) of a sinewave is equal to the reciprocal of its frequency, i.e. T = 1/f.
The above equation indicates that if one divides the angle described during one complete cycle of the sinewave (2p = 360o) into equal parts, then time period T of the wave will be divided into identical equal parts. Thus, it becomes fairly easy to set the different programmable timings synchronised with the AC mains sinewave at zero crossing. The main advantage of such an arrangement, as already mentioned earlier, is that only the firing time has to be programmed to set different firing angles. It is to be noted that the more precise the timer, the more precise will be the power being controlled.
In this circuit, the time period of mains waveform is divided into 20 equal parts. So, there is a time interval of 1 ms between two consecutive steps. The sampling voltage is unfiltered full-wave and is obtained from the diode bridge at the output of the power transformer. The timer is reset at every zero crossing of full wave and set again instantly for the next delay time. This arrangement helps the timer to be set for every half of mains wave—when the positive half of the mains waveform starts building up, the timer is set for that half and as it begins to cross zero, it gets reset and set again for negative half, when the negative half begins to build up. The process is repeated. Here, instead of using two zero crossing detectors—one for each half of mains wave—a single detector is used to perform both the functions. This is possible because the sampling wave for negative half is inverted by the rectifier diode bridge.
The 18V AC from power transformer is fed to the four diodes in bridge configuration, followed by the filter capacitor which is again followed by a three-terminal voltage regulator IC LM7812. The voltage so obtained drives the circuit. The unfiltered voltage is isolated from the filter capacitor by a diode and is fed to zener diode D8, which acts as a clipper to clip voltage above 6 volts.
This voltage is fed to the base of transistor T1, which is wired as zero crossing detector. When base voltage reaches the threshold, it conducts. It thus supplies a narrow positive pulse which resets the timer at every zero crossing. 
A 32.768kHz crystal is used to get stable output of nearly 1 kHz (1,024Hz) frequency after five stages of binary division by an oscillator-cum-divider IC CD4060. The 32.768kHz crystal is used because it can be found in unused quartz clocks and is readily available in the market. But use of a 1kHz crystal using a quad-NAND IC CD4093 as clock generator, as shown in Fig. 2, is better as it provides the exact time interval required. In that case, CD4060 oscillator/divider is not required.
The CD4017B counter-cum-decoder IC then divides this 1kHz signal into ten equal intervals, which are programmed via the single-pole, 10-way rotary switch. Once the delayed output reaches the desired time interval, the corresponding output of CD4017 inhibits the counter CD4017 (via pole of rotary switch and diode D6) and fires the Triac. Transistor T2 here acts as a driver transistor. The reset pin of 4017 is connected to zero crossing detector output to reset it at every zero crossing. (The load-current waveforms for a few positions of the rotary switch, as observed at EFY Lab, are shown in Fig. 3.) 
The Circuit can be used as power controller in lighting equipment, hot air oven, universal singal-phase AC motor, heater etc.


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  Wed, 24 Sep 2008 14:48:00 +0200
Phone Broadcaster
Here is a simple yet very useful circuit which can be used to eavesdrop on a telephone conversation. The circuit can also be used as a wireless telephone amplifier.
One important feature of this circuit is that the circuit derives its power directly from the active telephone lines, and thus avoids use of any external battery or other power supplies. This not only saves a lot of space but also money. It consumes very low current from telephone lines without disturbing its performance. The circuit is very tiny and can be built using a single-IC type veroboard that can be easily fitted inside a telephone connection box of 3.75 cm x 5 cm.
The circuit consists of two sections, namely, automatic switching section and FM transmitter section.
Automatic switching section comprises resistors R1 to R3, preset VR1, transistors T1 and T2, zener D2, and diode D1. Resistor R1, along with preset VR1, works as a voltage divider. When voltage across the telephone lines is 48V DC, the voltage available at wiper of preset VR1 ranges from 0 to 32V (adjustable). The switching voltage of the circuit depends on zener breakdown voltage (here 24V) and switching voltage of the transistor T1 (0.7V). Thus, if we adjust preset VR1 to get over 24.7 volts, it will cause the zener to breakdown and transistor T1 to conduct. As a result collector of transistor T1 will get pulled towards negative supply, to cut off transistor T2. At this stage, if you lift the handset of the telephone, the line voltage drops to about 11V and transistor T1 is cut off. As a result, transistor T2 gets forward biased through resistor R2, to provide a DC path for transistor T3 used in the following FM transmitter section.
The low-power FM transmitter section comprises oscillator transistor T3, coil L1, and a few other components. Transistor T3 works as a common-emitter RF oscillator, with transistor T2 serving as an electronic ‘on’/‘off’ switch. The audio signal available across the telephone lines automatically modulates oscillator frequency via transistor T2 along with its series biasing resistor R3. The modulated RF signal is fed to the antenna. The telephone conversation can be heard on an FM receiver remotely when it is tuned to FM transmitter frequency.
Lab Note: During testing of the circuit it was observed that the telephone used was giving an engaged tone
when dialed by any subscriber. Addition of resistor R5 and capacitor C6 was found necessary for rectification of the fault.


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  Wed, 24 Sep 2008 14:47:00 +0200
Telephone Call Meter Using Calculator And COB
In this circuit, a simple calculator, in conjunction with a COB (chip-on-board) from an analogue quartz clock, is used to make a telephone call meter. The calculator enables conversion of STD/ISD calls to local call equivalents and always displays current local call-meter reading.

The circuit is simple and presents an elegant look, with feather-touch operation. It consumes very low current and is fully battery operated. The batteries used last more than a year.

Another advantage of using this circuit is that it is compatible with any type of pulse rate format, i.e. pulse rate in whole number, or whole number with decimal value. Recently, the telephone department announced changes in pulse rate format, which included pulse rate in whole number plus decimal value. In such a case, this circuit proves very handy

To convert STD/ISD calls to local calls, this circuit needs accurate 1Hz clock pulses, generated by clock COB. This COB is found inside analogue quartz wall clocks or time-piece mechanisms. It consists of IC, chip capacitors, and crystal that one can retrieve from scrap quartz clock mechanisms. These can be purchased from watch-repairing shops for less than Rs 20

Normally, the COB inside clock mechanism will be in good condition. However, before using the COB, please check its serviceability by applying 1.5V DC across terminals C and D, as shown in the figure. Then check DC voltage across terminals A and B; these terminals in a clock are connected to a coil. If the COB is in good condition, the multimeter needle would deflect forward and backward once every second. In fact, 0.5Hz clock is available at terminals A and B, with a phase difference of 90o. The advantage of using this COB is that it works on a 1.5V DC source

The clock pulses available from terminal A and B are combined using a bridge, comprising diodes D1 to D4, to obtain 1Hz clock pulses. These clock pulses are applied to the base of transistor T1. The collector and emitter of transistor T1 are connected across calculator’s ‘=’ terminals

The number of pulses forming an equivalent call may be determined from the latest telephone directory. However, the pulse rate (PR) found in the directory cannot be used directly in this circuit. For compatibility with this circuit, the pulse rate applicable for a particular place/distance, based on time of the day/holidays, is converted to pulse rate equivalent (PRE) using the formula PRE = 1/PR.

You may prepare a look-up table for various pulse rates and their equivalents (see Table). Suppose you are going to make an STD call in pulse rate 4. Note down from the table the pulse rate equivalent for pulse rate 4, which is 0.25. Please note that on maturity of a call in the telephone exchange, the exchange call meter immediately advances to one call and it will be further incremented according to pulse rate. So one call should always be included before counting the calls.

For making call in pulse rate 4, slide switch S1 to ‘off’ (pulse set position) and press calculator buttons in the following order: 1, ‘+’, 0.25, ‘=’. Here, 1 is initial count, and 0.25 is PRE. Now calculator displays 1.025. This call meter is now ready to count. Now make the call, and as soon as the call matures, immediately slide switch S1 to ‘on’ (start/standby position). The COB starts generating clock pulses of 1 Hz. Transistor T1 conducts once every second, and thus ‘=’ button in calculator is activated electronically once every second. The calculator display
starts from 1.25, advancing every second as follows:
1.25, 1.5, 1.75, 2.00, 2.25, 2.50, and so on.
After finishing the call, immediately slide switch S1 to ‘off’ position (pulse set position) and note down the local call meter reading from the calculator display. If decimal value is more than or equal to 0.9, add another call to the whole number value. If decimal value is less than 0.9, neglect decimal value and note down only whole numbers.

To store this local call meter reading into calculator memory, press ‘M+’ button. Now local call meter reading is stored in memory and is added to the previous local call meter reading. For continuous display of current local call meter reading, press ‘MRC’ button and slide switch S1 to ‘on’ (start/standby position). The current local call meter reading will blink once every second.
In prototype circuit, the author used TAKSUN calculator that costs around Rs 80. The display height was 1 cm. In this calculator, he substituted the two button-type batteries with two externally connected 1.5V R6 type batteries to run the calculator for more than an year.

The power ‘off’ button terminals were made dummy by affixing cellotape on contacts to avoid erasing of memory, should someone accidentally press the power ‘off’ button. This calculator has auto ‘off’ facility. Therefore, some button needs to be pressed frequently to keep the calculator ‘on’. So, in the idle condition, the ‘=’ button is activated electronically once every second by transistor T1, to keep the calculator continuously ‘on’.
Useful hints. Solder the ‘=’ button terminals by drilling small holes in its vicinity on PCB pattern using thin copper wire and solder it neatly, such that the ‘=’ button could get activated electronically as well as manually. Take the copper wire through a hole to the backside of the PCB, from where it is taken out of the calculator as terminals G and H.

At calculator’s battery terminals, solder two wires to ‘+’ and ‘–’ terminals. These wires are also taken out from calculator as terminals E and F. Affix COB on a general-purpose PCB and solder the remaining components neatly. For giving the unit an elegant look, purchase a jewellery plastic box with flip-type cover (size 15cm x 15cm). Now fix the board, calculator, and batteries, along with holder inside the jewellery box. Then mount the box on the wall and paste the look-up table inside the box cover in such a way that on opening the box, it is visible on left side of the box.
Caution. The negative terminals of battery A and battery B are to be kept isolated from each other for proper operation of this circuit.

LookUp Table

Pulse rate (PR)

2

2.5

3

4

6

8

12

16

24

32

36

48

Pulse rate eqlt.

0.5000

0.4000

0.333

0.250

0.166

0.125

0.083

0.062

0.041

0.031

0.027

0.020

Note : Here PRE is shown up to three decimal places. In practice, one may use up to five or six decimal places.


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  Wed, 24 Sep 2008 14:46:00 +0200
Telephone  Conversation Recorder
This circuit enables automatic switching-on of the tape recorder when the handset is lifted. The tape recorder gets switched off when the handset is replaced. The signals are suitably attenuated to a level at which they can be recorded using the ‘MIC-IN’ socket of the tape recorder.

Points X and Y in the circuit are connected to the telephone lines. Resistors R1 and R2 act as a voltage divider. The voltage appearing across R2 is fed to the ‘MIC-IN’ socket of the tape recorder. The values of R1 and R2 may be changed depending on the input impedance of the tape recorder’s ‘MIC-IN’ terminals. Capacitor C1 is used for blocking the flow of DC.

The second part of the circuit controls relay RL1, which is used to switch on/off the tape recorder. A voltage of 48 volts appears across the telephone lines in on-hook condition. This voltage drops to about 9 volts when the handset is lifted. Diodes D1 through D4 constitute a bridge rectifier/polarity guard. This ensures that transistor T1 gets voltage of proper polarity, irrespective of the polarity of the telephone lines.
During on-hook condition, the output from the bridge (48V DC) passes through 12V zener D5 and is applied to the base of transistor T1 via the voltage divider comprising resistors R3 and R4. This switches on transistor T1 and its collector is pulled low. This, in turn, causes transistor T2 to cut off and relay RL1 is not energised.
When the telephone handset is lifted, the voltage across points X and Y falls below 12 volts and so zenor diode D5 does not conduct. As a result, base of transistor T1 is pulled to ground potential via resistor R4 and thus is cut off. Thus, base of transistor T2 gets forward biased via resistor R5, which results in the energisation of relay RL1. The tape recorder is switched ‘on’ and recording begins.
The tape recorder should be kept loaded with a cassette and the record button of the tape recorder should remain pressed to enable it to record the conversation as soon as the handset is lifted. Capacitor C2 ensures that the relay is not switched on-and-off repeatedly when a number is being dialled in pulse dialing mode.


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  Wed, 24 Sep 2008 14:45:00 +0200
Protection for your Electrical Appliances
Here is a very low-cost circuit to save your electrically operated appliances, such as tv, tape recorder, refrigerator, and other instruments during sudden tripping and resumption of mains supply. Appliances like refrigerators and air-conditioners are more prone to damage due to such conditions.
The simple circuit given here switches off the mains supply to the load as soon as the power trips. The supply can be resumed only by manual intervention. Thus, the supply may be switched on only after it has stabilised.
The circuit comprises a step-down transformer followed by a full-wave rectifier and smoothing capacitor C1 which acts as a supply source for relay rl1. Initially, when the circuit is switched on, the power supply path to the step-down transformer X1 as well as the load is incomplete, as the relay is in de-energised state. To energise the relay, press switch S1 for a short duration. This completes the path for the supply to transformer X1 as also the load via closed contacts of switch S1. Meanwhile, the supply to relay becomes available and it gets energised to provide a parallel path for the supply to the transformer as well as the load.
If there is any interruption in the power supply, the supply to the transformer is not available and the relay de-energises. Thus, once the supply is interrupted even for a brief period, the relay is de-energised and you have to press switch S1 momentarily (when the supply resumes) to make it available to the load.
Very-short-duration (say, 1 to 5 milliseconds) interruptions or fluctuations will not affect the circuit because of presence of large-value capacitor which has to discharge via the relay coil. Thus the circuit provides suitable safety against erratic power supply conditions.


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  Wed, 24 Sep 2008 14:44:00 +0200
Simple Code Lock
The circuit described here is of an electronic combination lock for daily use. It responds only to the right sequence of four digits that are keyed in remotely. If a wrong key is touched, it resets the lock. The lock code can be set by connecting the line wires to the pads a, b, c, and d in the figure. For example, if the code is 1756, connect line 1 to a, line 7 to b, line 5 to c, line 6 to d and rest of the lines—2, 3, 4, 8, and 9—to the reset pad as shown by dotted lines in the figure.
The circuit is built around two cd4013 dual-d flip-flop ics. The clock pins of the four flip-flops are connected to a, b, c, and d pads. The correct code sequence for energisation of relay rl1 is realised by clocking points a, b, c, and d in that order. The five remaining switches are connected to reset pad which resets all the flip-flops. Touching the key pad switch a/b/c/d briefly pulls the clock input pin high and the state of flip-flop is altered. The q output pin of each flip-flop is wired to d input pin of the next flip-flop while d pin of the first flip-flop is grounded. Thus, if correct clocking sequence is followed then low level appears at q2 output of ic2 which energises the relay through relay driver transistor t1. The reset keys are wired to set pins 6 and 8 of each ic. (Power-on-reset capacitor c1 has been added at efy during testing as the state of q output is indeterminate during switching on operation.)
This circuit can be usefully employed in cars so that the car can start only when the correct code sequence is keyed in via the key pad. The circuit can also be used in various other applications.


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  Wed, 24 Sep 2008 14:43:00 +0200
Long-range FM Transmitter
Several circuits for constructing FM transmitters have been published in EFY. The power output of most of these circuits are very low because no power amplifier stages were incorporated.

The transmitter circuit described here has an extra RF power amplifier stage, after the oscillator stage, to raise the power output to 200-250 milliwatts. With a good matching 50-ohm ground plane antenna or multi-element Yagi antenna, this transmitter can provide reasonably good signal strength up to a distance of about 2 kilometres.

The circuit built around transistor T1 (BF494) is a basic low-power variable-frequency VHF oscillator. A varicap diode circuit is included to change the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts. Transistor T2 (2N3866) forms a VHF-class A power amplifier. It boosts the oscillator signals’ power four to five times. Thus, 200-250 milliwatts of power is generated at the collector of transistor T2.

For better results, assemble the circuit on a good-quality glass epoxy board and house the transmitter inside an aluminium case. Shield the oscillator stage using an aluminium sheet.
Coil winding details are given below:
L1 - 4 turns of 20 SWG wire close wound over 8mm diameter plastic former.
L2 - 2 turns of 24 SWG wire near top end of L1.
(Note: No core (i.e. air core) is used for the above coils)
L3 - 7 turns of 24 SWG wire close wound with 4mm diameter air core.
L4 - 7 turns of 24 SWG wire-wound on a ferrite bead (as choke)

Potentiometer VR1 is used to vary the fundamental frequency whereas potentiometer VR2 is used as power control. For hum-free operation, operate the transmitter on a 12V rechargeable battery pack of 10 x 1.2-volt Ni-Cd cells. Transistor T2 must be mounted on a heat sink. Do not switch on the transmitter without a matching antenna. Adjust both trimmers (VC1 and VC2) for maximum transmission power. Adjust potentiometer VR1 to set the fundamental frequency near 100 MHz.
This transmitter should only be used for educational purposes. Regular transmission using such a transmitter without a licence is illegal in India.


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  Wed, 24 Sep 2008 14:43:00 +0200
Long-range FM Transmitter
Several circuits for constructing FM transmitters have been published in EFY. The power output of most of these circuits are very low because no power amplifier stages were incorporated.

The transmitter circuit described here has an extra RF power amplifier stage, after the oscillator stage, to raise the power output to 200-250 milliwatts. With a good matching 50-ohm ground plane antenna or multi-element Yagi antenna, this transmitter can provide reasonably good signal strength up to a distance of about 2 kilometres.

The circuit built around transistor T1 (BF494) is a basic low-power variable-frequency VHF oscillator. A varicap diode circuit is included to change the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts. Transistor T2 (2N3866) forms a VHF-class A power amplifier. It boosts the oscillator signals’ power four to five times. Thus, 200-250 milliwatts of power is generated at the collector of transistor T2.

For better results, assemble the circuit on a good-quality glass epoxy board and house the transmitter inside an aluminium case. Shield the oscillator stage using an aluminium sheet.
Coil winding details are given below:
L1 - 4 turns of 20 SWG wire close wound over 8mm diameter plastic former.
L2 - 2 turns of 24 SWG wire near top end of L1.
(Note: No core (i.e. air core) is used for the above coils)
L3 - 7 turns of 24 SWG wire close wound with 4mm diameter air core.
L4 - 7 turns of 24 SWG wire-wound on a ferrite bead (as choke)

Potentiometer VR1 is used to vary the fundamental frequency whereas potentiometer VR2 is used as power control. For hum-free operation, operate the transmitter on a 12V rechargeable battery pack of 10 x 1.2-volt Ni-Cd cells. Transistor T2 must be mounted on a heat sink. Do not switch on the transmitter without a matching antenna. Adjust both trimmers (VC1 and VC2) for maximum transmission power. Adjust potentiometer VR1 to set the fundamental frequency near 100 MHz.
This transmitter should only be used for educational purposes. Regular transmission using such a transmitter without a licence is illegal in India.


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  Wed, 24 Sep 2008 14:42:00 +0200
Self-switching Power Supply
One of the main features of the regulated power supply circuit being presented is that though fixed-voltage regulator LM7805 is used in the circuit, its output voltage is variable. This is achieved by connecting a potentiometer between common terminal of regulator IC and ground. For every 100-ohm increment in the in-circuit value of the resistance of potentiometer VR1, the output voltage increases by 1 volt. Thus, the output varies from 3.7V to 8.7V (taking into account 1.3-volt drop across diodes D1 and D2).
Another important feature of the supply is that it switches itself off when no load is connected across its output terminals. This is achieved with the help of transistors T1 and T2, diodes D1 and D2, and capacitor C2. When a load is connected at the output, potential drop across diodes D1 and D2 (approximately 1.3V) is sufficient for transistors T2 and T1 to conduct. As a result, the relay gets energised and remains in that state as long as the load remains connected. At the same time, capacitor C2 gets charged to around 7-8 volt potential through transistor T2. But when the load is disconnected, transistor T2 is cut off. However, capacitor C2 is still charged and it starts discharging through base of transistor T1. After some time (which is basically determined by value of C2), relay RL1 is de-energised, which switches off the mains input to primary of transformer X1. To resume the power again, switch S1 should be pressed momentarily. Higher the value of capacitor C2, more will be the delay in switching off the power supply on disconnection of the load, and vice versa.
Though in the prototype a transformer with a secondary voltage of 12V-0V, 250mA was used, it can nevertheless be changed as per user’s requirement (up to 30V maximum. and 1-ampere current rating). For drawing more than 300mA current, the regulator IC must be fitted with a small heat sink over a mica insulator. When the transformer’s secondary voltage increases beyond 12 volts (RMS), potentiometer VR1 must be redimensioned. Also, the relay voltage rating should be redetermined.


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  Wed, 24 Sep 2008 14:41:00 +0200
Electrical Equipment Control UsingPC: by P.V Vinod Kumar
Here is a novel idea for using the printer port of a PC, for con-trol application using software and some interface hardware. The interface circuit along with the given software can be used with the printer port of any PC for controlling up to eight equipment.

The interface circuit shown in the figure is drawn for only one device, being controlled by D0 bit at pin 2 of the 25-pin parallel port. Identical circuits for the remaining data bits D1 through D7 (available at pins 3 through 9) have to be similarly wired. The use of opto-coupler ensures complete isolation of the PC from the relay driver circuitry.

When the program is loaded and run, the monitor will show the control panel-with the control bar at the extreme left. The bar can be moved using the right and left arrow keys. Switching on/off of bits D0-D7 is done by bringing the bar over the appropriate square and then pressing the ‘Q’ key for ON and ‘W’ key for OFF operation. The monitor will show the status of the relevant switch by indicating ‘1’ for ON and ‘0’ for OFF status of the switch. In addition, the current date and time is also displayed on the screen.
Program Listing in Basic
CLS : SCREEN 2
KEY(1) ON: ON KEY(1) GOSUB FINIS 
KEY(5) ON: ON KEY(5) GOSUB RETIRE 
KEY(10) ON: ON KEY(10) GOSUB ALLON 
PORT% = &H378 
OUT PORT%, 0 
LOCATE 8, 10: PRINT "<--- --->"
V$ = STRING$(27, "²") 
LOCATE 5, 6: PRINT V$; SPC(1); "CONTROL PANEL"; SPC(2); V$ 
LINE (40, 31)-(600, 180), 1, B 
LINE (40, 40)-(600, 180), 1, B 
LINE (40, 100)-(600, 120), 1, BF 
LINE (140, 40)-(460, 110), 1, B 
LOCATE 8, 65: PRINT "ON-----Q" 
LOCATE 12, 65: PRINT "OFF----W" 
LOCATE 19, 15: PRINT "F1"; SPC(24); "F5"; SPC(27); "F10"
LOCATE 21, 10: PRINT "EMERGENCY OFF"; SPC(16); "LOGOUT"; SPC(24); "ALLON" 
D$ = DATE$ 
J$ = MID$(D$, 1, 3)
K$ = MID$(D$, 4, 3) 
L$ = MID$(D$, 9, 2) LOCATE 5, 7: PRINT SPC(1); K$; J$; L$; SPC(1); "" 
STAT: 
PSET (145, 85): DRAW "R20U10L20D10" 
PSET (185, 85): DRAW "R20U10L20D10" 
PSET (225, 85): DRAW "R20U10L20D10" 
PSET (265, 85): DRAW "R20U10L20D10" 
PSET (305, 85): DRAW "R20U10L20D10"
PSET (345, 85): DRAW "R20U10L20D10" 
PSET (385, 85): DRAW "R20U10L20D10" 
PSET (425, 85): DRAW "R20U10L20D10" 
T$ = TIME$
Y$ = MID$(T$, 1, 2)
Y = VAL(Y$) 
IF Y < 12 THEN PP$ = "AM" ELSE PP$ = "PM"
IF Y > 12 THEN Y = Y - 12
U$ = MID$(T$, 3, 3) 
LOCATE 5, 64: PRINT SPC(1); Y; U$; PP$; SPC(1); ""
LOCATE 9, 20: PRINT "1"; SPC(4); "2"; SPC(4); "3"; SPC(4); "4"; SPC(4); "5"; SPC(4); "6"; SPC(4); "7"; SPC(4); "8"
LOCATE 12, 19: PRINT AA; SPC(2); SS; SPC(2); DD; SPC(2); FF; SPC(2); GG; SPC(1); SPC(1); HH; 
SPC(2); JJ; SPC(2); KK 
X$ = INKEY$ 
X$ = RIGHT$(X$, 1)
N = INP(PORT%)
IF X$ = "K" THEN J = J - 40
IF X$ = "M" THEN J = J + 40 
PSET (J + 105, 85): DRAW 
"R20U10L20D10R2U10R2D10R2U10R2D10R2U10R2D10R2U10R2D10R2U10R2D10"
FOR T = 1 TO 400: NEXT 
PRESET (J + 105, 85): DRAW
"R20U10L20D10R2U10R2D10R2U10R2D10R2U10R2D10R2U10R2D10R2U10R2D10"
IF J + 105 < 105 THEN J = 0 
IF J >= 360 THEN J = 360
IF (J = 40) AND (X$ = "Q" OR X$ = "q") THEN GOSUB APPLE 
IF (J = 40) AND (X$ = "W" OR X$ = "w") THEN GOSUB APPLEOF
IF (J = 80) AND (X$ = "Q" OR X$ = "q") THEN GOSUB BAT 
IF (J = 80) AND (X$ = "W" OR X$ = "w") THEN GOSUB BATOF
IF (J = 120) AND (X$ = "Q" OR X$ = "q") THEN GOSUB TALE
IF (J = 120) AND (X$ = "W" OR X$ = "w") THEN GOSUB TALEOF 
IF (J = 160) AND (X$ = "Q" OR X$ = "q") THEN GOSUB FLAT 
IF (J = 160) AND (X$ = "W" OR X$ = "w") THEN GOSUB FLATOF
IF (J = 200) AND (X$ = "Q" OR X$ = "q") THEN GOSUB FAT 
IF (J = 200) AND (X$ = "W" OR X$ = "w") THEN GOSUB FATOF
IF (J = 240) AND (X$ = "Q" OR X$ = "q") THEN GOSUB SILK
IF (J = 240) AND (X$ = "W" OR X$ = "w") THEN GOSUB SILKOF
IF (J = 280) AND (X$ = "Q" OR X$ = "q") THEN GOSUB SEVEN
IF (J = 280) AND (X$ = "W" OR X$ = "w") THEN GOSUB SEVENOF 
IF (J = 320) AND (X$ = "Q" OR X$ = "q") THEN GOSUB LAST
IF (J = 320) AND (X$ = "W" OR X$ = "w") THEN GOSUB LASTOF 
GOTO STAT '------------ALL THE SUBROUTINES ARE BELOW--------------
APPLE: SOUND 500, 2 
AA = 1 
LOCATE 6, 50 
Q = 1 OR N 
OUT PORT%, Q 
RETURN 
BAT: SOUND 500, 2 
SS = 1 
W = 2 OR N 
OUT PORT%, W
RETURN 
TALE: SOUND 500, 2 
DD = 1 
Q = 4 OR N 
OUT PORT%, Q 
RETURN 
FLAT: SOUND 500, 2
FF = 1 
Q = 8 OR N 
OUT PORT%, Q 
RETURN
FAT: SOUND 500, 2
GG = 1 
Q = 16 OR N 
OUT PORT%, Q
RETURN
SILK: SOUND 500, 2
HH = 1 
Q = 32 OR N
OUT PORT%, Q 
RETURN 
SEVEN: SOUND 500, 2
JJ = 1
Q = 64 OR N 
OUT PORT%, Q
RETURN 
LAST: SOUND 500, 2
KK = 1 
Q = 128 OR N
OUT PORT%, Q
RETURN
TALEOF: SOUND 400, 1
IF DD = 0 THEN RETURN 
DD = 0 
IF N = 4 THEN P = 0 
IF N < 4 THEN P = N 
IF N > 4 THEN P = N - 4 
OUT PORT%, P RETURN 
APPLEOF: SOUND 400, 1
IF AA = 0 THEN RETURN
AA = 0 
IF N = 1 THEN I = 0
IF N > 1 THEN I = N - 1 
OUT PORT%, I 
RETURN BATOF: SOUND 400, 1
IF SS = 0 THEN RETURN 
SS = 0 
IF N = 2 THEN U = 0 
IF N > 2 THEN U = N - 2 
IF N < 2 THEN U = N 
OUT PORT%, U RETURN 
FLATOF: SOUND 400, 1 
IF FF = 0 THEN RETURN FF = 0
IF N = 8 THEN E = 0
IF N < 8 THEN E = N 
IF N > 8 THEN E = N - 8 
OUT PORT%, E 
RETURN 
FATOF: SOUND 400, 1 
IF GG = 0 THEN RETURN
GG = 0
IF N = 16 THEN Y = 0
IF N < 16 THEN Y = N 
IF N > 16 THEN Y = N - 16 
OUT PORT%, Y
RETURN
SILKOF: SOUND 400, 1 
IF HH = 0 THEN RETURN
HH = 0 IF N = 32 THEN Y = 0
IF N < 32 THEN Y = N 
IF N > 32 THEN Y = N - 32 
OUT PORT%, Y 
RETURN
SEVENOF: SOUND 400, 1 
IF JJ = 0 THEN RETURN
JJ = 0 
IF N = 64 THEN U = 0
IF N < 64 THEN U = N
IF N > 64 THEN U = N - 64
OUT PORT%, U 
RETURN 
LASTOF: SOUND 400, 1 
IF KK = 0 THEN RETURN
KK = 0
IF N = 128 THEN Z = 0 
IF N < 128 THEN Z = N
IF N > 128 THEN Z = N - 128 
OUT PORT%, Z 
RETURN 
ALLON: SOUND 500, 4 
OUT PORT%, 255
AA = 1: SS = 1: DD = 1: FF = 1: GG = 1: HH = 1: JJ = 1: KK = 1 
RETURN
FINIS: SOUND 400, 2 
OUT PORT%, 0 
AA = 0: SS = 0: DD = 0: FF = 0: GG = 0: HH = 0: JJ = 0: KK = 0 
RETURN 
RETIRE: 
OUT PORT%, 0 
END 


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  Wed, 24 Sep 2008 14:40:00 +0200
Teleremote Control
Here is a teleremote circuit which enables switching ‘on’ and ‘off’ of appliances through telephone lines. It can be used to switch appliances from any distance, overcoming the limited range of infrared and radio remote controls

The circuit described here can be used to switch up to nine appliances (corresponding to the digits 1 through 9 of the telephone key-pad). The DTMF signals on telephone instrument are used as control signals. The digit ‘0’ in DTMF mode is used to toggle between the appliance mode and normal telephone operation mode. Thus the telephone can be used to switch on or switch off the appliances also while being used for normal conversation.

The circuit uses IC KT3170 (DTMF-to-BCD converter), 74154 (4-to-16-line demult-iplexer), and five CD4013 (D flip-flop) ICs. The working of the circuit is as follows.
Once a call is established (after hearing ring-back tone), dial ‘0’ in DTMF mode. IC1 decodes this as ‘1010,’ which is further demultiplexed by IC2 as output O10 (at pin 11) of IC2 (74154). The active low output of IC2, after inversion by an inverter gate of IC3 (CD4049), becomes logic 1. This is used to toggle flip-flop-1 (F/F-1) and relay RL1 is energised. Relay RL1 has two changeover contacts, RL1(a) and RL1(b). The energised RL1(a) contacts provide a 220-ohm loop across the telephone line while RL1(b) contacts inject a 10kHz tone on the line, which indicates to the caller that appliance mode has been selected. The 220-ohm loop on telephone line disconnects the ringer from the telephone line in the exchange. The line is now connected for appliance mode of operation.
If digit ‘0’ is not dialed (in DTMF) after establishing the call, the ring continues and the telephone can be used for normal conversation. After selection of the appliance mode of operation, if digit ‘1’ is dialed, it is decoded by IC1 and its output is ‘0001’. This BCD code is then demultiplexed by 4-to-16-line demultiplexer IC2 whose corresponding output, after inversion by a CD4049 inverter gate, goes to logic 1 state. This pulse toggles the corresponding flip-flop to alternate state. The flip-flop output is used to drive a relay (RL2) which can switch on or switch off the appliance connected through its contacts. By dialing other digits in a similar way, other appliances can also be switched ‘on’ or ‘off.’
Once the switching operation is over, the 220-ohm loop resistance and 10kHz tone needs to be removed from the telephone line. To achieve this, digit ‘0’ (in DTMF mode) is dialed again to toggle flip-flop-1 to de-energise relay RL1, which terminates the loop on line and the 10kHz tone is also disconnected. The telephone line is thus again set free to receive normal calls.
This circuit is to be connected in parallel to the telephone instrument.


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  Wed, 24 Sep 2008 14:40:00 +0200
Low-cost Transistorised Intercom
Several intercom circuits have ap-peared in EFY using integrated circuits. The circuit described here uses three easily available transistors only. Even a beginner can easily assemble it on a piece of veroboard.

The circuit comprises a 3-stage resistor-capacitor coupled amplifier. When ring button S2 is pressed, the amplifier circuit formed around transistors T1 and T2 gets converted into an asymmetrical astable multivib-rator generating ring signals. These ring signals are amplified by transistor T3 to drive the speaker of earpiece.

Current consumption of this intercom is 10 to 15 mA only. Thus a 9-volt PP3 battery would have a long life, when used in this circuit.
For making a two-way intercom, two identical units, as shown in figure, are required to be used. Output of one amplifier unit goes to speaker of the other unit, and vice versa. For single-battery operation, join corresponding supply and ground terminals of both the units together.
The complete circuit, along with microphone and earpiece etc, can be housed inside the plastic body of a cellphone toy, which is easily available in the market. Suggested cellphone cabinet is shown.


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  Wed, 24 Sep 2008 14:39:00 +0200
Cordless Phone Backup

Normally the base of a cordless phone has an adaptor and the handset has Ni-Cd cells for its operation. The base unit becomes inoperative in case of power failure. In such conditions, it is better to provide a backup using Ni-Cd cells externally. Here is a simple circuit which can be used with cordless phone SANYO CLT-420 or similar sets.

The working is simple. When AC mains is present, Ni-Cd cells are charged through IC LM317L, which is wired as a current source. Also, diode D3 is reverse-biased, which keeps Ni-Cd cells isolated from positive rail. When AC mains goes off, the Ni-Cd cells provide supply to the cordless phone base unit through diode D3. A green LED is used to indicate the presence of AC mains.

Each Ni-Cd cell costs around Rs 34, and the cost of the backup unit, including the box and cells, would not exceed Rs 300. Hence the circuit is well worth the investment.

 


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  Wed, 24 Sep 2008 14:39:00 +0200
Sleep-switch cum Wake-up Timer

Here is a sleep-switch circuit that can be easily converted into a wake-up timer. A dual-mode time setting makes the system versatile. The circuit is low-cost and can function as a precise timer.

The heartbeat produced by IC1 is a sharp 1Hz square wave signal having a duty cycle of 50 per cent. This is achieved by using a 4.194304MHz crystal in combination with discrete components around it. The 1Hz output of IC1 is connected to IC2 as well as one of the terminals of switch S1. IC2 is configured as divide-by-6 counter while IC3 further divides the output of IC2 by ten to produce one-minute output at its pin 12. This is brought to the second terminal of two-way switch S1 to help select either the ‘minutes’ or the ‘seconds’ mode of operation for IC4.

The decade counter IC4 provides binary output as it counts up the input pulses and IC5 decodes/converts them to 1-of-10 outputs (units). Similarly, the IC6-IC7 pair provides tens output since IC6 clock input pin is connected to D output pin of IC4.

Rotary switches S2 and S3 can be set to select any time between either 0 to 99 seconds or 0 to 99 minutes, depending upon the position of mode switch S1. Switches S2 and S3 could also be replaced by thumb-wheel type switches or 10-position DIP switches with one of their side terminals shorted together to serve as a pole. Please note that IC5 and IC7 (74145) have active low outputs.

The outputs from switches S2 and S3 are input to a two-input OR gate inside IC8 (7432) to obtain active low output on completion of the set time delay to deactivate relay RL1 through relay driver transistor T1 (normally conducting) when set time is reached. When transistor T1 cuts off, its collector goes high to reset oscillator IC1, and thus count at output of IC4 and IC6 gets locked. For resetting or restarting, the power supply to the circuit should be switched off and then switched on again.

The BCD outputs of IC4 and IC6 are converted to seven-segment outputs by IC9 and IC10 to drive the units and tens displays respectively for indicating elapsed time continuously. The relay contacts (normally open and normally closed) can be suitably used to energise or de-energise an alarm after the preset delay. It can thus be used as wake-up alarm or sleep timer.

If you want to de-energise the relay, say after 30 minutes, then set switch S1 to minutes mode, S2 to 0 and S3 to 3, and then switch on the supply to the circuit. After 30 minutes the outputs at poles of switches S2 and S3 will go low and so also the output of OR gate (IC8). As a result, transistor T1 will be cut-off to de-energise the relay.

One can easily add 0-99 hours capability by cascading two counters as shown in the minutes counter section comprising IC2 and IC3. Input clock for hours counter would be the minutes clock available at pin 12 of IC3.

 


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  Wed, 24 Sep 2008 14:38:00 +0200
Simple Analog-to-digital Converter

Normally analogue-to-digital con-verter (ADC) needs interfacing through a microprocessor to convert analogue data into digital format. This requires hardware and necessary software, resulting in increased complexity and hence the total cost.

The circuit of A-to-D converter shown here is configured around ADC 0808, avoiding the use of a microprocessor. The ADC 0808 is an 8-bit A-to-D converter, having data lines D0-D7. It works on the principle of successive approximation. It has a total of eight analogue input channels, out of which any one can be selected using address lines A, B and C. Here, in this case, input channel IN0 is selected by grounding A, B and C address lines.

Usually the control signals EOC (end of conversion), SC (start conversion), ALE (address latch enable) and OE (output enable) are interfaced by means of a microprocessor. However, the circuit shown here is built to operate in its continuous mode without using any microprocessor. Therefore the input control signals ALE and OE, being active-high, are tied to Vcc (+5 volts). The input control signal SC, being active-low, initiates start of conversion at falling edge of the pulse, whereas the output signal EOC becomes high after completion of digitisation. This EOC output is coupled to SC input, where falling edge of EOC output acts as SC input to direct the ADC to start the conversion.

As the conversion starts, EOC signal goes high. At next clock pulse EOC output again goes low, and hence SC is enabled to start the next conversion. Thus, it provides continuous 8-bit digital output corresponding to instantaneous value of analogue input. The maximum level of analogue input voltage should be appropriately scaled down below positive reference (+5V) level.

The ADC 0808 IC requires clock signal of typically 550 kHz, which can be easily derived from an astable multivibrator constructed using 7404 inverter gates. In order to visualise the digital output, the row of eight LEDs (LED1 through LED8) have been used, wherein each LED is connected to respective data lines D0 through D7. Since ADC works in the continuous mode, it displays digital output as soon as analogue input is applied. The decimal equivalent digital output value D for a given analogue input voltage Vin can be calculated from the relationship

 


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  Wed, 24 Sep 2008 14:38:00 +0200
Magnetic Proximity Switch
There is practically no house without an overhead tank (OHT). People who use electrically-operated water pumps for filling the OHT find it very inconvenient to switch off the pump when their overhead tank starts overflowing, specially when they are busy. So there is plenty of water wastage as well as wastage of power (consumed by the pump). However, there is a solution to get rid of this headache. The circuit given here will switch off the pump and also generate a melodious tune when the overhead tank gets filled up to the maximum desired level. All you have to do is switch off the power supply to the circuit when you are relatively free. The heart of the circuit is the CMOS latch CD4001. Usually the latch can be operated in two modes, namely, set and reset mode, i.e. the latch output can be set to logic 1 or reset to logic 0 by applying appropriate active low level input signal to pins 1 and 13, respectively. Here, in the given circuit, the set point is pin 1 and the reset point is pin 13. The inverted output of the latches are obtained at pins 3 and 11, respectively. When the circuit is powered there is a voltage drop at pin 1 due to the resistor-capacitor R1-C1 combination. The values of resistor R1 and C1 are chosen in such a way that pin 1 is low for about two seconds which is sufficient to energise the relay through transistor T1 and thus the pump starts running. When sufficient water gets filled in the overhead tank, switch S1 in the sensing unit, in the overhead tank as shown in Fig. 2, sends an active low signal to pin 13 which resets latch gate N1 output to logic 0. This causes transistor T1 to stop conducting, thereby de-energising the relay and shutting down the pump. At the same time, the output at pin 11 of gate N2 will be logic 1. This results in conduction of transistor T2 and melodious buzzer sounds. The green LED also lights up while the red LED, which remains on as long as the relay remains energised, gets switched off when water reaches the specified level in the overhead tank. The circuit possesses the following advantages:
  1. Special sensing mechanism (easy to build) is used to sense the water level in the overhead tank.
  2. One can replace CD4011 with IC 7400. In that case, a 5.1V zener may be connected additionally between pin 14 of IC1 and the ground.
  3. The circuit can be easily fabricated on a general-purpose PCB.


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  Wed, 24 Sep 2008 14:36:00 +0200
Ultra Low Drop Linar Regulator
e circuit is a MOSFET based linear voltage regulator with a voltage drop of as low as 60 mV at 1 ampere. Drop of a fewer millivolts is possible with better MOSFETs having lower RDS(on) resistance. The circuit in Fig. 1 uses 15V-0-15V secondary from a step-down transformer and employs an n-channel MOSFET IRF 540 to get the regulated 12V output from DC input, which could be as low as 12.06V. The gate drive voltage required for the MOSFET is generated using a voltage doubler circuit consisting of diodes D1 and D2 and capacitors C1 and C4. To turn the MOSFET fully on, the gate terminal should be around 10V above the source terminal which is connected to the output here. The voltage doubler feeds this voltage to the gate through resistor R1. Adjustable shunt regulator TL431 (IC2) is used here as an error amplifier, and it dynamically adjusts the gate voltage to maintain the regulation at the output. With adequate heatsink for the MOSFET, the circuit can provide up to 3A output at slightly elevated minimum voltage drop. Trimpot VR1 in the circuit is used for fine adjustment of the output voltage. Combination of capacitor C5 and resistor R2 provides error-amplifier compensation. The circuit is provided with a short-circuit crow-bar protection to guard the components against over-stress during accidental short at the output. This crow-bar protection will work as follows: Under normal working conditions, the voltage across capacitor C3 will be 6.3V and diode D5 will be in the off state since it will be reverse-biased with the output voltage of 12V. However, during output short-circuit condition, the output will momentarily drop, causing D5 to conduct and the opto-triac MOC3011 (IC1) will get triggered, pulling down the gate voltage to ground, and thus limiting the output current. The circuit will remain latched in this state, and input voltage has to be switched off to reset the circuit. The circuit shown in Fig. 2 follows a similar scheme. It can be utilised when the regulator has to work from a DC rail in place of 15V-0-15V AC supply. The gate voltage here is generated using an LM555 charge pump circuit as follows: When 555 output is low, capacitor C2 will get charged through diode D1 to the input voltage. In the next half cycle, when the 555 output goes high, capacitor C3 will get charged to almost double the input voltage. The rest of the circuit works in a similar fashion as the circuit of Fig. 1. These circuits above will help reduce power-loss by allowing to keep lower input voltage range to the regulator during initial design or even in existing circuits. This will keep the output regulated with relatively low input voltage compared to the conventional regulators. The minimum voltage drop can be further reduced using low RDS(on) MOSFETs or by paralleling them.


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  Wed, 24 Sep 2008 14:36:00 +0200
Simple Sensitive Remote Control Tester
Here is a handy gadget for test- ing of infrared (IR) based re- mote control transmitters used for TVs and VCRs etc. The IR signals from a remote control transmitter are sensed by the IR sensor module in the tester and its output at pin 2 goes low. This in turn switches on transistor T1 and causes LED1 to blink. At the same time, the buzzer beeps at the same rate as the incoming signals from the remote control transmitter. The pressing of different buttons on the remote control will result in different pulse rates which would change the rate at which the LED blinks or the buzzer beeps. When no signal is sensed by the sensor module, output pin 2 of the sensor goes high and, as a result, transistor T1 switches off and hence LED1 and buzzer BZ1 go off. This circuit requires 5V regulated power supply which can be obtained from 9V eliminator and connected to the circuit through a jack. Capacitor C1 smoothes DC input while capacitor C2 suppresses any sudden spikes appearing in the input supply. Here, a plastic moulded sensor has been used so that it can easily stick out from a cut in the metal box in which it is housed. It requires less space. Proper grounding of the metal case will ensure that the electromagnetic emissions which are produced by tube-lights and electronic ballasts etc (which lie within the bandwidth of receiver circuit) are effectively grounded and do not interfere with the functioning of the circuit. The proposed layout of the box containing the circuit is shown in the figure. The 9-volt DC supply from the eliminator can be fed into the jack using a banana-type plug.

Tech. Editor’s note: In fact, the complete gadget can be assembled in the eliminator’s housing itself and a cut can be made in its body for exposing the IR module’s sensor part.


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  Wed, 24 Sep 2008 14:36:00 +0200
Electronic Scoring Game
You can play this game alone or with your friends. The circuit comprises a timer IC, two decade counters and a display driver along with a 7-segment display. The game is simple. As stated above, it is a scoring game and the competitor who scores 100 points rapidly (in short steps) is the winner. For scoring, one has the option of pressing either switch S2 or S3. Switch S2, when pressed, makes the counter count in the forward direction, while switch S3 helps to count downwards. Before starting a fresh game, and for that matter even a fresh move, you must press switch S1 to reset the circuit. Thereafter, press any of the two switches, i.e. S2 or S3. On pressing switch S2 or S3, the counter’s BCD outputs change very rapidly and when you release the switch, the last number remains latched at the output of IC2. The latched BCD number is input to BCD to 7-segment decoder/driver IC3 which drives a common-anode display DIS1. However, you can read this number only when you press switch S4. The sequence of operations for playing the game between, say two players ‘X’ and ‘Y’, is summarised below:

1. Player ‘X’ starts by momentary pressing of reset switch S1 followed by pressing and releasing of either switch S2 or S3. Thereafter he presses switch S4 to read the display (score) and notes down this number (say X1) manually.
2. Player ‘Y’ also starts by momentary pressing of switch S1 followed by pressing of switch S2 or S3 and then notes down his score (say Y1), after pressing switch S4, exactly in the same fashion as done by the first player.
3. Player ‘X’ again presses switch S1 and repeats the steps shown in step 1 above and notes down his new score (say, X2). He adds up this score to his previous score. The same procedure is repeated by player ‘Y’ in his turn.
4. The game carries on until the score attained by one of the two players totals up to or exceeds 100, to be declared as the winner.
Several players can participate in this game, with each getting a chance to score during his own turn. The assembly can be done using a multipurpose board. Fix the display (LEDs and 7-segment display) on top of the cabinet along with the three switches. The supply voltage for the circuit is 5V.


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  Wed, 24 Sep 2008 14:32:00 +0200
Tiny Dew Sensor
Dew (condensed moisture) ad- versely affects the normal per- formance of sensitive electronic devices. A low-cost circuit described here can be used to switch off any gadget automatically in case of excessive humidity. At the heart of the circuit is an inexpensive (resistor type) dew sensor element. Although dew sensor elements are widely used in video cassette players and recorders, these may not be easily available in local market. However, the same can be procured from authorised service centres of reputed companies. The author used the dew sensor for FUNAI VCP model No. V.I.P. 3000A (Part No: 6808-08-04, reference no. 336) in his prototype. In practice, it is observed that all dew sensors available for video application possess the same electrical characteristics irrespective of their physical shape/size, and hence are interchangeable and can be used in this project. The circuit is basically a switching type circuit made with the help of a popular dual op-amp IC LM358N which is configured here as a comparator. (Note that only one half of the IC is used here.) Under normal conditions, resistance of the dew sensor is low (1 kilo-ohm or so) and thus the voltage at its non-inverting terminal (pin 3) is low compared to that at its inverting input (pin 2) terminal. The corresponding output of the comparator (at pin 1) is accordingly low and thus nothing happens in the circuit. When humidity exceeds 80 per cent, the sensor resistance increases rapidly. As a result, the non-inverting pin becomes more positive than the inverting pin. This pushes up the output of IC1 to a high level. As a consequence, the LED inside the opto-coupler is energised. At the same time LED1 provides a visual indication. The opto-coupler can be suitably interfaced to any electronic device for switching purpose. Circuit comprising diode D2, resistors R5 and R6 and capacitor C1 forms a low-voltage, low-current power supply unit. This simple arrangement obviates the requirement for a bulky and expensive step-down transformer.


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  Wed, 24 Sep 2008 14:32:00 +0200
Precision 1Hz Clock Generator using Chip-on-Board
Usually the circuits for generation of 1Hz clock for applications in digital clock and counter circuits make use of ICs in conjunction with a crystal and trimmer capacitors, etc. However, the same or better accuracy can be achieved using a chip-on-board (COB) device found inside a digital clock which is readily available in the market for Rs 15-20. This COB consists of IC, capacitors and quartz crystal, etc which are mounted on its surface. It works on 1.4 volt DC source. This COB can be used to derive 1Hz clock. Resistor R1, capacitor C3, diodes D1 and D2 shown in the circuit convert 5V DC into 1.4V DC. A ½Hz clock is available at terminals A and B with a phase difference of 90o. The two outputs, are combined using capacitors C1 and C2 to obtain a complete 1Hz clock. This 1Hz clock pulse has a very low amplitude of the order of a few milli-volts which cannot be used to drive the digital circuits directly. This low-level voltage is amplified several times by op-amp IC CA3140. The op-amp CA3140 is connected in a non-inverting mode, and its gain is set by resistors R4 and R3. Capacitor C2 reduces the AC gain and unwanted stray pick-up and thus improves stability of the circuit. The input impedance of IC CA3140 is very high and thus there is no drop at the input when 1Hz clock signal of low level is connected across its input terminals from the COB. Amplified 1Hz clock pulse is available at its output pin 6, which is further amplified by transistors T1 and T2 to drive the digital clocks and timers. Preset VR1 is offset null control used to adjust proper 1Hz pulse at the output terminal ‘E’. Connect one LED in series with 220-ohm resistor between the terminal ‘E’ and ground and adjust preset VR1 till the LED blinks once every second. When using the COB, affix the same on a general-purpose PCB using rubber based adhesive and solder the terminals neatly using thin single-strand wire.
Lab Note: The COBs used in different watches may differ some-what in their configuration. But by trial-and-error one can always find out the appropriate points corresponding to points A, B, C and D. Figure of a second COB us-ed by EFY Lab is shown alongside. The points A and B (on the COB used by us) were observed to have complementary 1Hz outputs and hence anyone (only) could be used as input to opamp CA3140.


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  Wed, 24 Sep 2008 14:32:00 +0200
Digital Switching System
This circuit can control any one out of 16 devices with the help of two push-to-on switches. An up/down counter acts as a master-controller for the system. A visual indication in the form of LEDs is also available. IC1 (74LS193) is a presettable up/down counter. IC2 and IC3 (74LS154) (1 of 16 decoder/demultiplexer) perform different functions, i.e. IC2 is used to indicate the channel number while IC3 switches on the selected channel. Before using the circuit, press switch S1 to reset the circuit. Now the circuit is ready to receive the input clock. By pressing switch S2 once, the counter counts up by one count. Thus, each pressing of switch S2 enables the counter to count up by one count. Likewise, by pressing switch S3 the counter counts downwards. The counter provides BCD output. This BCD output is used as address input for IC2 and IC3 to switch one (desired channel) out of sixteen channels by turning on the appropriate triac and the corresponding LED to indicate the selected channel. The outputs of IC3 are passed through inverter gates (IC4 through IC6) because IC3 provides negative going pulses while for driving the triacs we need positive-going pulses. The high output of inverter gates turn on the npn transistors to drive the triacs. Diodes connected in series with triac gates serve to provide unidirectional current for the gate-drive.


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  Wed, 24 Sep 2008 14:31:00 +0200
40-Metre Direct Conversion Receiver
Using the circuit of 40-metre band direct-conversion receiver descr- ibed here, one can listen to amateur radio QSO signals in CW as well as in SSB mode in the 40-metre band. The circuit makes use of three n-channel FETs (BFW10). The first FET (T1) performs the function of ant./RF amplifier-cum-product detector, while the second and third FETs (T2 and T3) together form a VFO (variable frequency oscillator) whose output is injected into the gate of first FET (T1) through 10pF capacitor C16. The VFO is tuned to a frequency which differs from the incoming CW signal frequency by about 1 kHz to produce a beat frequency in the audio range at the output of transformer X1, which is an audio driver transformer of the type used in transistor radios.
The audio output from transformer X1 is connected to the input of audio amplifier built around IC1 (TBA820M) via volume control VR1. An audio output from the AF amplifier is connected to an 8-ohm, 1-watt speaker. 
The receiver can be powered by a 12-volt power-supply, capable of sourcing around 250mA current. Audio-output stage can be substituted with a readymade L-plate audio output circuit used in transistor amplifiers, if desired. The necessary data regarding the coils used i