FM Beacon Broadcast Transmitter

This circuit will transmit a continuous audio tone on the FM broadcast band (88-108 MHz) which could used for remote control or security purposes. Circuit draws about 30 mA from a 6-9 volt battery and can be received to about 100 yards. A 555 timer is used to produce the tone (about 600 Hz) which frequency modulates a Hartley oscillator. A second JFET transistor buffer stage is used to isolate the oscillator from the antenna so that the antenna position and length has less effect on the frequency. Fine frequency adjustment can be made by adjusting the 200 ohm resistor in series with the battery.

Oscillator frequency is set by a 5 turn tapped inductor and 13 pF capacitor. The inductor was wound around a #8 X 32 bolt (about 3/16 diameter) and then removed by unscrewing the bolt. The inductor was then stretched to about a 3/8 inch length and tapped near the center. The oscillator frequency should come out somewhere near the center of the band (98 MHz) and can be shifted higher or lower by slightly expanding or compressing the inductor. A small signal diode (1N914 or 1N4148) is used as a varactor diode so that the total capacity in parallel with the inductor varies slightly at the audio rate thus causing the oscillator frequency to change at the audio rate (600 Hz). The ramping waveform at pins 2 and 6 of the timer is applied to the reversed biased diode through a large (1 Meg) resistor so that the capacitance of the diode changes as the ramping voltage changes thus altering the frequency of the tank circuit. Alternately, an audio signal could be applied to the 1 Meg resistor to modulate the oscillator but it may require an additional pull up resistor to reverse bias the diode. The N channel JFET transistors used should be high frequency VHF or UHF types or similar.

Teleconfrencing System

Here is a low-cost teleconferencing system that lets you talk to two persons at a time in any part of the world over two telephone lines. The circuit makes use of a coupling transformer and some passive components.
The circuit is connected between the two telephone lines. It works like this: When ‘X’ calls ‘A’ on the first telephone line, ‘A’ puts this call on hold, dials ‘Y’ on the other telephone line (which is free) and keeps this call too on hold, and slides switches S1 and S2 to ‘on’ position. Now ‘X,’ ‘A’ and ‘Y’ can talk to one another simultaneously over the two telephone lines.

Both the primary and secondary coils of the coupling transformer consist of 500 turns of 40SWG insulated copper wire. At the secondary side, a small circuit is used for DC holding. This circuit is built around transistor T1 (BC547), resistors R2 and R3 (15 kiloohms and 100 ohms, respectively), condenser C3 (22μF, 63V) and two LEDs as indicators for both the primary and secondary sides. It provides proper DC characteristic to hold second telephone line in operation even though no telephone on that line is present
Here, transistor T1 acts like a resistor to DC and as high impedance for audio signals. The high impedance of the circuit is provided by condenser C3, which prevents any audio signal from appearing at the base of T1. Thus any audio voltage appearing across telephone line No. 2 will not cause a corresponding current in the transistor..

FM-Transmitter

Nothing critical here. To get a bit of tuning out of the coil you could put a 4-40pF trimmer capacitor (optional) parallel over the 1 μH coil, L1. C1/C4 and C5/C6 are ceramic capacitors, preferably NPO (low noise) types. C2/C3 are electrolytic or can be tantalum types. The antenna is nothing more than a piece of 12" wire or a piece of piano wire from 6" to 12".

To find the signal on your receiver, make sure there is a signal coming into the microphone, otherwise the circuit won't work. I use an old mechanical alarm clock (you know, with those two large bells on it). I put this clock by the microphone which picks up the loud tick-tock. I'm sure you get the idea... Or you can just lightly tap the microphone while searching for the location of the signal on your receiver.

Parts List:

R1,R3 = 100K

R2 = 10K

R4 = 470 ohm

C1,C4 = 470pF

C2,C3 = 4.7μF, 16V, electrolytic

C5,C6 = 4.7pF

C7 = 4-40pF trimmer cap (optional, see text)

L1 = 1μH

Q1,Q2 = 2N2222, NPN transistor

Mic = Electret Microphone

B1 = 9 Volt, Alkaline battery

Watch Dog For Telephones

Most of the telephone security devices available in market are simple but quite expensive. These devices provide blinking or beeping type line-tap/misuse indications. Quite often they do not offer guaranteed protection against unauthorized operation. A very simple and unique circuit of a telephone watch-dog to safeguard subscriber telephone lines against any fraud is described here. This little circuit keeps continuous watch over the telephone lines and sounds an alarm in case of any misuse. In addition it transmits a loud tone through the telephone lines to prevent further misuse. When switch S1 is turned on, the normal (on-hook) telephone line voltage at the output of bridge-rectifier diodes D1 to D4 is approximately 48 volts, which being well above the break-down voltage of zener diode D5, the diode conducts. As a result transistor T2 gets forward biased. This effectively grounds the base of transistor T1 which is thus cut off and the remaining circuit does not get any power supply. In this state, only a small (negligible) current is taken by the circuit, which will not affect the telephone line condition. However, when handset of any telephone connected to the telephone lines is lifted (off-hook), line voltage suddenly drops to about 10 volts. As a result, transistor T2 is switched off and transistor T1 gets forward biased via resistor R1. Now, the astablemultivibrator built around timer IC1 starts oscillating and the speaker starts sounding. Output of the astablemultivibrator is also connected to the base of transistor T1 through capacitor C5. As a result, only a loud (and irritating) tone is heard in the ear-piece of the unauthorized telephone instrument. This circuit can be constructed on a veroboard using easily available low cost components and it can be connected to any telephone line without the fear of malfunctioning. No extra power supply is required as it draws power from the telephone line for operation. Note: Please disconnect the gadget when you are yourself using the telephone as it cannot distinguish between authorized and unauthorized operation.

Clapp Switch

The circuit shown here for clap operated switch is inexpensive and easy to assemble by hobbyists of any level. Clap sound signals picked up by condenser microphone are first amplified by transistor T1, which is a simple common-emitter amplifier. Amplified signals are rectified by diode D1. Positive half cycle of clap signal is applied to a 3-stage DC amplifier formed by transistors T2, T3 and T4. Output from transistor T4 is used as a clock for flip-flop IC1 (7472). For each clap, the output of IC 7472 toggles alternately to on/off state. Transistor T5 functions as relay driver since output level of IC1 is insufficient to drive a relay directly. The entire circuit, except the relay driver, operates at 5 volts regulated.

The 5-volt supply is derived from 12V supply (used for relay driver transistor T5) using a zener diode with series resistor R11 (150-ohm, 1-watt). Using this circuit any electrical or electronic load can be controlled by just clapping in front of the microphone. The microphone should be housed inside a suitable funnel shaped enclosure to improve sensitivity. Potmeter VR1 is used as sensitivity control. The entire circuit can be powered from simple 12-volt unregulated supply using 12-volt step-down transformer, followed by full-wave rectifier and a filter capacitor of about 1000μF, 25V.

Photo Electric Strret Light

This is basically a Schmitt Trigger circuit which receives input from a photo cell and controls a relay that can be used to switch off and on a street lamp at dawn and dusk. I have built the circuit with a 120 ohm/12volt relay and monitored performance using a lamp dimmer, but did not connect the relay to an outside light

The photocell should be mounted above the light on top of a reflector and pointed upward at the sky so the lamp light does not strike the photo cell and switch off the lamp

The switching points are about 8 volts and 4 volts using the resistor values shown but could be brought closer together by using a lower value for the 7.5K resistor. 3.3K would move the levels to about 3.5 and 5.5 for a range of 2 volts instead of 4 so the relay turns on and off closer to the same ambient light level. The potentiometer would need to be readjusted so that the voltage is around 4.5 at the desired ambient condition.

Simple Infra-Red Detector

Circuit description:

This circuit is a simple IR detector for testing IR remote controllers. The circuit is based on one phototransistor which receives the IR beam. The NPN transistor works as an amplifier which feeds current to the led. When this circuit detects IR or light, the LED is on. So you need to shield the phototransistor from ambient light if you don't want to do your tests in the dark. The best way is to fit the phototransistor in a small black tube.

Component list:

Q1 BP109 (or similar phototransitor)

Q2 BC238C or BC547

D1 RED LED

R1 390 ohms 0.25W