Wiring circuit

88-108 MHz FM Broadcast Transmitter circuit

This circuit will transmit a continuous tone audio FM broadcast band (88-108 MHz) which could be used for remote control or security. Circuit draws about 30 mA with a 6.9-volt battery and can receive about 100 meters. A 555 timer is used to produce the tone (about 600 Hz), which modulates the frequency of a Hartley oscillator. A second JFET transistor buffer stage is used to isolate the oscillator from the antenna to the antenna position and length has less effect on the frequency.

Fine adjustment can often be done by adjusting the 200 Ohm resistor in series with the battery. Oscillator frequency is set by an inducer of turn 5 and 13 pF capacitor operated. The inductor is wound around a # 8 x 32 bolt (about 3 / 16 in diameter) and then removed by unscrewing the screw. The inductor was then streached to about a length of 3 / 8 inches and hit near the center. The oscillator frequency should come out somewhere near the center of the band (98 MHz) and can be moved up or down slightly the expansion or compression of the coil.

A small signal diode (1N914 or 1N4148) is used as a varactor diode so that the total capacity in parallel with the coil varies slightly in the type of audio causing the oscillator frequency to change the audio rate (600 Hz) . The ramp waveform at pins 2 and 6 of timer applies to the reverse diode across a large bias (1 Meg) resistor so that the capacity of the diode changes as voltage changes thus increasing altering the frequency of tank circuit.

Alternatively, an audio signal could be applied to the 1 Meg resistor to modulate the oscillator but may require a pull-up resistor to reverse bias additional diode. The N channel JFET transistors used should be high frequency VHF or UHF types (Radio Shack # 276-2062 MPF102) or similar.