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FM radio beacon transmitter circuit

2016-04-04 01:47  
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This article describes the FM radio beacon transmitter circuit. Schematics can combine text look better grasp of this principle. This circuit will transmit a continuous audio signal, the FM broadcast band (88 - 108 MHz), it may be used for remote control or security purposes. Circuit draws approximately 30 mA from June - 9 volt battery, you can receive about 100 yards. A 555 timer is used to generate the tone (about 600 Hz), the frequency adjustment a Hartley oscillator. The second stage is the transistorsthe JFET transistor buffer is used to isolate the oscillator from the antenna, the antenna position and length has little effect on the frequency. Fine frequency adjustment can be adjusted by 200 ohm resistor in series with the battery. Oscillator frequency is set five tapped inductor and 13 pF capacitors. The inductor is wound a # 8 X 32 bolts (about 3/16 diameter), and then tighten the bolts. Then inductance streached about 3/8 inch length and utilization near the center. The oscillator frequency should come out near the center of the band (98 MHz), can be transferred either high or low, slightly expanded or compressed inductors. A small signal diode (1N914  or 1N4148), is used as a variable capacitance diode, so the total capacity of the parallel inductor audio rate slightly different oscillator frequency changes resulting audio frequency (600 Hz). Intermittent waveform pins 2 and 6 of the timer is used to reverse bias the diode through a large (1 meg) resistors, capacitors, diodes, the voltage change is thus changing the oscillation frequency changing circuit. Alternatively, an audio signal can be applied to a 1 meg resistor adjusts the oscillator but it may require additional pull-up resistor diode reverse bias. N-Channel JFET transistors used should be of transistorsthe HF VHF or UHF types (Radio Equipment Company # 276 - 2062 MPF102) or similar. The principle is simple and effective.

FM Transmitter


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