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Sunday, November 28, 2010

Low Power FM Transmitter


This article should satisfy those who might want to build a low power FM transmitter. It is designed to use an input from another sound source (such as a guitar or microphone), and transmits on the commercial FM band - it is actually quite powerful, so make sure that you don't use it to transmit anything sensitive - it could easily be picked up from several hundred metres away. The FM band is 88 to 108MHz, and although it is getting fairly crowded nearly everywhere, you should still be able to find a blank spot on the dial.
NOTE: A few people have had trouble with this circuit. The biggest problem is not knowing if it is even oscillating, since the frequency is outside the range of most simple oscilloscopes. See Project 74 for a simple RF probe that will (or should) tell you that you have a useful signal at the antenna. If so, then you know it oscillates, and just have to find out at what frequency. This may require the use of an RF frequency counter if you just cannot locate the FM band.
The circuit of the transmitter is shown in Figure 1, and as you can see it is quite simple. The first stage is the oscillator, and is tuned with the variable capacitor. Select an unused frequency, and carefully adjust C3 until the background noise stops (you have to disable the FM receiver's mute circuit to hear this).