This transmitter is very similar to the simplest RF Transmitter. The Different is the photoresistor placed in series with R1. This could be also a thermistor or other variable resistance sensor. The frequency of the tone that is transmitted varies with the amount of light falling on tha photoresistor.
L1 is 20 to 30 turn of 24 to 32 ga. Magnet wire close wound around a 1/8 to 1/4" diameter non-conductive form and tapped 1/3 or the way from one end. The tap is connected to the emitter of Q1.
The user should be able to pick up the signal from this transmitter on any regular FM or VHF receiver. By increasing the number of turns on L1, the RF frequency can be dropped down all the way into the AM broadcast band.
via Tony Van Roon
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