Presentation of the FM transmitter:
The kind of particularly popular electronic gadget lovers … even if it rarely works the first time!
FM Transmitter (FM = Frequency Modulation) also sometimes called “Micro Spy”, because it can transmit through a microphone, a conversation from one point to another without wire connection. But let’s call it so here FM transmitter because it is obvious that you know it is forbidden to spy on someone without telling him (what a strange idea, too, that someone spying after be informed of your intentions). This arrangement is, therefore, a transmitter, and you’ll need a receiver to complete the transmission chain. Any conventional FM receiver to receive the range 88 to 108 MHz (VHF) is perfect.
The basic scheme for crystal microphone, power supply 9 V.
FM broadcast in stereo or mono ?
The transmitters described here provide a diffusion of an audio signal in mono, even those with a “stereo” input.
If you take the time to search on the net you will find lots of patterns of FM transmitters. Myself have made of several types. One whose description follows, but other smaller operating with a supply voltage as low as 1.5 or 3 volts. This scheme gives acceptable results, but I still have to warn you: its scope is very correct, but its consumption is not negligible. The saline 9V battery will last approximately two hours continuously, an alkaline may last up to 5 hours. Its main advantage is a safe operation and in its simplicity. In addition, all components are common and inexpensive, which will finally convince the beginner to try it.
The oscillator is realized with a transistor (Q2) mounted in an oscillating circuit composed of an inductor ( L1) and trimmers (VC1 and VC2). The adjustable capacitor VC1, coupled in parallel to the inductor L1, constitutes the resonance circuit which will set the transmission frequency. There will be two ways to change this transmission frequency either by varying the value of the adjustable capacitor VC1, either by spacing or by tightening a few turns of the inductor L1. The adjustable capacitor VC2 adjusts the reaction rate and ensure reliable starting of the oscillation at each power up (it is well known that an amplifier should not oscillate oscillates, and an oscillator which should oscillate might not start). Personally, I have replaced several times this trimmer with a fixed or 6,8pF 8,2pF and it always worked. But just as I tell you this for your copy makes it difficult. So do as you feel. The polarization of the transistor is provided by the emitter resistance of 100 ohms and 22 ohms of the base resistance. It’s hard to make it simpler.
The frequency modulation is effected by varying the polarization of the oscillation transistor (Q2). This is ensured by Q1, whose collector-emitter resistance will vary depending on the signal applied between its base and its collector, signal from a microphone type “crystal”.Polarization continuously from the base of transistor Q1 is provided by resistor R1 68K.