This FM transmitter works in the 88-108 MHz frequency band, low-power electronics assembly and easy to implement for use in our homes. And this circuit is great for us to start practicing a little radio transmissions.
Frequency modulation (FM).
The FM modulation is performed on the base-emitter of transistor Q2 and at this point there is a sum of the frequency of arrival (audio) and the output frequency of the oscillator.
Antenna : We will soldering one meter length of cable at the end of a first round of the loop that is closest to the collector of transistor Q2.
Adjustment : To work properly, we must adjust VR1 so that we have the Q1 collector voltage between 1.5 and 1.9 volts.
Frequency setting : Tune to an FM radio about 95 MHz (in an area where there is no station) after a few meters from the circuit. We then apply a voltage to the transmitter and began to slowly change the settings of C2 until background noise disappears, once this is achieved, we can test the performance of our voice modulation into the microphone. We also try to change the frequency of transmission separately or together the turns of the coil.
The Micro Wireless Spy FM 88-108 coil construction:
N is the number of turns = 5 turns
D: is the coil radius in inches = 0.5 cm
L is the length in inches = 1 cm
