This is a simple and small Two Transistors FM Transmitter. It is ideal for ready espionage for the strip from radio Fm or receiver of VHF. Of course, the recreational purpose also exists and the children will adore having a transmitter that allows speaking for a radio FM placed at a distant place and like this pretend the secret agent.
Operation of the Two Transistors FM Transmitter
The voice/music captured by the electret microphone, your voice or close people’s chats, are mischievous to the base of the PNP transistor Q1 through C1 (10uF) where is amplified. The resistor R2 together with R3 it determines this amplification.
After having amplified, the sign is removed from the collector of Q1 transistor and mischievous to the base of Q2 transistor (NPN), where it modulates in frequency the sign generated by this stage. This stage is an oscillator of high frequency that operates in the strip of FM. Your frequency is determined by L1 (0.3uH) and for the adjustment on the variable capacitor (CV).
The obtained signs, already modulated in frequency, they are mischievous to a linked antenna in the collector of the transistor Q2 (BC557 Transistor).
The range of the transmitter depends on this antenna, of the feeding of the circuit and of the place where it is operating. In the open field, with 6 V battery and an antenna of about 40 cm of length, this range of this Tow Transistors FM Transmitter can be 1000 meters. With a smaller antenna, the range of this transistor will reduce. A common wall is not an obstacle for this transmitter.
Q1 – BC557
Q2 – BF494
MIC–microphone electret two terminals
Resistors: (1/4 W, 5%)
C1–10 µF/16 V – ELECTROLYTIC
C2–4,7 µF/16 V – ELECTROLYTIC
C3–10 nF – ceramic
C4–4,7 pF – ceramic
C5–100 nF – ceramic
CV–trimmer from 3-30 to 5-50 pF
L1 – 0.3uH
B1 –3 to 9 V
Printed Circuit Board With Full Detail is given Below
Full Discussion about this circuit you can freely discuss.
Thanx a lot