Circuit of potent transmitter of FM with transistor 2n2218
This small transmitter can reach more than 1 km in favorable conditions of emission. Modulation can with an electret microphone or another audio source. The frequency is given by CV (common trimmer of 2-20 or 3-30pF) and L1 4 or 5 turns of 22 AWG enameled wire, with a diameter of 1 cm with air core.
Watch out for the capacitors that should be ceramic. The antenna should have 72 cm. For transmission connect a nearby FM radio receiver to a half volume at a free frequency (with no radio operating), with a wooden / plastic wrench, turn the CV screw until you pick up the strongest signal from the transmitter, Because a spurious signal can be picked up. If you have difficulty adjusting, remove the coil and wind it up again with more or less turns. Below we have the circuit diagram of the FM transmitter. Mic1 is a two-terminal electret microphone. The R1 resistor biases the microphone; you may need to change the value of R1 to fit your microphone, values from 1k to 10k can be tested. Preferably use glass fiber board, which is suitable for high frequency.
Below we have the schematic of the one of the circuit of the transmitter of FM
List of Material for assembly of the FM transmitter
Resistors 5% 1/4 w:
R1 = 3 a 10kΩ (Brown, black, orange, gold)
R2 = 6,8kΩ (Blue, Gray, red, gold).
R3 = 4,7kΩ (yellow, violet, red, gold. )
R4 = 39Ω (Orange, White, black, gold.
C1,C3 = 100nF (104) Ceramic
C2= 6.8nF (682) ceramic
C3 = 6.8pF (6p8) ceramic
C4 = 100nF (100n, or 0.1 or 104)
CV = TRIMMER 3-30 PF.
T1 = 2n2218, 2n2219 or equivalent.
L1 = see the text
B1 = 9V battery.
Mic1 = electret microphone.
Several = printed circuit board, antenna, box, etc.
2n2218 FM transmitter PCB layout
Pinout of the transistor 2N2218
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