This is the schematic for an FM transmitter with 3 to 3.5 W output power that can be used between 90 and 110 MHz. Although the stability isn't so bad, a PLL can be used on this circuit.

Content list :

R1 27k


R2 56k


R3 12k


R4 100


C1 1u

Colpitts oscillator. Its frequency depends on the capacitance of the vary cap diode. The center frequency is changed by varying the biasing voltage of the vary cap through the 47K pot. You can use a 75cm telescopic antenna or simply a length of hook-up wire. Mine worked fine with a 6cm hook-up wire and gave a range of 100m with a good FM receiver. The approx. cost of the circuit is around Rs.35

This circuit will transmit a continuous audio tone on the FM broadcast band (88-108 MHz) which could used for remote control or security purposes. Circuit draws about 30 mA from a 6-9 volt battery and can be received to about 100 yards. A 555 timer is used to produce the tone (about 600 Hz) which frequency modulates a Hartley oscillator. A second JFET transistor buffer stage is used to isolate the oscillator from the antenna so that the antenna position and length has less effect on the frequency.

Transmitter Parts:

R1, R4, R6 10K resistor


R2 1Meg resistor


R3 100K resistor


R5 100 ohm resistor


R7 1K resistor


C1, C2 0.1uf capacitor


C3 0.01uf capacitor


C4 4-40pf variable capacitor


C5 4.7pf capacitor


Q1, Q2 2N3904 transistor


L1 8-10 turns of 22gauge wire around non-conductive 1/4" diameter form


Elec Mike Electret Mic

Parts List:

R1,R3 = 100K


R2 = 10K


R4 = 470 ohm


C1,C4 = 470pF


C2,C3 = 4.7µF, 16V, electrolytic


C5,C6 = 4.7pF


C7 = 4-40pF trimmer cap (optional, see text)


L1 = 1µH


Q1,Q2 = 2N2222, NPN transistor


Mic = Electret Microphone


B1 = 9 Volt, Alkaline battery