Having a range of up to a quarter mile, it’s great for a house security system, baby monitoring device or simply a listening gadget that you can place anywhere!

C4 is a small, screw-adjustable, trimmer capacitor. Set your FM receiver for a clear, blank space. Then, with a non-conductive tool, adjust this capacitor for the clearest reception. Although this transmitter is designed for the FM broadcast band, it can be tuned to 2 meters, and other VHF bands by changing values of C4 and L1.

L1 is 9 turns of #22 gauge solid wire (air-wound) 1/4 inch diameter coil. Use a 1/4 inch diameter bolt and wrap the wire in the threads. After mounting the coil, back out the bolt.

C1, C2, C3, and C5 are ceramic type capacitors, preferably npo (low noise) or equivalent. However, you can use any type you have around, but do not use electrolytic or tantalum capacitors.

A 2N3904 transistor was used for Q1 and Q2. The 2N3904 is a general purpose silicon NPN bipolar transistor used for switching and amplifier applications. However, you may substitute the 2N3904 with a 2N2222 or a 2N3906, these are also general purpose transistors.

The antenna is 8” to 18” of any type wire.

Try to keep all leads as short as possible to prevent stray capacitance.

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 5 – 30pf variable capacitor
C5 4.7pf capacitor
Q1, Q2 2N3904 transistor
L1 9 turns of #22 gauge
Microphone Electret Mic
Circuit Board
Battery Clip

You can buy these parts at any electronics store.

Source: Free Kit Plan – FM Transmitter