FM Voice Transmitter

The following diagram is the circuit diagram of voice transmitter which use FM signal carrier to transmit the vioce signal to the FM receiver device.

Components List:
R1 = 4.7K
R2 = 330 ohm
C1 = 0.001uF (1nF)
C2 = 10-40pF
C3 = 4.7pF
Q1 = 2N3904
L1 = see text
Misc = Electret mike, antenna, 3V battery (button cell)

Circuit Construction:

This is one more easy-to-build miniature transmitter that uses a minimum of parts. Construction is straight forward and non-critical. Although this design uses a 3-volt power source (such as a lithium coin or button cell), a 9-volt battery can be utilized, instead, by growing the value of R1 to 15K and R2 to 1K. C4 is an optional RF bypass capacitor that might assist enhance performance and improve the range a bit. Experiment to find greatest results.

L1 was produced by stripping 22 gauge hookup wire of it’s insulation, then wrapping it in the grooves of the screw threads of a 1/4 diameter bolt, and then back-screwing the bolt out of the resulting coil. 8 turns were produced around the bolt. By wrapping the turns within the threads, a uniform seperation was created between the coil windings.

If you decide to substitute transistors with some thing similar you already have, it perhaps necessary adjust the collector voltage of Q1 by changing the value of R2 or R3 (because you change transistors, it adjustments this bias on the base of Q1). It should be about 1/2 the supply voltage (about 4 or 5v).

Circuit Notes:

The default for the capacitors sort is ceramic, preferably the npo 1% sort or equivalent. But essentially nothing crucial here. Use any capacitor you’ve got laying arround, but no electrolytic or tantalum capacitors. Do not go out and rush to the store. Most parts may be salvaged from somewhere. Only in the event you intend to make use of this circuit outside the home you may need to choose much more temperature stable capacitors.

I’m not positive about the range. With the 3V supply it’s most likely around 100 feet or so. The 9V supply will beef up the range considerably, again not tested, but probably within the 300 feet range or so.

To find out the signal on your receiver, ensure there is a signal coming into the microphone, otherwise the circuit will not function. I use an old mechanical alarm clock (you know, with those two significant bells on it). I put this clock by the microphone which picks up the loud tick-tock. I’m positive you get the notion… Or it is possible to just lightly tap the microphone whilst searching for the location of the signal on your receiver.

FM voice transmitter circuit source: http://www.sentex.ca/~mec1995/circ/fmt2.htm