A working low-power FM transmitter requires little more than the connection of a series of transistors, capacitors and resistors on a printed circuit board. Though the design of the circuit is quite simple, assembling it requires some skills, including familiarity with a soldering iron and the ability to read a circuit diagram. Also, either during construction or after, changes can be made to the circuit to modify the transmitter in various ways, to adapt its output to better serve your needs.

Instructions

1.Change the output frequency of the transmitter by changing the number of coils in the inductor for the oscillating stage of the circuit (the L1 position). Wrap a section of No. 22 gauge wire around a 1/4-inch diameter bolt to create the coils, with the number of loops determining the frequency, ranging anywhere from the 88 to 108 MHz of the FM band. Replace only this inductor if you choose, or you may opt to start from scratch on a different circuit board if you plan on making other modifications as well. Solder the inductor into place if replacing only this component.

2.Increase the power of the transmitter by adding additional stages, either by expanding onto the existing circuit board or assembling a new one. Obtain a circuit board diagram for this more-powerful transmitter, and insert the various capacitors, resistors, transistors and inductors according to their positions on the diagram. Turn the circuit board over to solder the various elements together when finished. Also use a stronger battery, upgrading to a 9-, 12- or even 15-volt battery to increase power if the circuitry of the transmitter is equipped to handle it.

3.Extend the range of the transmitter by adding a BALUN (BALanced to UNbalanced) transformer at the junction of the C11 and C12 positions -- using an example of a four-stage transmitter -- to further amplify the power of the transmitter, and replace a standard wire antenna with a 300-ohm impedance dipole antenna.

4.Change the resistor nearest the microphone to adjust the sensitivity of the microphone as well as its audio quality. Select either a 1, 5 or 10-K resistor, with the 1-K resistor allowing for the greatest sensitivity, and the 10-K resistor the least.

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