Microphone FM Transmitter

This is a simple microphone fm transmitter. It operates from a 9V battery as shown above. The old FM mic system was not a perfect solution, so I built this kit and a mixer for the receivers. using three separate transmitter circuits and a mixer on the receiever, the choir voices were perfect and cleanly reproduced.

See detail of microphone FM Transmitter schematic below.


From the notes I made experimentally, C3 is vital to the circuit and without it the circuit may become unstable. C4 is in parallel with C5 and presents a moderate load impedance. Finally all transistors are NPN. The circuit works well and has proved reliable.

In the circuit L1 has 6 turns and has a diameter of 5.5mm and has a length of 4.5mm. Based on the formula for an air cored inductor:

Inductor formula:

Where:
L1 = inductance in uH
r = radius of coil in inches
l = length of coil in millimetres (mm)
n = number of turns on coil

Using the above values, the inductance of L1 can be calculated. A diameter of 5.5mm = 0.22 inches, the radius is half this value or 0.11 inch, the length is 4.5mm and number of turns, n = 6. This gives L1 a value of: