Circuit : David Celestin, Ghana W. Africa
A portable FM transmitter that can be used to replace a FM microphone.
This simple transmitter operates from a 9V battery as shown above. I personally built this for a
purpose. This is how it happened:- "members of my singing group find it difficult to handle a FM mic
in Church. The choir sings in acapella form and the congregation expect us to perform well. 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."
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:
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: