FM wireless Mike is a small electronic project based on a radio transmitter. It
is a wonderful idea to make a wacky talky through which you can talk to your
friend, next door. Just speak or play into the microphone and you’ll broadcast
to an FM receiver at distances up to 50 feet (maybe 100 feet if the wind is
right). Use a standard FM radio to receive the sound.
circuit is a combination of a Microphone preamplifier and a FM transmitter.
Microphone Preamplifier is a simple audio amplifier. Transistor Q1 is operated
in Voltage Divider Mode. We use a dynamic microphone in the amplifier. The small
alternating current produced in the microphone is amplified and fed in to the FM
FM transmitter is a tuned collector radio frequency oscillator. The coil and
trimmer is in parallel tank circuit. We specify the value of trimmer and size of
the coil such that the resonant frequency will be near 88-108 MHz (in the FM
radio Band). The oscillator oscillates with a constant resonant frequency and we
can change the operating frequency by changing the trimmer capacitance. This
frequency is the center frequency of the FM. If we want the FM as a modulation
we have to apply the modulating signal (audio signal from amplifier) to the base
of the transistor of the oscillator. We all know the base-emitter junction acts
like a diode. If we apply the signal the voltage across the junction will change
and this particular transistor have some property of changing base capacitance
according to the applied input voltage. Thus we change the capacitance which is
in series with the resonant tank circuit causing a change in operating
frequency. This is actually the FM modulation. Now the modulated signal will be
transmitted via a small radio antenna.
List for FM Wireless Mike
B1 - 9.V battery, Type
C1 - 0.05.uF, 3.V DC capacitor
C2 - 20.uF, 3 V DC electrolytic capacitor
C3 - 5.uF, 12 V DC electrolytic capacitor
C4 - 47.pF, 25 V DC capacitor
C5 - 5.30 pF trimmer capacitor
C6 - 6.3.pF ceramic capacitor
C7 - 0.01.uF, 10 V DC capacitor
L1 - see pictorial detail
MIC - Crystal or ceramic microphone element
Q1,Q2 - npn transistor HEP - 720
R1 - 47,000.ohm, 1/2. watt resistor
R2 - 33,000.ohm, 1/2. watt resistor
R3 - 1500.ohm, 1/2. watt resistor
R4 - 3300.ohm, 1/2. watt resistor
R5 - 100,000.ohm, 1/2. watt resistor
R6 - 470.ohm, 1/2. watt resistor
S1 - SPST switch
Use standard RF wiring precautions and
make coil L1 exactly as shown. Best speech clearly is obtained by using crystal
or ceramic mike. For music reproduction, substitute a dynamic mike element.
The unit can be assembled on a cardboard using push in terminals for tie points.
The case must be metal to prevent hand capacitance from continuously changing
the output frequency. Pass the 6-in. solid ware antenna through the metal case
using 1/4th- in. whole and a matching rubber grommet for an insulator.
Use the circuit to broadcast your voice to your neighborhood. Talk to your
friend next door. One thing you should keep in your mind -don't tune in a
frequency where a station is broad casting otherwise you may disturb other FM
listeners in your neighborhood.