2 Transistor FM Voice Transmitter
Take care with transmitter circuits. It is illegal in most countries to
operate radio transmitters without a license. Although only low power this
circuit may be tuned to operate over the range 87-108MHz with a range of 20 or
30 metres.
Notes:
I have used a pair of BC548 transistors in this circuit. Although not strictly
RF transistors, they still give good results. I have used an ECM Mic insert from
Maplin Electronics, order code FS43W. It is a two terminal ECM, but ordinary
dynamic mic inserts can also be used, simply omit the front 10k resistor. The
coil L1 was again from Maplin, part no. UF68Y and consists of 7 turns on a
quarter inch plastic former with a tuning slug. The tuning slug is adjusted to
tune the transmitter. Actual range on my prototype tuned from 70MHz to around
120MHz. The aerial is a few inches of wire. Lengths of wire greater than 2 feet
may damp oscillations and not allow the circuit to work. Although RF circuits
are best constructed on a PCB, you can get away with veroboard, keep all leads
short, and break tracks at appropriate points.
One final point, don't hold the circuit in your hand and try to speak. Body
capacitance is equivalent to a 200pF capacitor shunted to earth, damping all
oscillations. I have had some first hand experience of this problem.
Title: 2 Transistor FM Voice Transmitter
electronic circuit
Source: www.electronics-lab.com
Published on: 2005-02-03
Reads: 2416
Print version:
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