FM Wireless Microphone Circuit Design Explained

Making a wireless mic at home can be real fun, here we learn one such simple project which can be used for recording and paying back your voice wirelessly.
Cordless microphone and amplifier units are generally used during public address programs, stage entertainment programs or in all forms of occasions where voice signals are required to be amplified so as to make them audible over a wider area and distance. However since microphones are normally held by the hand while speaking, the unit needs to be perfectly hassle free so that the individual holding it is able to move about the premise freely. In this article we learn how to construct a simple wireless microphone circuit and use exactly for the above intended purpose.
A microphone is a device which is able to convert voice or sound vibrations in the air into electrical pulses. They are generally used for public address purposes and entertainment programs. Here we learn a very simple way of making an FM wireless microphone circuit that requires no wires for the specified operation
Older types of mics carried a wire or an electrical cord from the mic up to the amplifier, making things very cumbersome and inconvenient for the user. The cord used to dangerously dangle about the legs of the user making him vulnerable to entanglement and even stumbling because of the mess.
This led to the invention of much sophisticated wireless types of mics which became much comfortable to handle and use on any platform, moreover the distance of the user from the amplifier also was no longer an issue now.
However the invention could take place only after the invention and improvements in the FM broadcast technology, because the wireless mic actually incorporated a small FM transmitter which sent the voice signals in the form of FM waves to the FM receiver before it could get amplified in to the loudspeakers.
These wireless mics are still being used effectively for the intended applications and have become quite indispensable with the specific users.
Though the device may look quite sophisticated with its operations, but did you know it is actually very easy to construct at home and therefore can be made by any electronic enthusiast?
It is definitely one of the best fun electronic projects as it not only provides thorough amusement while making it but can be proudly used by the constructor for displaying the impressive wireless transmission capabilities of the built device.

Construction Clues

Let’s try to understand how to build a wireless FM microphone circuit.
The mic section actually consists of a mini FM transmitter which is so small that literally it can be accommodated in a space of less a square inch and if its made using SMD, it could well be made within an area of 1 square cm.
Actually the unit can be experimented in many different ways as the parameters involved are truly flexible. The power consumption being negligible allows us to use button cells for the operations. However pencil cells would be more preferable if the unit is intended to be used for long hours of speech transmission.
The main active part of the circuit is the general purpose transistor, while the other supporting passive parts are also very few making the item very compact as far as part count is concerned.
The circuit assembly strictly does not require a designed PCB, nope! And in fact is not recommended either. The whole circuit can be fitted over a small piece of veroboard, or probably if you have a good hand with soldering, you would be able to stitch the parts together over a thin piece of plastic or rubber strip.
The figure shown alongside illustrates the details of the transmitter part, all that’s required for completing the wireless microphone section. A plastic pipe or any similar enclosure may be used for housing the circuit along with the battery and the switch.

The transistor, the inductor and the relevant capacitors are mainly responsible for generating the FM carrier waves; the configuration quite resembles a Colpitts oscillator. The capacitors C1, C2 and C3 mainly determine the oscillator frequency and can be altered for changing the reception positions over the FM receiver band.
The MIC converts the voice signals spoken close to it into electrical pulses. These electrical pulses hit the base of the transistor, which now suddenly functions as an audio amplifier, amplifying the signals at its collector arm.
However since the tank configuration responsible for manufacturing the carrier waves is also included at the collector arm get influenced by these amplified voice signals.
The carrier waves now start getting modulated or rather ridden by the audio signals constituting the transmission of the audio in the air.
The transmitted waves can be received over any standard FM radio receiver, or if the unit is to be operated directly in association with a high power amplifier unit then probably a FM receiver module may have to be built with a headphone jack integrated for allowing an easy plug-in with the amplifier LINE IN socket.
The FM module are easily available ready-made in the market with presets for the necessary frequency adjustments.
These are quite small PCB assemblies having built-in presets and discrete outputs for volume control, audio, and antenna. The only section that does not become a part of these assemblies is the amplifier which any way we don’t need as the amplification function is primarily associated with the PA system where the FM module needs to be fixed through the relevant LINE input sockets.
The FM module can be easily accommodated inside a small plastic square box with the embedded large jack protruding out of the box and also the antenna in the form of a neatly wrapped flexible piece of wire.
However for hobby purpose you may use your home FM radio for the receptions.

Testing and Setting up the Microphone Transmitter

Once the transmitter is built, it may be tested with the following few simple steps:
Connect a 3 volts supply to the circuit, preferably from two AAA pencil cells.
Keep a FM receiver somewhere around the transmitter at about 2 meters from it initially and start tuning the receiver until you find the “null” spot where the “hissing” from the radio suddenly becomes zero.
Now tap or speak loudly over the mic of the transmitter, which should be audible over the receiver clear and loud.
Now take the FM radio further away from the transmitter to about 10 meters and repeat the procedure by readjusting the tuning of the radio until the reception is crystal clear.
The testing of the wireless mic is done and it’s ready to be used.
House the e entire assembly inside a suitable enclosure as described in the above section and you are all ready with an efficient cordless microphone…….Well,.. now nobody can stop you from becoming a home-brewed karaoke rock star.


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