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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.



92 Comments

  1. hi am new here but got interested with your ccts please can you help with the reciever section to enable attachment with an audio amplifier

    • Normally an FM radio will produce a "hissing" noise in the absence of a station. On detection of the RF signal waves from the above transmitter circuit, this noise instantly stops and gets converted into a pitch silence, termed as the "null" spot here.

  2. hello good evening sir
    am a engineering student of electrical i wanna to make a 2km capacity of a FM transmitter i don't know how to make that please can you help me am very interested to do such things can you please help me my E-mail is khadervali4u@gmail.com plz am looking for your replay plz sir!

    • Hello,
      Good day!

      Presently I do not have any long range transmitter design, but you can find many good circuits online if you do some Googling.

      If I personally get one, will surely post it in this blog.

  3. hello,Good afternoon here in Philippines. I just want to know if copper wire can be use to be an antenna..thanks in advance and GODbless 🙂

  4. Hi Swagatam,
    BC547/548 You used as RF Oscillator it oscillates in FM band (around 100MHZ all time) signals propagate according to the base & C3+L1 to PIN-POINT the frequency. Can this same circuit used as FM RECEIVER? Reverse the circuit —All Signals from AIR (Say 100Mhz is Required FREQ.) fed into base of BC547/548 OSCILLATES@100Mhz rest Please tell me is it possible? Working principle of Radio TX and RX is same!!!!! +FREQ in TX, -FREQ in RX

  5. Hi Swagatam,
    BC547/548 You used as RF Oscillator it oscillates in FM band (around 100MHZ all time) signals propagate according to the base & C3+L1 to PIN-POINT the frequency. Can this same circuit used as FM RECEIVER? Reverse the circuit —All Signals from AIR (Say 100Mhz is Required FREQ.) fed into base of BC547/548 OSCILLATES@100Mhz rest Please tell me is it possible? Working principle of Radio TX and RX is same???
    mixing AUDIO FREQ to OSCILLATOR in TX Section, Filter AUDIO FREQ to OSCILLATOR in RX Section???

    I tried lots of circuits, transmitter section is always good just stretching the coil is all needed can get you in the middle of the band. But @Receiver Section signal sensitivity is the problem in village/jungle/hilly areas. Post a CIRCUIT considering this.

    • Hi Max,

      Yes what you are saying is feasible, but since the FM signals from the transmitter are very weak will need a lot of amplification before it can be heard over the speaker, that's the difficult part.

      Best way to learn the working is to buy a ready made walkie talkie unit, open it and study all the stages.

  6. Hello Swagatam

    Check this out http://siva666siva.webs.com/FM.PDF
    Same as FM Transmitter Oscillator with filter & Pr-Amp, I built it with 2 BC548 (w/w/o LM386) work like charm Up-to 10 meters it receives audio from homemade FMTX beautifully but can't receive further difficult to tune. i NEVER used trimmer/Variable/GANG Capacitors to tune… Just 10PF ceramic & Coil size adjusting to find the band. BOTH FM TX & RX work for a certain distances.

    a guy from talking Electronics said the same.
    There is no point in buying Ready-made Units you can just study the chip & its features!
    And i think there is no working SIMPLE FM RX circuit match to THE SIMPLE FM TX in INTERNET.
    AND there is no fun making TRANSMITTER without a RECEIVER. Please Check out the link and POST A RECEIVER CIRCUIT soon….

    • Hello Max,

      Thanks for the information and the link, yes indeed the circuits are very interesting and very easy to build.

      I'll possibly create a new article topic and post the given information there.

      I had suggested a conventional walkie talkie because they are able to communicate across at least 100 meters distance.

      Anyway, I appreciate your interest, thanks!

  7. HELLO SWAGATAM

    Its Unbelievable that this WIRELESS MIC FM TRANSMITTER is actually a FM RECEIVER!
    all you got to do is remove the audio section, put an antenna (IMPORTANT PART) in coil-collector junction thats it, then the output (original antenna wire) to 104 cap to LM386 Amplifier (LIKE * SAID JUST REVERSE). AND TUNE IT. Even its difficult to receive the Popular FM Stations, You will 100% receive data from WIRELESS FM TRANSMITTER.

    To Improve add more Common-Emitter Oscillators series with this (Output to base) from 2nd stage the coil turns must be between 8-10, and compressed like no space between each turns. The efficiency of this type of configuration is 95%.

    I got this concept recently from HecaWorld One transistor FM Receiver. by hecanet from YouTube. AND its working with every FM TRANSMITTER that i build. Especially transistor Ckts
    Soon I'm gonna post a video about this.

    +Length of ANTENNA is important for the regeneration effect. its between 15-35 cm. precisely
    +No ground is needed!!!!!
    +Its FM its carry more data.
    +RECEPTION IS VERY VERY VERY GOOD!
    +NO INTERFERENCE & DISTORTION CLEAR SOUND/ CLARITY!
    +If its in Line-of-Sight area the range is max. (better for RC operations)
    +MAIN THING THAT ITS WORKING IN "3 VOLT" ALL DAY RX+LM386/Earphone ALL IN 3VOLT

    BC548
    After making TX i searched for RX circuits. and i come to this point that TX IS RX.

    TRANSMITTER IS RECEIVER.

    • Thanks very much Max for this great info.

      Actually I had this in my mind, and tried to implement this concept long ago, about 5 years ago while designing a cheap RFID circuit, however I couldn't succeed in it.

      Your information is very valuable and I am sure folks reading this would hugely appreciate this.

      Thanks again.

  8. Hello Swagatam

    Recently i got so many bug zapper. Mosquito bat circuit, they producing 1-5KV from 4.5vdc, and these circuits working more than 6 hours without any charge problems, is there any way to use them as inverter. at least 230 volt from 4.5vdc to light a 60WATT bulb. And how to make enough Amps ?

    • Hello Max,

      No it won't work, because a mosquito battery would have a max capacity of 4.5 x 1 = 4.5 watts which is nowhere near to your required specifications.

      You may probably use it to power many LEDs in series from a 4.5V source.

  9. i made transmitter. using above circuit.am using normal FM receiver. once i turned off mic noise is coming.can you tell me how to cancel that noise. i need to use like normal wireless mic.

  10. Hi Sir , I have made this Compact FM transmitter by replacing mic with 3.5 mm Jack to connect to mobile phones , mp3 player … I wanted to play music wirelessly through Car music system and this is a good circuit I wanted . Good thing about this ckt is that it can transmit good bass and at very low volumes . But the problem is that I have used 6 turns of .5 mm thick Copper wire . I get good sound within 2 metr range , moving away I can hear lot of disturbances . I even added antenna but no use . So , I wanted to amplify the signal from transmitter . Can this be done using transistors ???

    Also I wanted to mix L+R of stereo to get good mono signal . Is it possible ??? Please help and thanks for the circuit

    • you can do it by slightly tweaking L1 or C2. Either reduce the value of C2 or increase one turn on L1, or reduce the diameter of L1, or squeeze/press L1 sightly with your fingers to tighten the gaps between the turns.

  11. PLEASE HELP: I have a stalker doing this to me in my houses and in my cars. Please help me to know where to look to find it. I think it may be somewhere around my AC. Would that be a good place to start? What all would I look for?

    • Bilal, all the resistors are 1/4 watt 5%, and all the capacitors are disc ceramic type, except which is 1uF/16V non-polar electrolytic, the MIC is an "electret MIC"

  12. Nice one… But i would like the mic to transmit the data over wifi to a receptor…probably a phone so the phone plays it.. Is there a way to do that or will i have to build a mobile application that can convert the sent data from the mic to the mobile phone?

  13. sir can i connect my mobile to play music in FM ( music that plays in mobile should transmitted to fm reciving module)
    that mean can i use my mobile as an fm transmitter

    • Manjunath, yes definitely you can do that! you can use your cellphone's headphone music output with the the above explained transmitter for listening to it on an FM radio wirelessly.

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