Mini Audio Transmitter




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In this instructable I'm going to show you how to build your own portable audio transmitter. This transmits FM waves so you could easily get the signals on your mobile phone, radios, etc. As the name and the picture indicates it is very small and is approximately the size of a 9v battery clip.

This transmitter is like the ones in the movies which are used to spy on people or try to record conversations, well don't use this this to spy on anyone, its just for educational purpose only.

What does this do?

  • Well all this is a FM transmitter so you could start your own mini FM station.

I'm going to build a mini FM receiver soon, to go with the mini FM transmitter

How does this work?

Well all of us have heard of "frequency modulation", most commonly known as "FM", this circuit works on the very same principal to transmit audio signals captured by the microphone. This circuit uses BC547 transistor to amplify the signal and then frequency modulate it. Since it is tiny and powered with just 9v the signal range is limited to only 15m.

If you like my project you can vote for me in the battery powered contest.

OK now enough of the talking and let's start building.

Step 1: Tools and Components

Like always lets start with getting all the parts, the list is quite simple all you need is



Step 2: Components

I got almost all of the components from a pile of old PCBs I had in an old forgotten box. All I had to get was the BC547 and the electret microphone. Actually I did find the BC547 in an old PCB but i was not sure if it would work. It looked quite burnt to me. The old PCBs had many components resistors, crystals, diodes, etc. I may use them some day and for know back in the box.

I had to de-solder the parts of the old board, for those who don't know how to solder and de-solder there is a bunch of instructables that describe how to do this and learning to solder is not a hard task.

Step 3: PCB

First of all lets start with cutting up a PCB to the required size. The size to compare is a 9v battery clip, it might look quite small in the beginning but don't worry it would hold all the components just fine. Use a sand paper for smoothing the sides of the PCB and to clear out any rough edges.

Make sure to get a PCB with big holes as the variable capacitor pins won't go in the standard size holes.

Step 4: Microphone

You can get the microphone at a local hardware store. And be sure to get some male pins to hold the microphone in place refer the picture as to how to solder the microphone in place.

Why not use some wires to hold the microphone?

I would not suggest wires as when you tape the circuit if the last few steps you would not get a clear audio. I tried it and got a lot of noise. I got lesser noise when I used the male pins soldered to the microphone.

Step 5: Circuit

Once you're done with the PCB and know where and how to solder the microphone now it's time to complete the rest of the circuit. Follow the circuit above and solder all of the components. Make sure not to leave any space between any of the components if you need to get the circuit small. For the inductor use 0.5mm wire and 8 turns, with each turn with a diameter of 6mm.

And for the antenna just use a thin 5cm long wire. For more stability you could center tap the coil and solder the antenna to the center tap.

Also If you notice the circuit has a LED in it, it is used to show when the circuit is functional. I did not add the LED in my circuit because it was draining my battery faster.

Step 6: Taping Time

Once you got the circuit like the one in the above picture, it's time to cover it with tape. I used wiring tape to cover the whole circuit except the microphone and the variable capacitor. This is an important step as when you proceed to the next step, where you tune to the required bandwidth. Touching the circuit (mainly the coil) with your fingers would lead to severe noise.

You could also use a heat sink instead of tape, I used tape because I wanted to experiment withe circuit so I did not want it to be permanent.

Step 7: Tuning to the Required Bandwidth

Now it's time to tune the circuit to a required bandwidth, you could do this in two ways.

  1. Use your mobile phone to find the signal
  2. Manually tune the variable capacitor to match a frequency

The first step is recommended all you have to do is power the circuit and turn on auto find bands on your mobile. Your mobile would scan for channels and all you have to do is look for your transmitter (play some music in front of the transmitter) on that list.

The second method is time consuming, in this method you have to turn on your radio and the circuit. Keep the radio at a specific channel, and then tune the variable capacitor extremely slowly. When you hear stuff on the radio maybe a song that you are playing stop and the bandwidth on the radio is the required bandwidth.

Step 8: Re-Chargeable Version

After using the 9v battery circuit for some time I thought of replacing the battery with rechargeable Li-ion batteries. If you have viewed my previous instructables you would have seen that I use these batteries a lot.

The rechargeable batteries provide longer transmission than the common 9v battery.

If you liked this instructable then you would also like this one.

3 People Made This Project!


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147 Discussions


4 years ago on Introduction

AWESOME! I've built dozens of FM transmitters but never have I seen one that's soldered on top of a 9v clip. Your project made me build another one :)


BTW, try the two-stage 'Super Spy Bug". I think you'll get more gain and rage out of it. Here's a schematic :)

4 replies

Reply 4 years ago on Introduction

Thanks ASCAS

Any idea on how much range you got from it. Currently I'm working on a similar FM receiver and I may make 'Super Spy Bug" once that's done. Then you would get a complete spy set.

Gospelustas ThisIsSteve

Reply 3 years ago

Please can I have the circuit diagram of this mini Audio Transmitter? Thanks this is my email Gospelustas@yahoo.Com. Thanks for the Love


Reply 4 years ago on Introduction

The original author of the circuit said that it can transmit FM signals up to a quarter mile (depending on your antenna). We ran a test on the football field from my previous school. We received a clear signal across it, about 120 meters. I guess it can still cover a wider range, although I still haven't tested at lengths beyond 120m.


Reply 3 years ago

can i use on this fm transmitter bc547b becase i can't find 2n3904 transistor ?


4 years ago on Introduction

I'm very interested on radio commutations. Your circuit looks really neat and simple. Double vote for you.

2 replies

Question 6 months ago on Step 7

What is the exact frequency u got it working?


1 year ago on Step 1

Is the 4.7 k resistor important, because I don't currently have it.


1 year ago

what's the mhz of this transmitter?


2 years ago

Hey! I'm new here! I would like to make this kind of transmitter but I'm afraid that it will not work properly! Is there any tips on how exactly I should to do every step? I need to know that because the soldering, the position of any of this components including the battery (I don't know why) the turns of the inductor, all this have matter and depending on this all facts the transmitter will work! It is not like battery and the light bulb, which position you want to join them they will still work! I don't know I confuse myself! I had made the circuit given by ASCAS but it didn't work properly! I need help on this! What about your circuit how you solder all the components together? With cables or something else?


2 years ago

how i online bay electronic Components?
plz give me any website link because i want to bay electronic components .


4 years ago

this is a very nice n awesome project I am going to make it..
I have two question...
1). is transmitting FM signal is legal in india?
2). and how I attach a 3.5 mm headphone jack on it instead of microphone...
thanks in advance n keep posting instructables like it.....:-)

1 reply

Reply 2 years ago

1. Unless you are using it block specific radio stations, you will be okay. Small, low-power radio transmitters are almost always legal in most countries.


2 years ago

how to make the inductor?and what is its inductance?


2 years ago

First thanks for the great 'ible.
So i bought one of those cheap fm transmitters for cars and i wanted to hook up a normal mic, the kind with 3.5mm jack, to it. So that i can broadcast my voice to an FM frequency in short range. The transmitter has an AUX, USB and micro SD slot. Any ideas?


2 years ago

It's not working plz help me


3 years ago

How to connect the wire antenna ?