FM wireless transmitter circuit

This is a simple FM wireless transmitter circuit.
Why use it?

Imagine you and friends are going to sing a song. All are happy.
But often the cable of the microphones tangle.

Is it better if no cable?

Yes, this circuit can do.

It is suitable for the beginner to learn the RF transmitter circuit, too.

Do not keep worried that it is difficult or expensive.

It is easy and cheaper than a normal high-quality microphone with cable.

Are you up for it?

How it works

Basic principle:

We speak to Microphone. The circuit will get this sound signal to mix with FM frequency to transmit the broadcast spectrum.

Then, we get the audio sound from FM radio receiver In the matching radio frequency channel only.

Real circuit working

Look at a circuit diagram below. It is a small circuit. But is a good teacher for a transmitter circuit.

FM wireless transmitter circuit

This is a step by step process:

First, we speak into a MIC1. It is special part called a condenser microphone.

Inside it has an amplifier module to amplify our voice. To be a higher level before sending it to the output leg.

For this reason, this MIC requires a little current to power them.

We will see the R1-resistor 27K pass voltage to a positive leg of a microphone.

At the same, this leg is its output, too.

Second, a C1 is a coupling capacitor. It passes only AC signal
to a base of Q1.

Which the Q1 acts as two things are.

  • a radio frequency(RF) generator
  • an audio mixer

To mix the sound signal and the RF together.

And, at base of Q1 has both resistor R2-27K and R3-4.7K. They are a divider circuit. To set a level voltage to bias current of Q1.

For capacitor C2-0.01uF connects to the ground to bypass the high frequency.

And, collector of the Q1 connects with positive voltage pass the coil.

For this coil, we use a copper wire No. 19-20 AWG, wrapped around the air core diameter of 7 mm.

You should wrap 7 rounds, similar to coil spring.

Then, stretching out to a length of 15 mm.

At close, this coil has a capacitor-C4, 5-20pF. It is the adjustable capacitor from 5pF to 20pF. We called timmer. It is connected to a frequency tuning

While the collector of Q1 connects to the antenna. Which in this circuit, we use an antenna that is built with yourself.

By using the copper wire 16-20 AWG, about 7 inches in length, and curled into a circle like a spring, about 6 mm in diameter.

The length of the wire coil to the end. Then take one end soldered to the antenna connection.

How to build

This project you may use universal PCB or look at PCB layout on image below.


Figure 2: Copper PCB layout

PCB component layout of FM transmitter circuit
Figure 3: Components layout

Creating an electronic project often has similar steps.

You should install a small device first.

Connect a shielded cable length about 5 cm to connect a condenser and the PCB.

We use the coated copper wire for to prevent rust green from moisture in the air.

At the end of the wire should scrape out with a razor blade before solder.

Use a lead solder a better mix of 60/40, and requires some skill in soldering.

Otherwise, the spectrum not work. It is a high frequency.

How to tune

When completed. Check and Check again.

Let’s tune. Here are the steps:

Take the FM radio, open station radio dial in the middle.

Turn on the FM wireless transmitter circuit.
Use a screwdriver plastic or plastic sheet flat spin adjust the trimmer.

Speak into the microphone

Tune until it has the sound to the FM radio.

If tune the trimmer until full cycles, Not yet match wave. Try stretching or shrinking of the coil.

Then, adjust the new one. And try some stretching or shrinking the antenna, in order to deliver the best results.

If all the fine-tuning does still not work. Also, try turning the radio waves, may rotate to the minimum frequency and maximum frequency, or turn around the station.

Then try to adjust it again.

When adjusting until available.

Then, install the circuit into a plastic box or aluminum box.
Or any box ready to build up own. By folding a sheet of aluminum or a plastic sheet is glued to the box for convenience to you.

Note:

As a principle, this circuit sends far about 100 meters. It depends on:

  • The location.
  • Low or high noise oscillator.
  • Low or high-quality component
  • The soldering is good or not.

So, it is difficult to determine how far?

But from my experience, in the center of the floor to the two floors of the building.

It is enough for the small and cheap circuit like this.

Suggestion

If you are interested in this circuit.
But fear that it will not work. You can see the details of…

FM wireless microphone circuit diagram

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