How to make FM Transmitter

 Here it is shown how you can make your own FM transmitter. The circuit schematic and PCB(Printed Circuit Board) layout is created in Proteus professional software. 3D view of the expected FM transmitter is also generated to get view of how the FM transmitter looks like in the end. 

The FM transmitter is made up basically of pre-amplifier and oscillator circuit. Two general purpose transistor BC547 is used for the pre-amplifer and the oscillator circuit. Besides these two transistor other passive components are used which are listed below. It is also shown how to make the coil needed for the oscillator part of the FM transmitter with the help of online calculators. Video showing how the FM transmitter works is shown at the end.

The completely build FM transmitter bug picture is shown below.

Materials Required

Parts required are as follows:

- two BC547 transistors

- electret microphone

- battery 3V to 6V

- 15KOhm resistor, 10KOhm resistor, 47KOhm resistor, 1MOhm resistor, 470Ohm resistor

- 100nF ceramic capacitor, 22nF ceramic capacitor, 2.2uF electrolytic capacitor, 1nF ceramic capacitor, 47pF ceramic capacitor, 5pF ceramic capacitor, 22pF ceramic capacitor

- 10 turns of laminated copper wire for inductor

- 25cm wire for antenna 

- some wires

- Nail enamel remover

- Ethanol(Ethyl Alcohol)

- Ferric Chloride

Circuit Schematic and Working Principle

The circuit diagram of the FM bug transmitter is shown below.

In the above circuit diagram, MIC1 is the electret condenser microphone. R1 is required to power up the microphone because the electret microphone has a FET(Field Effect Transistor) inside it which requires power. Audio enters the transmitter through this microphone. The audio enters the pre-amplifier via the coupling capacitor C1. The first transistor Q1, the BC547 transistor, which is biased using self biasing method with resistors R2 and R3 is the pre-amplifier. The tutorial Testing of self biased BJT amplifier on breadboard with PC oscilloscope shows how to design and test self biased amplifier. The pre-amplifier amplifies the small audio signal of around 10-15mV amplitude from the microphone to higher amplitude. The amplified signal is then coupled into the oscillator circuit using the electrolytic capacitor C2. The oscillator circuit is made up the second BC547 transistor configured with inductor L1, C4 and C3 in colpitts oscillator configuration. The value of L1, C4 and C3 are chosen such that it oscillates in the FM frequency range which is 86 to 106MHz. Here the value of the L1, C4 and C3 are such that the oscillator oscillates at 88MHz. The coil inductor is made of 10 turns with 4mm diameter. 

Calculating Coil inductor for FM radio

To calculate this number of turns required for given coil diameter first we need to calculate the inductor inductance required value for given frequency and two capacitor values. For this we use the Oscillator Online Calculator and select the Colpitts oscillator. In the calculator we enter the desired RF frequency which is 88MHz and also enter the value of the two capacitors which is 47pF and 5pF. This is as shown below.

 From the calculator we get inductor inductance value of 723.79nH. 

Then we use Air Core Inductance Online Calculator to calculate the number of turns required for given diameter of the coil. We enter the above calculated inductance value in uH which is 0.72379uH, enter the length of 0.5cm(5mm) and the radius of 0.2cm(radius=diameter/2=4mm//2=2mm=0.2cm).

From the calculator we get number of turns required N as 9.94 which is approximately 10 turns. 

We wind 10 turns of laminated copper wire around a 4mm diameter shaft such as screw driver to make the inductor coil.

PCB Design

The PCB layout can be created on many online cads or free PCB software. Here the PCB layout was created in Proteus professional electronics design software. The completed PCB layout is shown below.

To have a prototype view of the designed FM transmitter 3D model of the expected FM bug transmitter was created also which is shown below.

Once the PCB layout design is complete and you are satisfied with the layout and 3D prototype it can then be printed on white photo paper. The printer used here was HP LaserJet P1102. The following shows the printed PCB layout on the photo paper.

 The next step is to imprint this layout mask onto the a printed Circuit Board. For this we cutoff equal area required by the PCB layout mask from a Copper Clad Laminate PCB Circuit Board. 

Then we use equal mixture of nail polisher and Ethyl alcohol(also popularly known as Ethanol) to put the printed side of the layout onto the copper clad PCB board. 

We have press the layout onto the PCB board for some time and then put the board into water. We let it remain in water for some time, take it out and take off the photo paper. If everything went good, then the PCB layout print should be printed on the copper PCB as illustrated below.

Next we make a solution of ferric chloride by dissolving ferric chloride anhydrous into water.

The printed copper PCB is then immersed into the ferric chloride solution. This will etch the all the copper except the parts underneath the printed layout. 

 To get faster etching response we can stir the pcb while inside the solution. Some warm water also accelerate etching process. 

Once the etching is complete we take it out and the remaining black print layout is removed using nail polisher liquid. After that we get the PCB layout of the FM transmitter as shown below.
 

This copper PCB layout can oxidize and get rusted. So in order to prevent this we tin this copper layout by soldering it with soldering wire. The tinned PCB layout after that looks like the following.

The next step is then to drill the holes and place the components on the PCB. The next picture shows the FM transmitter with all the components populated onto PCB board.

In my design the microphone was not permanently soldered onto the PCB board. In place of the microphone a two header pins was used and the electret microphone was connected to the board using a headphone jack and using male to male audio jack as shown in the above picture. You can instead solder the microphone onto its place on PCB board. I did make this plugable microphone jack so that I can use the microphone in other FM and radio projects. The DIY microphone jack is shown below.

Testing FM Transmitter

The next step is to test the build FM transmitter. Power supply from 3V to 6V can be used. Here 6V power supply was used. A tuning receiver radio is required. Audio from any source is placed and played near the electret microphone of the FM transmitter. The FM receiver radio is then tuned. This FM transmitter was designed for 88MHz FM frequency. So here it was tuned near the 88MHz. In fact, the designed FM transmitter transmitted at 88MHz which is demonstrated in the following video. The receiver radio and the FM transmitter were placed 50cm apart. The transmitter and receiver should not be placed very close to each other. One can experiment with different distance between them.

Video demonstration

The following video demonstrates the working of the FM transmitter at 88MHz. 

In this tutorial we showed how to make FM transmitter along with schematic diagram, PCB design process and testing it. The FM transmitter build here used two transistors. Another FM transmitter design with two transistors is shown in the tutorial Two Transistor FM Transmitter. One can also make FM transmitter using a single transistor which is shown in the tutorial 2N3904 One Transistor FM Transmitter and Single Transistor FM Transmitter.