Build Your Own Micro FM Transmitter

By Sean Michael Ragan
Make Projects

This electronics project is based on a circuit that has been credited to multimedia artist Tetsuo Kogawa. Through a 1/4" phono jack, it takes audio input without the optional antenna connections to broadcast an FM radio signal about 30'.

The standard model (simplest FM transmitter) is slightly more complex in that it includes a trim capacitor to adjust the transmitting frequency, can be powered by a 9V battery and uses a hand-turned copper coil.

Micro FM Transmitter

I decided to use the PCB parts from the Sonodrome's old kit. The circuit is simple and can also be built on a perfboard or on a panel. If you want to etch your own board, visit Bay Area Circuits.

Required Tools:

3/8" Drill bit
Audio signal source, with 1/4" phono out
Bolt or machine screw, 1/4-20 thread for use as a mandrel in forming the coil.
FM radio
Third hand
Needle Nose Pliers
Cutters
Small Screwdriver, to fit trim capacitor.
Side-cutting pliers
Soldering Iron
Rosin core solder
Wire cutter/stripper
Hand-held electric drill
Bubble wrap (6" square)
Double-sided foam tape (1.5")
Case: glass, wood, or plastic (mine was salvaged from a thrift-store digital clock, jelly jar works great, too!)

Parts List:

Copper wire (4"), enameled, solid, 19 AWG or 20 AWG
1/4" TRS jack, (phono jack) Only tip and shield connections are used.
Battery holder clips, 9V, with 4" leads
PCB, bought or home-etched or use perfboard and jumpers
9V Battery
Mini trim capacitor, 20pF
10pF Ceramic capacitor
2 0.01µF Ceramic capacitor
NPN silicon transistor, BC337
1µF Electrolytic capacitor
470Ω Metal film resistor
10kΩ Metal film resistor
27kΩ Metal film resistor
Multi-strand hookup wire (8"), 24 AWG
Double-sided foam tape (1.5")

Step 1: Prepare the Case

FM Transmitter Case

Drill Mounting Holes in Case

Disassemble your case.

Mark and drill a 3/8" hole, in an appropriate location, for the TRS jack.

At this time, you'll want to drill mounting holes for a power switch and/or a power jack if you choose to use an external supply.

Step 2: Prepare the Jack

Of the two 4" pieces of 24 AWG hookup wire, strip about 1/2" from each end and tin.

Solder one end of one lead to the front leg of the 1/4" TRS jack and one end of the other lead to the back leg.

Step 3: Form the Coil

Use a Screw to Form the Coil

Formed coil

Take about 4" of the 19 AWG enameled copper wire and wind at least four turns around the threads of a 1/4-20 bolt or machine screw.

To remove the screw, rotate the bolt counterclockwise.

You want a total of four turns in the coil. Bend the two legs down, as shown, and clip them to about 1" long.

The mounting holes for the coil legs should be 12mm apart on the surface of the PCB. The act of installing the coil on the board should stretch it to the correct length, but you may have to tweak it a bit with pliers or a screwdriver to make sure the rate of coiling is even between the two legs.

Step 4: Install the Components

Assembly Diagram

Components Assembly

Bend and slip the component leads into the correct holes in the PCB.

Verify the correct orientation of the electrolytic capacitor and wire leads.

Note: Bend the leads on the solder side of the board to temporarily secure them in place and clip them to about 1/4" to open up room to solder in.

Step 5: Soldering the Components Together

To get ready for solder,

Clip the PCB, trace-side up onto your helping hand.

Flux, heat and solder each lead in place.

Once the solder has cooled, clip any protruding leads with side or end-cutting pliers.

Note: Be sure to work with plenty of ventilation and avoid inhaling fumes when soldering.

Step 6: Mount PCB

Soldered Components

Mount the PCB to the Case

Mounting details will vary with the case you choose. (The foam-tape method worked great for my salvaged clock case, but your mileage may vary).

Attach a strip of double-sided foam tape to the featureless corners of the PCB on the trace side.

Do not attach tape directly over the traces, or you may damage them if you ever have to remove the tape.

Remove the backing from the foam tape strips on the PCB.

Step 7: Tune the Circuit

To adjust the capacitor,

Connect a fresh 9V battery to the battery clip and an audio signal to the TRS jack. I used the headphone jack of my laptop for an audio source.

Turn on your FM radio and scan around looking for your signal. It may be quite static and noisy at first, but I found mine at around 99.8 MHz.

Until your signal comes through loud and clear, use a small screwdriver to adjust thetrim capacitor.

Step 8: Final Assembly

Mounting the Phono Jack

Wrap the Battery in Bubble Wrap for Padding

Remove the washer and nut from the phono jack and thread it from inside the case through the hole you drilled in Step 1.

To secure the jack, put the washer over the threads and tighten the nut down from outside the case. Finger-tight is fine.

Attach a 9V battery to the clip, pad it with a scrap of bubble wrap, and stuff it in the case before sealing up.

I'm going to modify my transmitter with a jack for an external regulated power supply. You may want to do the same or at least add a power switch between the battery and the board.

Step 9: Use It!

Turn your radio on again, pick it up and walk away from the bench until the signal fails. Mine was loud and clear to about 30'.

Note: Depending on where you live, operating even a very short range FM transmitter like this, without a license, may conflict with applicable laws and/or regulations. Be sure to investigate carefully before turning it on and err on the side of caution if in doubt.