Have you ever wanted to broadcast your own radio station within your neighborhood? This small and simple 87-108MHz FM transmitter is the toy that geeks have always wanted. This tutorial includes the PCB layout and the schematics. It has a range of up to 1/4 mile or more. It's great for room monitoring, baby listening and nature research.
- 1/4 Mile Radius Range
- Powered By A 9V Battery
- Lasts For Several Days
- Adjustable 87-108MHz
- Copper Clad PCB/ Perfboard
- Solid Gauge # 18 Wire
- Electret Microphone
- ¼" Bolt
- 2N3904 General NPN Transistor (2x)
- 15pF or 40pF Trimmer Capacitor
- 100nF Ceramic Capacitor (2x)
- 10nF Ceramic Capacitor
- 4pF Ceramic Capacitor
- 1M Ohm ¼w Resistor
- 100K Ohm ¼w Resistor
- 10K Ohm ¼w Resistor (3x)
- 1K Ohm ¼w Resistor
- 100 Ohm ¼w Resistor
- A Pair Of Pliers
- Soldering Iron
- Hot Glue Gun
Turn on your radio then tune it to your desired channel frequency. You'll get more range from the vacant channels. Don't touch the coil, just turn the trimmer capacitor until you hear a feedback from the radio.
Any Alternatives For The Trimmer Capacitor?
Most likely, you'll find one from a scrap transistor radios however you can replace it with a 20pF ceramic capacitor then rely on the coil's adjustment for compensation. Trimmer caps are color coded, only use the Red, Green or Yellow. I used the yellow since it covers a wider range.
Which Wire Should I Use?
Only use solid wires that has a thickness of 18 - 22 gauge. When I was still a novice hobbyist, I was worried about the strict selection of parts. Don't worry too much, eventually you'll learn to improvise when parts are not available.
Why Does The Frequency Change After Tuning?
Let's admit it, tuning the radio is a bloody challenge! If you use a metal screw driver to tune the trimmer cap, chances are you'll end up having a different frequency broadcast the moment you lift the screw driver. This is why non conductive screw drivers are recommended.
Where Can I Find A Non-Conductive Screwdriver?
You'll easily find one from a PC repair shop, or maybe form a hardware store. If you really can't find one, the famous Little Bits kit comes with it.
Is It Possible To Connect A 3.5mm Audio Jack?
Yes, it's possible! All you need to do is to link the common ground then solder a 1K ohm resistor each channel (L & R) forming a junction. Now solder a wire from the junction to U1's collector pin (refer to the schematic). You can now connect your MP3 player! I'll post an additional step regarding this mod.
Can I Hook This On A 12v Car Battery?
This project would still operate at voltages between 7v-14v, so yeah it's compatible. If you want to stay safe and prevent the circuit from burning, solder a 10 ohm resistor in series with the FM transmitter and 12v battery.