Hi everyone,this is the best device I had ever made in my life I think because it is useful for my daily use.I had an old FM radio which was of no use catching dust in the cupboard.So,I thought of making an FM transmitter because that radio had no input port and I need to buy a new set of speakers.This was the perfect and cheapest solution of the problem.You can transmit sound in the range of about 250 meters.CLARITY WILL BE HIGHEST ON YOUR FM RADIO(NOT SMARTPHONE RADIO FUNCTION) AND EVEN IT WILL EXTEND THE RANGE TO 320-350 METERS.The schematic is from the website www.makezine.com. In my transmitter the frequency was between 97.8-98.8
- Long battery life
- Long range
- Connect to your smartphone
- Crystal clear sound on FM radio
- Compatible to any device with 3.5 mm jack output
- Cheapest 0.6 $
- Easy setup with easily available parts
Here's a video of it working :-
Step 1: Parts Required
For this project you will need :-
- 2 X 10 pf capacitor (ceramic with written "10" on it)
- 2N3904 transistor
- Mono audio pin 3.5 mm
- 10 uf electrolytic capacitor (minimum 16 v)
- 2 X 0.01 uf ceramic capacitor (written 103 on it)
- 3.5 mm pin socket (optional)
- 18-22 gauge solid copper wire
- Perf board
- 9 volt battery
- 10 kilo ohm resistor
- 27 kilo ohm resistor
- 470 ohm resistor
- Some wires
- 9 volt battery connector
- Soldering iron
- A pair of scissors
- Hot glue gun
Step 2: Perf Board and Schematics
Circuits assembled on pref board are not necessarily fragile but may be less impact-resistant than printed circuit boards.Here's a schematic of the FM transmitter.Place the components correctly as displayed in the schematic.Capacitor (10 uf) has polarity so keep in mind the terminals of the capacitor negative is for audio pin and the positive is joined to resistor.You may face some problems while doing it on pref board. Firstly place all components with some space between them.If you are a beginner then the space between components should be more.It would prevent risk of shot circuit.While placing the transistor keep in mind the pin out of 2N3904 transistor.Place all components and then solder them carefully and cut off the extra pins using a nipper.
Step 3: Making the Coil
Take a 1/4 inch bolt and wrap around a 18 - 22 gauge wire carefully.Make about 4-5 turns and then unscrew it form the bolt and cut off extra wire.It is preferred to have stripped wire instead of insulated one.And your coil is ready!
Step 4: Connecting Wires to 3.5 Mm Plug
Open the cap take two wires and solder both of them to pins of plug.The smaller pin wire would be connected to negative side of electrolytic capacitor and the other to ground.After soldering screw down the cap ,Choose the length of wires according to your choice.
Step 5: Perf Board Soldering
Solder the audio plug to corresponding places.Now solder all the ground pins of the components together.Make sure that the ground connections should not touch positive pins of components.In the picture the black wires are all the ground connections.At last solder the 9 volt battery connector to board remembering the correct place for both terminals.
Step 6: Checking
After soldering everything ,insert the plug in your phone that have FM radio.Play any easily recognizable track and on the other side connect the 9 volt battery to connector.Connect earphones to other mobile phone or use any FM radio and search for track changing frequency on radio and when you find the clearest sound stop there.That's the right frequency on which the transmitter sends signals.Mine was 98.4 Mhz. Sometimes between 97.8-98.8 Mhz.To increase range add two antennas to the circuit but make sure that the two antennas don't make contact with each other.
Step 7: Making Enclosure
Use Duct tape,cardboard and some other tools to make a box for transmitter. Leave some space for switch,wires of audio plug and I prefer to leave the top open.
Step 8: Rest Place for Audio Plug!
Take 3.5 mm headphone jack port and join it to the same side where audio plug comes out.You need not to connect the port pins anywhere it is just a rest place for plug to make the device more handy and portable preventing mess of wires.I had increased the length of audio plug and also had twisted both wires to make them more compact.
Step 9: Making Strap
Make the strap out of any material which suits you,I had used elastic material so that it can fit everyone's hand making it usable for everyone.To see how to make this see my another instructable Caretaker's Gear v2.0 (more portable).
Step 10: Done!
This is a real working project which would not only reduce normal expenses but also gives education about working of radio transmission.It is a perfect device for people who love radio communication.It prevents usage of loud speaker in our home to play music in each and every room.Hence, reducing noise pollution.Queries and improvements are most welcome you can ask me by commenting!
Please vote if you like!
Make and enjoy!