Introduction: Simple Low Power AM Radio Transmitter
Many of us love building crystal radio. But you need to live close to the radio station and you need a very good antenna and ground. City Dwellers like me do not have the luxury of pulling long antennas and a decent earth.
How about building this low power AM Transmitter, solely for the educational purpose to test your crystal radios ?
Warning! Using or Owning a radio transmitter without a license is illegal in some countries. Please check and decide whether to use one at your own risk. Depending on the length of the antenna , it can cover within 3 meters by a crystal radio. If you lay wires around the room, you can listen to crystal radio around the room. For conventional off the shelf radio , 10 meters with reasonably good audio quality. If you lay wires around the room, you can listen to crystal radio around the room.
Follow this video for the instructions:
Step 1: How to Build It
How to build this low power AM Transmitter, solely for the educational purpose to test your crystal radios. The whole transmitter can be fit into this small box. The screws are for connection to Antenna (or telescopic antenna) and ground. Current consumption is 3mA using CR2032 3V button cell.
1.000 Mhz Crystal Oscillator operating from 3V to 6V.
The lower left pin is Nc (the pin with a Dot).
Upper left pin is inputUpper right is output
Lower right is ground
455khz IF Transformer( RED).
Use the side with 3 pins (left pin and middle pin) ~350uH for 3 Meter Antenna)
Insert the crystal oscillator onto the breadboard with the dot positioned at bottom left.
Connect the left pin of the coil with the output of the crystal oscillator
ST32 8 ohm : 1.2K transformer.
The side with two wires is for 8 ohm.
The side with three wire is for 1.2K.
Connect the Ground of cryatal oscillator to -3V.
Connect one of the pin of the 1.2k side of the transformer to +3V.
Connect the other pin from the 1.2K transformer to the input of the crystal oscilator.
Connect audio input to the two pins on the 8 ohm side of the transformer.
Plug the audio cable into the phone's headphone jack.
Play a song from your phone and turn the volume up to 90% of Max.
This circuit has no amplification, so your phone need to be tuned to 90% volume to produce enough signal level to drive the transformer.
A regular Mic will not work.
Connect a 3 meter Antenna to the middle pin of the coil.
Connect ground of the circuit to your earth connection for crystal radio testing (or your water pipe).
Place your AM crystal radio close to the transmitter and tune to the frequency of the transmitter ~ 1000Khz.
Tune the 455khz IF transformer until you hear the loudest audio from the crystal radio.
Move your crystal radio further away to test the range of the transmitter.
To avoid disturbing your neighbours and breaking laws in your country, your transmission should not leave the room.
The audio impedance matching transformer plays a big part in the transmission.
Suitable audio transformers in decending order of fitness.
ST32 8 : 1.2K
ST81 8 : 1K
ST401 8 : 600
ST82 8 : 500ST83 8 : 400
Output is lower and distorted when connecting the crystal oscillator output directly to the antenna as the antenna cannot resonant with the transmitter.
I initially used 6V, but after changing from 6V to 3V power supply, the transmitter becomes more stable.
Mounted components to a circuit board according to the circuit diagram.
Put the circuit board into the tiny plastic box