Super simple AVR AM transmitter
(Wireless communication and power) (avr) (Computer programming)AVR clock output pin
The AVR has a clock output pin, which if configured, outputs the system clock.
The clock is by default 1MHz on most (all?) AVR microcontroler, right in the middle of the AM broadcast range!
I added -U lfuse:w:0x22:m to avrdude’s flags to enable the pin.
AM, the simplest modulation type
AM is the simplest scheme to send sound over a radio wave, by varying the signal strength according to the instantaneous voltage of the audio signal.
If a transmitter if turned on and off at 1KHz, the result is a 1KHz (square wave) AM tone!
The modulator
To send an AM transmission, the signal coming from CLKO (Clock output pin), must be modulated.
This can be done with a NPN transistor which can be obtained for less then .10$ per unit (If you buy in bulk).
A resister between 100Ω and 1KΩ to ground on the antenna seems to improve results when used with a dipole antenna (long wire).
Antenna
I got best results with a magnetic loop antenna, made from 3 - 10 turns of thin wire (Optimal turn count depends on wire thickness and voltage) with a diameter of ~50 cm.
A human body (just touch the output with a finger) or a pair of long wires also worked.
Range
The avr with a 5v supply and loop antenna got a range of around 2m with a cheap off-the-shelf radio.
Legality
In the US, operating an ultra low power transmitter on the AM bands is legal as long as you stay under 0.05 W (50 mW).
It is very unlikely that you exceed the limit with just the AVR’s output and crude antenna, but you shouldn’t connect this to an amplifier.
If you do, you will be noticed as the square wave produces has a lot of harmonics, which will interfere with a lot of other channels.
