Ridiculously Simple AVR AM Radio Transmitter

Posted on Feb 7, 2014

Create a rapid transmitter/receiver pair that anyone would have around their house. I decided that AM or FM radio would be good since everyone can receive that, and pondered how best to generate the necessary radio signal and modulate it appropriately. After a few LC oscillator designs, I thought about the RC oscillators built into most micro-controllers. I grabbed an ATMEL AVR I had on hand (an ATTiny44A)

Ridiculously Simple AVR AM Radio Transmitter
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and checked the datasheet. It had an 8MHz RC oscillator, which could be divided-down to 1MHz, and output on a CKOUT pin all configurable with a few hardware fuses! Note that commercial AM radio stations are between 0. 52 and 1. 61 MHz, so a 1MHz signal would be smack-dab in the middle of our radio dial! I had to build a prototype to see how well it would work. Once concern was that the RC oscillator wouldn`t be stable enough to produce reliable audio boy was I wrong! The circuitry is textbook simple. Appropriately configured, the AVR generates 5V square waves from its CKOUT pin. Although a pretty shape, they`re not powerful enough on their own to be heard across a room, so I needed an amplifier stage. A class C amplifier provided by a 2n7000 is commonly done in the low power amateur radio (QRP) community, so I went with it. A 2n7000 N-channel MOSFET with a 220-ohm resistor on the drain and the CKOUT directly into the gate did a fine job (I`ve used this design for 10MHz QRSS transmitters before), and I was able to modulate its amplitude by feeding the voltage from a MCU pin (turned on/off rapidly) through a decoupling capacitor into the drain of the MOSFET. I couldn`t have asked for a simpler result! This code sends a message in Morse code. It seems too easy! Applications are endless, as this is one heck of an easy way to send audio from a micro-controller to a radio, and possibly to a computer. Morse code is easy, and since...

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