RF data link and remote control
Posted on Oct 31, 2012 3739
This section describes an experimental low power, low bandwidth data signaling system that was initially made to operate at 55 MHz (television channel 2 in the U.S.). Before operating a radio transmitter, find out what kind of transmitter operation, if any, is permitted in your locality. Radio transmitter operation is a serious legal matter. This design can be readily adapted to different frequencies and lower power levels. If you choose to build and operate the transmitter described here, you do so at your own risk. I'm only publishing this as an example of what can be done, and to show how easy it is to build a simple but functional receiver.
This is a simple, low cost RF data link that can send data reliably over a distance of one to two meters, enough for bench top or desktop use. The data protocol supported by the encoder and decoder provides 16 device addresses, three message types, and a toggle bit. Pulse frequency modulation is employed to make the receiver design less demanding and to reduce the susceptibility to noise. The encoder and decoder are based on Atmel microcontrollers, though the protocol can be easily implemented on most micro controllers. The transmitter is an oscillator that is biased on and off by the encoder chip. The receiver is a tuned radio frequency (TRF) detector without preamp followed by an 40 db AF amplifier that drives a comparator, which provides pulses suitable for driving the digital input pin on the decoder. Most of the gain is provided by the op amp, where gain is easy and cheap, and the RF section is minimized and simplified. The only things that really need any tweaking to get the demonstration circuits to work is one hand wound inductor in the transmitter and one hand wound inductor in the receiver. The trick is to get the transmitter and the receiver on the same frequency. Having them "almost" on the same frequency will result in poor performance - very short range and unreliable decoding. Use a small wooden or plastic tool to deform the coils. Expect to spend some time at it, and don't be disappointed if you have to...