RF control to switch low voltage power, and RF serial link.
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Remote Power Switch

This small project makes use of Hope RF modules sold by Futurlec and others to enable remote switching of low voltage power.

(NOTE: These modules seem to be unobtainable. However similar modules such as the ALPHA-TRX433S, a rebadged version of the 433MHz RFM12B, are available. The project needs to be redesigned for these and has not been further developed.)

The background is a 12V power distribution system in a building which is isolated from the grid and uses solar batteries for all power. The low voltage used in the system requires care to be taken in regard to losses, so it was decided to avoid using wall switches which would require additional cable length with its attendant loss.

A small board uses an RFM02 transmitter module in a hand unit with an Atmel ATTiny2313. A receiver board using an RFM01 receiver module is mounted at the power distribution box. While the RF modules supposedly used SPI, the RFM02 modules actually uses a modification of this, meaning that an inbuilt SPI peripheral on the microcontroller cannot be used. The transmission system is FM in the 433MHz band.

The system operates by transmitting a single ASCII number continuously when a button is pressed on the remote unit. The receiver will collect all received symbols and uses a set of "bins", one for each possible symbol, to hold a count of the number of each symbol received. A decision is made after 10 symbols have arrived by selecting the symbol with the most counts. No more decisions are made until a preset delay has elapsed following the last reception.

More information is available on Jiggerjuice.

The receiver board is designed to take RFM12 transceiver modules so that the hardware can double as a serial communications system between two base stations.

More information is available on Jiggerjuice.

K. Sarkies 4 January 2015