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An 8-Channel PC Relay Board

Control devices via your PC parallel port with this eight-channel relay board.

By Ross Tester

Have you ever wanted to turn something on or off with a command from your computer?

Here’s a simple project that does just that – in fact, it will control up to eight external devices via your computer’s parallel port.

Click for larger image
The project is capable of switching eight different devices, with the two larger relays (right end of PC board) being mains rated. The cable underneath which connects to your PC's parallel port must be a "straight through" D-25 male to D-25 female type, not a crossover cable often used with 25-pin serial ports.

Most of us have at some stage thought "wouldn’t it be nice if I could get the computer to do X" (fill in your own "X"!) but then have done nothing more because, well, how do you interface a computer with the real world?

It’s surprisingly easy!

As you would no doubt realise, your computer already "talks" to external devices via its parallel port (more than likely a printer; perhaps a scanner or some other peripheral).

It’s that same parallel port which you can use to control external "things". It’s especially easy if those things only need to be turned on and off.

You’ve probably seen parallel port interfaces in the past which have used perhaps half a dozen chips and scads of other components. This one uses a tad less – zero ICs and less than ten other components per "channel".

Normally, to talk to a printer the computer sends various pins in its parallel port high or low at breakneck speed. The various combinations of highs and lows are interpreted by the printer to place ink at a certain position on the page.

In this simple interface, we also send various pins high or low – in fact, eight pins (D0-D7). But there’s no combination of pins to worry about. If any of those pins go high, a relay associated with it closes. If the pin is low, the relay opens.

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