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.
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
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.