February 1982, Volume 2, Number 1

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by J. C. Lord

Some months ago I was given a Hoover Keymatic front loading washing machine which needed a new controller. The cost of such a controller being at that time about £60, I decided to have a go at controlling the machine from my Nascom. The project has proved successful, and I think that readers may be interested to see how the job was done using ten P.I.0. lines.

I removed from the washing machine the main motorised controller unit, the motor drive board, and all of the wiring. This left the two level switches (empty and full), the three solenoids (two for cold water, and one for hot), the pump and the main motors. I then built a board to interface between the Nascom and the various items on the washing machine.

Five of the P.I.O. lines are used as on/off outputs. The output system consists of a Darlington driver chip operating five transistorised relays. The Darlington driver acts as a buffer to provide 7 mA for the relays; the P.I.0. should not source or sink more than 2 mA. The relays switch 240 V. A.C. for the various machine components, as shown in figure 1. The 240 V. to microprocessor isolation in this circuit is provided by the relays. An alternative method would be to use optically isolated solid state relays.

This is an OCR’d version of the scanned page and likely contains recognition errors.

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