NASCOM CONTROLS WASHING MACHINE
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.O. 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.O. 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.