Circuit Construction

The original prototype was built on stripboard ("vero" board) and mounted in an ABS plastic enclosure.

All components and materials came from RS.

I used a 20A(contacts) 24V(coil) relay for switching, the power supply delivered about 18-20 volts across the 470mF capacitor with the relay activated, so the 78L12 was dropping 6-8 volts, current drain of the chips is insignificant so I did not need to use a heatsink on the regulator.

I have not specified the transformer and bridge rectifier, just use something to deliver what is required (for example, using a 7812 1 amp regulator, with a 12 volt relay coil you can probably get away with as low as 16 volts across the 470mF capacitor). If you use a different regulator, you can have a higher input voltage and drop more accross it without any damage, also the chips will take a 15 volt regulator, if you prefer.

Overview and Circuit Operation
Schematic Diagram

Back to my Homepage.

© copyright 1996-97 Stephen E. Mendes, Barbados.