The automatic stopwatch timer is a cinch to build and resetting the count is as easy as pushing a button. And by using a snap-action thermostat (salvaged, of course!), it’s even possible to automatically measure how long a temperature is above or below a set-point – ideal for environmental, solar heating and machinery monitoring.

Because it uses salvaged parts, the complete project should cost you only a few dollars. Let’s see what’s involved.

The components

You’ll need an old digital watch with a stopwatch function, two relays (one a double-throw design), a capacitor and a pushbutton switch. Oh yes, and something to mount the components on! We used a small piece of pre-punched board laminate (unclad) and that was literally the only component that we had to buy new!

This digital stopwatch timer will automatically measure how long low voltage equipment is running, making a cumulative count until reset. Most of the parts can be salvaged for nothing.

It’s quite likely that you already have a digital stopwatch tucked away in a drawer – as fashions change, lots of people buy new watches even when the old one is still working fine.

If you have to buy a new one, shop around at discount stores – it shouldn’t be too hard to find a watch that includes a stopwatch function for about $5. The unit shown here came from a watch I’d not worn in years – in fact, until I went looking, I’d forgotten I even had it!

Relays can be found in a huge amount of discarded electrical equipment. The ones used here are quality Omron designs that were salvaged from an old radio transmitter that had been sent to the tip. Although they use 24V coils, they work fine in this application which is based around a 12V supply.