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Regarding motor/solenoid control using relays, it's always smart to use a 'flyback diode', even if you're using mechanical relays.

This probably isn't the place to explain the (physics) phenomenon, but when you turn OFF any inductive load (anything with a coil of wire), there's potential for a big voltage spike. If you don't have a flyback diode in place, the voltage spike can either just cause a bunch of electrical noise (which could mess up your routine), or it could also destroy some of your electronics (in the relay area).

So, it's a good idea to install the diode, even if using mechanical relays. #1, it'll keep the rest of your electronics from being subjected to the large electrical noise spike (i.e.: source of 'glitching'). #2, the voltage spikes could eventually burn out contacts on even a mechanical relay.

If I'm speaking jibberish, look up 'flyback diode' on wikipedia. You hook it up across the two leads of your motor/relay/solenoid with the 'Cathode' side of the diode connected to the positive side of your motor/relay/solenoid. The Cathode side of a diode usually has a line on it.

Oh yeah.... this ONLY works for DC. DON'T do it on an AC load :)

- Hook
 
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