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TRIAC

8-bit optocoupled TRIAC circuit

A little more intricate, but still simplistic, is the TRIAC design.

Why TRIAC's?

Why TRIAC? you ask. Semiconductor Relays are expensive, whereas a simple TRIAC and an optoisolated TRIAC driver, such as the MOC3041 from Motorola can still do the job quite nicely.

Huh? Opto isolator?

An optoisolator is a device which is used to keep low-voltage microprocessor circuits (such as the very delicate parallel port) and high-voltage (mains) apart. They generally consist of 1 or more Infrared LED's pointing at a detector. The detector is used to trigger the TRIAC or SCRs. Sometimes, as is the case with the MOC3041, there is a Zero-crossing detector also incorporated into the same package.

Which triac should I use?

Generally, to be on the safe side, choose a TRIAC which has more or less double your mains voltage rating. I.e. for 220V use a 400V TRIAC. Check out the pricing on TRIACs to see which choice is economical for you.