Triac

Triac work with AC only. In fact it turns the circuit on until the voltage is above a minimum level... that is constant in DC, so it will never turn off! As for AC, beware if it has no zero-cross system. Zero cross are smart and will turn off the circuit at the moment the AC goes 0 (or so). Otherwise, beware of magnetic waves if the current stops abruptly in the middle of a cycle.

Misc doc

• http://www.signindustry.com/electric/articles/2000-11_ALDORtriacexplainted.php3
• http://www.epanorama.net/circuits/semiconductor_relays.html#triacuse
• http://home.howstuffworks.com/dimmer-switch3.htm
• http://computerchristmas.com/christmas/link-how_to/HowToId-7/How_to_build_a_triac_switching_unit_SSR

Opto coupler / photocoupler

Optocoupler works with DC. It's a led and a photo-transistor in a box. When light is on, the transistor transmits the current. Simple as that. Very fast switching speed.
Taken from wikipedia

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