Diac

And while the OP is on the Google page, he should look at the sixth link:

particularly Fig. 8.1 of the Teccor data sheet. Nearly all diacs are used for simple phase control of AC line voltage loads with triacs as triac triggers. (By the way, the extra few cents for a Q4015 is worth it -- don't use the Q2015 rated at 200V max. with 120VAC).

Good luck Chris

you can use a diac to give privacy on phone sockets wired in parallel

- i.e. stop someone picking a phone up in another room and listen to your call.

insert the diac in one of the wires to each socket and done.

in normal mode, the phone line sits there with about 50V on it (polarity is not an issue) which means when the phone is picked up the diac will conduct (>30V) but once the phone is off the hook, the line voltage falls to about 12-15V which is below the cunduction threshold for all the other diacs... the practical upshot is that you can call but with the line voltage below the threshold, anyone else picks up a handset gets a dead line. good huh?