How do I build a low voltage relay?

In this thread I discover I need a relay. So I head on over to Radio Shack thinking they’d be a dime a dozen. The only relays they had are 12V. I need one that will trip with 1V to 3V. I’m not sure if there are other electronics stores around that may have a low voltage relay (I’ll look).

But I’ve got a drawer full of diodes, resistors, transistors from back when I used to know what to do with them. There’s gotta be a way I can string some of these things together to close a switch in the presence of a current as described, i.e. the current going to the little watch speaker.

I did find a schematic for a solid state relay as well as this one (pdf), but one tripped by 5V, and both have overkill design for a way-higher load than I need.

I’ll keep looking, of course, but if you can either give me a quick how-to or point me to a schematic I’d be much obliged.

I just noticed both linked-to designs are “optically isolated” Is that the only way?

You need a relay driver. The 1-3v input drives the base of a transistor into saturation and the relay coil is in series with the collector or emitter current.

You’ll need to know the coil resistance in order to make sure the transistor doesn’t burn up, and work in an emitter resistor if necessary.

:smack: Yes! Now why didn’t I think of that? Thanks!