> > I'd suggest a power MOSFET or even a small relay.
>
> Yeah, MOS is what I was thinking originally but it seems that most of
these
> transistor/relay circuits are BJT.

Ok, while that is true, don't worry about it.  Here's what you do:

1.Find out what the activation current/voltage is for the relay
2.Use the transconductance of the MOSFET to determine how high a voltage it
needs to be driven with to get that activation current, the drive it at 5V
if the voltage required is less than that.

3. Supply enough voltage to the coil to make that work (refer to my ASCII
art, previous post)
4. Remember to wire a diode across the relay, or you'll let the smoke out.

Remember Electronic devices don't work without the smoke; if you let the
smoke out, your project doesn't work anymore. ;)

Oh, just a note, a diode across the MOSFET is not what you want here.  The
diode needs to go across the coil.  If that's not how you work it, there's
problems.

If the diode across the MOSFET is wired as drain/cathode to source/anode,
then current never flows, and if there's a current spike, it won't conduct.

If it's wired as drain/anode to source cathode, your relay is always on.

A diode across the MOSFET won't help.


Hope that helps,

--Brendan

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