I'm trying to use my little AVR to control up to 12 120VAC loads, up to 5 amps. Does anyone know of a cheap (< $5 US) relay (SPST-NO) that only needs 40 mA or less to activate? My Tyco T77 relays need 90 mA at 5VDC to pull in the contacts, it turns out, so the processor dies. I don't have a harsh environment, or rapid switching requirements, just need a simple on-off relay. I'm switching banks of lights, should look cool if I can get it to work. Thanks for any advice.
Mike Ross
You might want to look into one of the parallel port relay boards like below and use it attached to your AVR instead of the parallel port.
This looks like it might work for you:
Ah, here's another data sheet that does say 40 ma for the coil at 5VDC:
Oh, man. That is the perfect answer. I asked the question yesterday, and here you show me the exact thing I need, for very cheap, $0.99 ea. Get this, I just finished ordering 12 of that exact same relay from DigiKey for $5 each! If I'd read your post first, I would have saved $50. Live and learn. At least now I know where to get replacements if I need 'em.
Anyway, thanks for the fine example of how Usenet adds value. That was probably the most prompt and appropriate answer I've ever gotten on Usenet.
Mike Ross
Mike Ross
You're gonna run into problems if you try to drive [too many] relays directly off an AVR or PIC chip, esp as many as 12 relays. Besides load currents in individual pins, you also have to look at the max load on the chips.
What you want to do instead is insert a couple of ULN2803 darlington driver chips between the uC and relays. The drivers are cheap, under $1, and they only require 1 mA or so current from the uC, plus they have integrated clamping diodes for the relay coils [absolute necessity], plus they also add an extra degree of isolation and protection for the uC. Better to blow a $0.50 part than a $5 uC chip. This is really the best way to go.
Also, you can buy relays cheap from places like electronic goldmine, but these are usually surplus items, and they'll likely not have the same parts next time you go back. Digikey will have the same replacement parts 99.9% of the time.
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