Any hobbyist electronics gurus out there?

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

Need some help.

I have an electronics device that I am modifying. It has a 2.5v light that illuminates that I want to have switch on a 12v light (500mA) in parallel. So I need some kind of 2.5v micro relay or amp which will switch on a 12v source.

Any ideas of how I can accomplish this? I picked up a Radio Shack 5v micro relay, but it's got a minimum 3.5v switch-on voltage, doesn't work. I will need four of these circuits, and I am building a breadboard, so something in a DIP package or otherwise that can handle 500mA per circuit would be perfect.

Any ideas? Thanks.

Donnie Barnes · **Posted:** Thu Jul 07, 2011 9:47 am

I'm away from my reference materials until next week, but what you need is a pretty simple transistor circuit to turn on your relay (at a half amp I'd go with a small relay...your 5v one you already have would work). This might give you a good start:

http://www.kpsec.freeuk.com/trancirc.htm

--Donnie

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

Donnie, thanks, it's funny I just read that page over breakfast this morning

I think I have a plan -- a Radio Shack TIP31 NPN high current transistor should work perfectly for this. I found a very useful page on how to use a high current transistor as a cheap alternative to a relay: http://www.rason.org/Projects/transwit/transwit.htm

Turns out the TIP31 has a minimum gain (Hfe) of 25 so I should only be using about 20mA or less from my 2.4v source, which shouldn't overload it.

Product page: http://www.radioshack.com/product/index ... Id=2062610
Datasheet: http://people.ece.cornell.edu/land/cour ... /TIP31.pdf

Off to Rat Shack at lunch...

Donnie Barnes · **Posted:** Thu Jul 07, 2011 10:36 am

Yeah, I knew you could easily handle that with a higher current transistor, I just figured since you had the relay... But yeah, without the relay you don't need the stupid diode, too, and there are no moving parts, both of which are pluses (it's kind of funny...LEDs have huge lifetimes, but the type of diode you want to use in that circuit around a coil actually does NOT have a good life expectancy, relatively speaking, and can ultimately cause you trouble that's hard to diagnose).

I'm pretty sure I'd actually have all the parts you need at home for this, and in fact have the exact circuit you need sitting built (times four) on a breadboard. It's got a higher component count, though, because it uses a tiny transistor to turn on a bigger one, but the bigger one is big enough to do an amp or more, IIRC. But you're on the right track...hardest part might be finding that transistor actually in stock at a Radio Shack.

By the way, it seems there's a decent chance that Radio Shack is going to actually try to go back to their roots a little over the next year or two and start carrying more hobbyist related items. And I don't just mean actually carrying some transistors and resistors and capacitors, I mean those things PLUS stuff like Arduinos and such. True hacker items. Could be pretty neat.

--Donnie

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

Update - I figured out I was wrong, the box I'm wiring into actually sinks the minus side of the bulb to ground ... so plan is still on but I'm going to use a PNP transistor which switches on when the base is low.

Rat shack has the TIP42G for $1.69, gonna try those this evening. It's fun, haven't tinkered with electronics since my job in high school working on data logging at a thin-film solar cell manufacturing company

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