logic level to switch 12v.

hertelbrian wrote: ok... not a electrical engineer by ANY MEANS.. very basic knowledge of electronics...  i have a logic level control line 5v    2.5mA     i need to turn on a 12v source... i tried a car relay.. but friends said that was stupid and can burn out my source...  said there are relays that can be switched w/ logic level to turn on 12v??? anyone have any ideas???

With such a low current relays will be out of the question.  Your only hope is the use a transistor.  Grab a 2n3904 NPN transistor from Radio shack.  Take your 5vdc feed and run it through a ~4000 resistor (limits to roughly 0.00125A) to the base of the transistor. 

Ground the Emitter of the transistor

Your output will be on the collector side of the transistor, and should be ran through a 47 ohm resistor (limits current to 100ma).  After the resistor this ground source will be strong enough to fire a 12vdc relay.  Feel free to let me know if you have any other questions.  Parts cost from Radioshack should be around $3, not counting the relay!

Kevin Pierson

Another option to the 2N3904; I bought a bagful of 2N7000 MOSFETs for general purpose use, it has basically no draw on your drive source, and should have no problem sourcing enough current to turn on your 12V relay. You can even turn them on and off just by touching the gate with your finger and touching 12V or ground. They're switched using voltage rather than current, which means you don't have to worry about drawing too much drive current from your circuit. They're 8 cents a piece from mouser.com when you get 100, good for a lot of different things. A whole bagful to play with for $8- The only thing you have to watch out for is that these things are very sensitive to static charges, so you have to be careful when handling them to prevent ESD damage. They're pretty reliable when you install them properly; I'd install a supression diode across the source and drain leads to clamp down the big voltage spike that occurs when the relay turns off. Some of the high current relays can deliver quite a kick when they turn off, and the solid state stuff doesn't take kindly to that..

Here's a link to the datasheet; the TO-92 package is the standard packaging- 2N7000 Datasheet