NPN Transistor switching a relay help!

Thread Starter


Joined Jun 12, 2010
I am trying to build a simple circuit where a NPN transistor switches a relay.

my input is coming from an infrared photo-transistor.

My problem is the NPN (2n2222), passes 9.4v and relay engages with out any V+ on the base. When i apply a voltage to the base i get the full 12V out.

I have read for hours, including the tutorials on this site and think i understand this, but apparently i am missing something.

Any help would be greatly appreciated.

Thank you in advance for sharing your wisdom.

- James


Joined Jul 17, 2007
Hi James,
It isn't rocket science. However, you need to give us a bit more in the way of specifics of what you have. Values and part numbers would be a big help.


Joined Mar 12, 2010
Putting a resistor from base to ground might help by draining off any leakage currents, but it's more likely the transistor is kaput or you've installed a wiring error.

Ron H

Joined Apr 14, 2005
My guess is that the collector and emitter leads are swapped. Base-emitter breakdown will explain the behavior you are seeing.
Check it out, onejames.