Dual Transistor Circuit Doesn't work

Thread Starter


Joined Jul 15, 2015
Hi All

I made a Transistor switching circuit awhile ago for my car and it seemed to have worked (Attached as 'Original') . The idea was to allow two separate constant triggers (not at the same time), control ONE relay. The relay would turn on other 12V equipment in my car. The 12V input trigger seems to work while, but the Ground trigger is hit or miss. Mostly misses.

So I came up with a with the other attached circuit (Modified 1) to have the PNP drive the NPN for the Ground Trigger and it worked. .. But now the 12V input doesn't work anymore. The one thing that did work.

So now I'm thinking, maybe to use a P-CH mosfet in place of the PNP in the Original drawing, but use a 10K pull-up resistor on the gate.

Any input would be much appreciated.

Thank you




Joined Aug 21, 2008
It looks like you figured out why the ground trigger was intermittent. Your fix will assure that there is more voltage available for the relay.


Joined Aug 21, 2008
Yes, in the way shown on your "Modified 1.png". I don't have a way of knowing whether you will have enough voltage, but at least you will be increasing it by nearly two diode drops.

By the way, I am only looking at the voltage drops, not the actual voltages nor am I tracing out your logic.