A voltage controlled switch

Thread Starter


Joined Nov 7, 2007
I want to implement the following logic: a 1.5 V voltage triggers a circuit to connect a 12 V battery to a relay.(the circuit is turned off as soon as the voltage goes below 1.5) A transistor and a diode along with a 12 v relay problem can solve the problem. But i am unable to figure out the circuit. Also, how do i achieve reliability in the circuit? Probably would need a circuit breaker too....to isolate the 1.5 volt source (the source is a microchip) from the 12 V DC battery.
Thank you for your time!!!


Joined Aug 15, 2007
We did something like this at my last job (long time ago -- late 80's!) We had an 8085 (5-VDC supply) write to a latch (the 8085 did not have any ports, so we faked it with the latch), and the latch output fed into a ULN-2003 driver, which grounded one side of the relay. The other went to +12 volts. There was also a small (1N914?) diode across the relay's coil. I think the ULN-2003 had enough built-in resistance that a separate resistor was not needed, but, hey, that was 15+ years ago! Worked like a champ.

I would guess you want to feed the microcontroller's signal into a transistor. I'd hook up an LED and current-limiting resistor between the transistor's collector and the +12V source, and make sure things work the way I expected. (A circuit breaker will probably be too slow to protect the micro.) Actually, I'd probably first set up the transistor-LED-Resistor, and with a current-limiting resistor on the base of the transistor, see if I can toggle the LED on & OFF by feeding the base of the transistor with 0 & 1.5 volts.



Joined Jun 13, 2007
Microcontroller 1.5V to base via a resistor. Emitter grounded. Relay coil connected to 12V and in series with the collector. Put the diode reverse biased across the relay coil to absorb inductive kick-back.

A circuit breaker is too slow and you won't find one rated for 1.5V anyway. I doubt you'd need any such device in any case. If you really want isolation between the micro and the 12V circuit then employ an optoisolator.


Joined May 16, 2005
A transistor and a diode along with a 12 v relay problem can solve the problem. But i am unable to figure out the circuit.
If the threshold were just a bit lower, I would suggest putting the diode in series with the transistor base. Transistor would conduct when i/p was high enough to forward bias both the diode and the EB junction.