A voltage controlled switch

Discussion in 'The Projects Forum' started by ZAP, Nov 7, 2007.

  1. ZAP

    Thread Starter New Member

    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!!!
  2. RiJoRI

    Well-Known Member

    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.

  3. niftydog

    AAC Fanatic!

    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.
  4. ZAP

    Thread Starter New Member

    Nov 7, 2007
    thank you guys for the tips!!!
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    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.