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Trigger a relay at certain voltage?

Discussion in 'General Electronics Discussion' started by supak111, Feb 3, 2018.

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  1. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

    Apr 29, 2012
    Hey guys, I have a device which has a varying output voltage between 1v and 5v.

    I am trying to turn ON a relay from this device when the voltage is above 3v. It would be even better if I could control at what voltage the relay turns NO.

    What would be hopefully easy way to design his?
  2. duke37


    Jan 9, 2011
    1. You can get low voltage relays, there may be one to suit you.
    2. A single transistor with a zener diode in the emitter lead will need more base voltage to turn it on.
    3. An op-amp or comparator with hysteresis will do what you want and can be adjusted as required.

    You do not give details of the power supply or the relay requirements. Depending on the application, a mosfet may be better than a relay.
    What accuracy and hysteresis is required.
  3. BobK


    Jan 5, 2010
    Anything other than a comparator with hysteresis would likely result is less than satisfactory operation. Hysteresis allows you to define a "dead zone" around the trigger point so that, if the voltage is wavering right around 3V it will not keep triggering on and off.

    Here is a circuit:


    R2 and R3 are a voltage divider that set the trigger point. In this case, 3V. You can replace them with a potentiometer to allow you to adjust the trigger point anywhere from 0V to 5V. Or, you can place a potentiometer between them, say 10K to allow you to adjust it over the range of about 2.5V to 3.5V.

    R1 provides the hysteresis. In this case, it is about 2mV, so it triggers on the way up at 3.02V and on the way down at 2.98V. Make this resistor lower to increase the hysteresis, higher to decrease it.

    L1 represents the relay coil. Depending on the relay you use, you may need to use a transistor to drive the coil.

    The comparator is a random one built into LTSPICE. The LM339 is a popular widely available one that would work.

    hevans1944 likes this.
  4. BobK


    Jan 5, 2010
    Here is an updated circuit with a transistor driving the relay. You will almost certainly need this since comparators cannot provide enough current to drive anything but a very sensitive relay.


    Note that, because the transistor is an inverter, the + and - inputs of the comparator are reversed in this circuit.
    I have also added a diode across the relay coil, which is necessary to prevent a voltage spike when the relay turns off.

    With a bit of math, you can calculate values of R1, R2 and R3 to get any lower and upper trigger points that you want.

    shrtrnd and hevans1944 like this.
  5. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

    Apr 29, 2012
    Thanks for your help guys. I was able to find very cheap ml393 voltage comparator modules on eBay which are adjustable that should do the job.
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