Driving relay with security light motion sensor

Discussion in 'General Electronics Chat' started by 3ds3, Jul 26, 2013.

  1. 3ds3

    Thread Starter New Member

    Jun 15, 2013
    I'm using a motion sensor for security lighting to drive a relay to break a circuit.
    It works but only for a few operational cycles of the device, when the motion detector trips and breaks the circuit.
    I believe it has to do with the current/voltage spikes that occur during each cycle feeding back into the motion sensor causing it to "trip"
  2. MaxHeadRoom


    Jul 18, 2013
    Maybe a little more information as to circuit details?
    It could be anything, no relay BEMF diode or what??
  3. electronis whiz

    Well-Known Member

    Jul 29, 2010
    the lamps normally have relays in them to trigger the lamp. hard to tell if your using external relay or integrated one. I would look into the settings on the sensor. sensitivity, timer, etc. possible your using a relay that's too big for it. they seem to use like an average size electronics relay. like you may find in a UPS, car systems like windows wipers, fan. these normally run like 9-12V DC I believe that's what these sensors use too. you may need to use a small reed relay to trigger bigger one, or use some form of power transistor.
  4. 3ds3

    Thread Starter New Member

    Jun 15, 2013
    The sensor runs on 120 volts
    The relay runs on 120 volts
    I believe the sensor has a triac, based on that it would not operate the relay without a 500 ohm resistor to draw enough current to latch the triac closed.
    I even tried a 450 micro farad cap to try and ease the surges but it seems to not really help.
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Its possibly the relay taking too much current when dropping out, your Sensor will use a zero voltage detector opto Triac , you will have to slave another triac to fire the lamp, or smaller relay coil.
  6. 3ds3

    Thread Starter New Member

    Jun 15, 2013
    The coil is 4.3 meg ohms.
    The triac would not energize the coil because the minimum latching current for the triac was not met