need help with 741IC comparator

Discussion in 'The Projects Forum' started by arq, Jun 9, 2009.

  1. arq

    Thread Starter Member

    Nov 2, 2008
    I am building light sensor based circuit driving a 12v relay. LDR is used as sensor. 741 Ic is used as voltage comparator. On non inverting input reference voltage is set equal to 8 volts. and on inverting input resistive combination of LDR and variable resister of 1M ohm is connected. Hysteresis is also added to the circuit with 100k ohm resister. All conditions for saturation are satisfied and circuit is working well. I have checked voltages on all the pins . Correct voltages are obtained. On the output whenever i connect LED . LED turns on and off according to the conditions and requirements . But when i tried to connect the relay through transistors. Relay does not triggers. I have checked with the multimeter, current flow is totally zero. I cannot understand why this is so. If someone one can help.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    We need to see the schematic to see how it might be improved. Also, the LM741 is an op amp, not a comparator. You might obtain better/correct results by using something like an LM311 comparator.

    If you want to try one, look at the data sheet to see how to use it properly.
  3. mik3

    Senior Member

    Feb 4, 2008
    Post the circuit and the condition you want the relay to turn on.
  4. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    did you not like my answer in the other post, or is this another attempt?