need help with 741IC comparator

Thread Starter


Joined 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.


Joined 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.