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Circuit to Compare Light Level or Temperature using an LM741 op-amp

The Circuit Diagrams for using and LDR or Thermistor



This is the circuit designed on fritzing for the LDR


This circuit uses an LM741 OP AMP to make a light level indicator so that when the light level dips below an acceptable level, the LED lights up.

You can set the level by turning the 22k potentiometer and tuning the cut off to what you want.

This circuit can be adapted so as to use a thermistor instead, but also any other analog resistive sensor you can find but you may find that you need to change the 22k potentiameter to 100k for certain sensors.

Brief explanation:

The 741 is set up as a comparator. This means that the 741 is comparing the input voltage of + and the - so that if the + has a higher input than the -, then the output will be high so it will light up the LED, if the - line has a higher voltage than the +, then the output will be low.

This circuit was designed by "Jim" and tested at Mallinson Electrical, we can verify that it works.

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