Schmitt Trigger Light Sensor
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Build the circuit without the 22K positive feedback resistor.
R1 and R2 form a voltage divider.
R3 and the LDR form another potential divider.
When the potential at B is greater than the potential at A, the 741 operational amplifier output goes high and the LEDs come on.
R4 limits the current through the LED to less than 20mA.
Two LEDs are needed because, when the Op Amp pin 6 voltage is low, there may still be as much as two volts present.
Adding the 22K resistor adds some positive feedback and the on and off threshold voltages will now differ slightly. Adding this resistor converts the circuit from a Comparator to a Schmitt Trigger.
Set the supply voltage to +12 Volts as accurately as you can.
If you shade the LDR just right, you can make the LEDs come half on.
When you add the 22K resistor, the problem is solved.
This new circuit is called a Schmitt Trigger.
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