This circuit detects sudden drops in LUX level. Slow changes are ignored. It could be used as an intruder detector if the intruder interrupted a light beam. It was originally designed as a fish bite detector. When the fish pulls on the fishing line, a card is moved, covering an LED shining on the LDR. This causes a drop in the light reaching the LDR.
To allow the circuit to run on a single 12 Volt supply, R5 and R6 act as a Voltage divider providing a 6 Volt line. This is half the power supply voltage because the resistors are equal. This 6 Volt line is decoupled by C2. This removes any AC signals from this line.
The diode D1 protects the circuit from accidental connection of power the wrong way round.
The LDR and R1 form a voltage divider. If the light level increases, the resistance of the LDR decreases. This causes the subsystem output voltage to increase.
C1 blocks DC but couples changes in voltage from subsystem 1 to the comparator.
R2 and R3 form a fixed voltage divider. The DC output voltage is about 13 millivolts greater than half the power supply voltage. The DC output voltage will be 6 + 6 x (10x103 / (10x103 + 4.7x106)) = 6.013 Volts.
Subsystem 5 is a comparator.
If the light level is constant, the non inverting input will be at a potential 13 mV greater than the inverting input so the output will be high at 10 to 12 Volts.
The LEDs will be on.
If the light level drops a little, the inverting input voltage will also drop.
If it drops more than 13 mV, the comparator output will go low and the LEDs will go out.
This low output pulse is designed to trigger a 555 monostable.
R4 limits the current through the LEDs to a safe level.
Two LEDs are used because the comparator output never drops below 2 Volts.
With one LED, it would always be lit.
With two LEDs in series, nearly four volts are needed to light them so they turn off properly when the comparator output is low.
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