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Detect Changes in Light Level

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.