Mar 182015

Shadow detector alarm circuit with 2 LDRs

This shadow detector alarm circuit, detects if there is a difference of lighting between two LDRs, and sends an audible and visual warning. Shadow detector alarms, that use two LDRs, are more reliable than those that use only one.

How the Shadow detector alarm circuit works?

The main elements of this circuit are the two LDRs and operational amplifier that works as a comparator.

The non inverting input of operational amplifier is set to 4.5 volts, using a voltage divider formed by R3 and R4. The inverting input of the operational amplifier has a voltage setted by the voltage divider formed by the LDRs: R1 and R2.

Shadow Detector Alarm Circuit

In normal lighting conditions (no shadow), the two LDR receive the same amount of light, and the output of the operational amplifier has a low voltage level. When one of the LDR (on this case: R1) receives less light than the other, the voltage at the inverting input of operational amplifier, is lower than in the non-inverting input. When this happens, the voltage at the output of the operational amplifier goes high.

Then the output voltage of the operational amplifier enable the transistor Q1 (it goes into saturation) to turn on the LED and activates the relay. The LED gives a visual warning and the relay activates the audible warning.

Is advisable to place a semiconductor diode (D2) in parallel with the relay, as shown in the diagram, to protect the transistor Q1.

– The circuit uses a 9 volt battery or a power supply of the same voltage.
– The LDRs should be placed with a separation of no more than 3 cm between them.

Shadow detector alarm circuit components

  • 1 Operational amplifier: LM741 (IC1)
  • 2 LDRs (photoresistor) (R1, R2)
  • 1 2N2222 NPN transistor or similar (Q1)
  • 1 1N4007 semiconductor diode (D1)
  • 1 red LED (D2)
  • 1 9 volt relay (RL1)
  • 2 10K resistors (R3 and R4)
  • 1 1K resistor (R5)
  • 1 470 resistor (R6)
  • 1 100 nF capacitor (C1)


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