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DIY Hobby projects, diagram, schematic

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DIY Hobby projects, diagram, schematic

LDR Light Detector

electronicecircuits, March 16, 2010September 14, 2023
LDR Light Detector

LDR Light Detector Circuit using 741 Op-Amp

This LDR light detector circuit’s threshold is set by resistor VR1. When the intensity of the light falling on the LDR is lowered, resistance of that unit increases, and lowers the voltage applied to the inverting input of the 741.

LDR Light Detector circuit

PARTS LIST
R1100kΩ
R2100Ω
VR1100kΩ
Q12N2222
RL19V Relay
IC1741
LDR

The reference voltage at the noninverting input of the 741 is set (via Vr1) so that the comparator switch from low to high  when the light falling on the LDR is reduced. That high activates transistor Q1, Which causes the relay contacts to close.

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Comments (5)

  1. seetharaman says:
    April 29, 2010 at 3:27 pm

    Hi R2 should be 100 ohms not 100K ohms.

    Reply
    1. Ravindra says:
      April 30, 2010 at 10:16 am

      Thank you Seetharaman, I correct it.

      Reply
  2. TomPier says:
    May 4, 2010 at 1:26 pm

    great post as usual!

    Reply
  3. autism symptoms says:
    November 8, 2010 at 10:11 pm

    What a great resource!

    Reply
  4. kong says:
    March 11, 2011 at 8:18 am

    Should be have the free wheeling diode?

    Reply

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