Light Level Sensing

Discussion in 'General Electronics Chat' started by screen1988, Apr 22, 2013.

  1. screen1988

    Thread Starter Member

    Mar 7, 2013
    I want to make the Light Level Sensing Circuit as bellow but I don't know how to choose device and value for resistors, Opam.

    Could you tell me what values for R1, R2, R3, R4, RF?
    What type of opam, transistor that is appropriate for the circuit?
    I have just simulated the circuit in proteus with R1=R2=RF = 10K, R3=R4 =330Ω
    transistor 2N1711, diode 1N4148, opam 741and the resistor series with LDR is also 10K. It works but I wonder if these values and devices are appropriate to do in real world?
    Thanks for help.
    P.S In this circuit I want to replace potentiometer by a resistor.
    Last edited: Apr 22, 2013
  2. Meixner


    Sep 26, 2011
    Many of the component values are difficult to determine because nothing in your schematic is defined. The values of R3 and TR1 depend on the relay that is used and the value of Vcc. The value of VR1 is determined by what LDR is used. Just by experience I would start with R1 and R2 set to 10K, and RF at 100K and see how it works. I dont think R4 is needed. I would use a 1N4001 for the diode. If you replace the potentiometer with a resistor there will be no way you can adjust the switching level.
    screen1988 likes this.
  3. PackratKing

    Well-Known Member

    Jul 13, 2008
    What particular units of measure for light level are you looking for ?

    I have schematics for several photographic light meters...
    all indicate which lens f-stop to employ for a given exposure.
    based on the " EV " or light value protocol...
    Last edited: Apr 26, 2013
    screen1988 likes this.