ldr use

Discussion in 'General Electronics Chat' started by abjay, Apr 1, 2014.

  1. abjay

    Thread Starter New Member

    Mar 14, 2014
    I'm trying to build a 9v circuit that will operate a 8-12v latch at first light. The idea is to use a ldr to put power through to the latch, when the latch has opened a tilt switch then cuts the circuit until reset at night ready for the next morning. The only function of the ldr is to activate the latch in the morning, the trouble is I can't get it to work, I've wired the ldr in series with the latch and the latch will work if you short the ldr. Am I being too simplistic here? Can anyone help with a circuit diagram that would do the trick?
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    Do you have a diagram?
    Can you show us how you are wiring it.

    LDR cannot supply enough current . You need a transistor switch
    abjay likes this.
  3. ian field

    Distinguished Member

    Oct 27, 2012
    The old Philips EE kits had various projects using LDRs, there's a website:
    http://ee.old.no/library/ with scans of manuals in numerous languages. Some of the manuals don't include component values on the index of schematics - you either have to search through the manuals for one that does or search around for the component layout overlay cards.

    Apart from the older kits that used germanium transistors (AF116 & AC126), there are no hard to find parts.

    Just out of curiosity - didn't RoHS ban CdS LDRs?!
    abjay likes this.
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    We need more details, such as the resistance of the latch or the current it needs at 9V, and some idea of the resistance (light or dark) for the LDR.

    Attached is a PDF with three versions of a basic approach. The left one uses a standard small transistor, the center one uses a darlington transistor. The darlington has more gain, which means it will switch more quickly or sharply as the sunlight gradually increases, which should improve battery life. The right one uses an SCR, silicon controlled rectifier. This is a DC switch that latches on when its gate is triggered. It resets when it loses power. C106 is a popular series in a TO-92 package like small transistors.

    For each of these, you'll probably have to adjust the resistor value based on the characteristics of your LDR.

    abjay likes this.
  5. #12


    Nov 30, 2010
    That's a good starting place. :)
    Drawings tell so much, but few people here post drawings.:confused:
    A few more details and we'll have enough to work with.
    abjay likes this.