LDR becomming insensitive

Discussion in 'General Electronics Chat' started by chaos51, Jun 14, 2012.

  1. chaos51

    Thread Starter Active Member

    Jun 18, 2011

    Without providing circuit details (I have no schema, I would need to draw one, I'll do that if it becomes absolutely necessary),
    I have a question about LDRs.

    I have made a little circuit, that switches a relay on or off, depending if it is dark or light... Simple stuff. LDR + some resistors + Schmitt Trigger + transistor + relay.

    It works pretty well, and has been doing so for 2 months or so.

    However, the LDR seems to become less sensitive for each week that goes. The delta resistance seem to decrease for light and dark.

    I had this problem before, and redid the schematic to provide some protection (diode + capacitor) to the LDR, so it would not get any peak voltages from the coils in the relay going on or off.

    The first experiment lasted two weeks, and then the LDR was basically useless. The second, 2 months or so, and the LDR is still going. But it does seem to degrade, though more slowly.

    Some details:

    The power supply is 12v, and because of the setup, the LDR should be getting about half of that.

    The Relay needs 12 volt to switch.

    The LDR is placed outside, to catch daylight. It is protected by some cellophane, and it it's place in a little box so it does not get wet when it rains. Together this gives it double moisture protection (I hope)...

    So, has anyone any idea what could be going on?

    Last edited: Jun 14, 2012
  2. Audioguru


    Dec 20, 2007
    The LDRs in my cheap solar garden lights get sunburned and rust away within 1 year.
  3. chaos51

    Thread Starter Active Member

    Jun 18, 2011

    Sunburned, or rusted is I think not the problem.
    We're I live there is not so much sun, and secondly they are carefully placed to be always in the shadow.

    I haven't seen any rust on them yet either. I think they are as moisture protected as is reasonably necessarily.

  4. bertus


    Apr 5, 2008

    How much current is flowing through the LDR?
    To much current may burn the LDR over time.

  5. #12


    Nov 30, 2010
    I've had a LDR based photocell on the side of my house, running my outdoor lights for 30 years. Lately, I noticed that it is getting pretty dark at night before the lights come on. This indicates that a properly designed circuit might last 30 years, not a few weeks.