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    Replacing failed photo resistor. How to determine value?

    I just replaced an outdoor motion activated light for my friend and decided to troubleshoot the old one. Within a few minutes I determined the LDR(photo resistor) was bad. It only changes about 5 ohms between light and dark, and after removing it, the light works again. I would like to fix it because I have a use for it, but the LDR has no markings. With the LDR removed, the resistance of the circuit is about 80k. Any ideas on how to figure out the value?
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    #2
    Re: Replacing failed photo resistor. How to determine value?

    I would put a pot (may be 10K to start with) in its place and vary the resistance to simulate Dark and light so at you will know what the delta is, then you may be able to do some modification using photo Transistor.
    https://www.badcaps.net/forum/attach...0&d=1554877524 where is the connection to the sensor? I cannot clearly see the traces on that section where the sensor is.
    You should also take some Voltage measurement on the two pads where the sensor is connected to.
    Last edited by budm; 04-10-2019, 12:42 AM.
    Never stop learning
    Basic LCD TV and Monitor troubleshooting guides.
    http://www.badcaps.net/forum/showthr...956#post305956

    Voltage Regulator (LDO) testing:
    http://www.badcaps.net/forum/showthr...999#post300999

    Inverter testing using old CFL:
    http://www.badcaps.net/forum/showthr...er+testing+cfl

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      #3
      Re: Replacing failed photo resistor. How to determine value?

      Guessing the chip is BISS0001 which is a popular PIR motion detector IC.
      CdS cell input triggers at 0.9V/1.7V and you'd have to check your circuit what the resistor is to +V power, and if it is 3.3V or 5V the IC runs off.
      For 3.3V power if R is 1MEG then a 500kohm dark CDS (or higher). I think you have a 1MEG trimpot for R.
      Some people put in a photo-transistor and lower say ~100k resistor.
      You can do the voltage divider math once you know +V and R.
      Careful the electronics is running from hazardous live so there is a shock hazard.

      edit: the chinese cheapola CdS cells are GL55 series:
      GL5528 is 10-20k light, 1MEG dark
      GL5516 is 5-10k light, 500k dark
      Last edited by redwire; 04-10-2019, 12:57 AM.

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        #4
        Re: Replacing failed photo resistor. How to determine value?

        Originally posted by budm View Post
        Iwhere is the connection to the sensor? I cannot clearly see the traces on that section where the sensor is.
        You should also take some Voltage measurement on the two pads where the sensor is connected to.
        New pictures below. Red circle is the LDR connections. Voltage is about 5.7v with the LDR removed.
        Attached Files
        Last edited by lookimback; 04-10-2019, 01:20 AM.
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          #5
          Re: Replacing failed photo resistor. How to determine value?

          Originally posted by redwire View Post
          Guessing the chip is BISS0001 which is a popular PIR motion detector IC.
          CdS cell input triggers at 0.9V/1.7V and you'd have to check your circuit what the resistor is to +V power, and if it is 3.3V or 5V the IC runs off.
          For 3.3V power if R is 1MEG then a 500kohm dark CDS (or higher). I think you have a 1MEG trimpot for R.
          Some people put in a photo-transistor and lower say ~100k resistor.
          You can do the voltage divider math once you know +V and R.
          Careful the electronics is running from hazardous live so there is a shock hazard.

          edit: the chinese cheapola CdS cells are GL55 series:
          GL5528 is 10-20k light, 1MEG dark
          GL5516 is 5-10k light, 500k dark
          It's one of those chip under blob things, so I don't have any way of knowing. The trimmer on the board is for range. Where am I measuring R?
          Last edited by lookimback; 04-10-2019, 01:11 AM.
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            #6
            Re: Replacing failed photo resistor. How to determine value?

            I don't think you can really make a good guess from here how the blob is made and what its light/dark cutpoint is. Probably have to resort to guess and check, I'd probably start with the GL5528 and see how it works.

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              #7
              Re: Replacing failed photo resistor. How to determine value?

              try to remove the coating, it could be the problem.

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                #8
                Re: Replacing failed photo resistor. How to determine value?

                I've been playing around with different resistances and it seems that somewhere around 33k is the tripping point. Anything above that will make it turn on.
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                  #9
                  Re: Replacing failed photo resistor. How to determine value?

                  Originally posted by stj View Post
                  try to remove the coating, it could be the problem.
                  I tried cleaning it up, but no change.
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                    #10
                    Re: Replacing failed photo resistor. How to determine value?

                    You can test the CdS photo-resistor out of circuit. They are slow and it can take 10 seconds when they get dark for resistance to settle. Very small chance the capacitors in the circuit by C16 C17 are leaky or failed instead.

                    I think any replacement CdS would work- photo-resistors have a logarithmic ohms scale and around 100:1 change between light and dark. So there isn't big difference between them if you are only looking at on/off, light/dark sensing.

                    A GL5528 looks good, or a GL5516 if that's all you have in the junk box.
                    It would affect the exact trip point at dusk, the GL5516 would make it need to be darker out before the motion sense is enabled.

                    Another method I faintly recall is counting squiggles and size on the CdS cell to compare them, but that's grasping at straws.
                    GL55xx CdS datasheet

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                      #11
                      Re: Replacing failed photo resistor. How to determine value?

                      Originally posted by redwire View Post
                      You can test the CdS photo-resistor out of circuit. They are slow and it can take 10 seconds when they get dark for resistance to settle. Very small chance the capacitors in the circuit by C16 C17 are leaky or failed instead.

                      I think any replacement CdS would work- photo-resistors have a logarithmic ohms scale and around 100:1 change between light and dark. So there isn't big difference between them if you are only looking at on/off, light/dark sensing.

                      A GL5528 looks good, or a GL5516 if that's all you have in the junk box.
                      It would affect the exact trip point at dusk, the GL5516 would make it need to be darker out before the motion sense is enabled.

                      Another method I faintly recall is counting squiggles and size on the CdS cell to compare them, but that's grasping at straws.
                      GL55xx CdS datasheet
                      I'm 100% certain it's the photo-resistor. I've tested it out of circuit and it barely changes between light and dark. Also, I've simulated it in circuit with a resistor and that makes the light work again. I just ordered 20 GL5528s for 82 cents from AliExpress. I guess it will be about a month before I have them.
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                        #12
                        Re: Replacing failed photo resistor. How to determine value?

                        You can use it with the CdS taken out, if you need a security light until the part comes in. The motion sensor IC will think it's dark out all the time and it will trigger during the day.

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                          #13
                          Re: Replacing failed photo resistor. How to determine value?

                          Originally posted by redwire View Post
                          You can use it with the CdS taken out, if you need a security light until the part comes in. The motion sensor IC will think it's dark out all the time and it will trigger during the day.
                          I'm thinking about doing that. I still have to run the circuit to where I need it, and I may not get to that for a couple weeks anyway. Pretty cool that I'm fixing this for about 4 cents.
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