Using Photoresistor Sensor in Place of Motion Sensor

Discussion in 'General Electronics Chat' started by dreamzboy, Nov 1, 2013.

  1. dreamzboy

    Thread Starter New Member

    Nov 1, 2013
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    I recently bought a Driveway Patrol Motion Detector but I wanted to use it to detect lights instead of motion. I have the Cds Photoresistor but unsure how to make this work. I bought it from Adafruit. The reason being that I wanted to know when someone enters the room. Motion sensor will work but not reliable because it depends on movement and when someone stands still for a few seconds, it failed. In contrast, this room required light in order to see. There's no window or any other interfering light source. I'm utilizing the motion sensor detector for its wireless transmitter/receiver capability as I will have the receiver mounted to where I can see it. Think of it as an occupancy detector.

    Here's what I understand:

    • Photosensor is really a resistor which depends on the light source.
    • Photosensor will need to be used with 10KΩ resistor to make it either a pull-up or pull-down resistor.
    • IR sensor is dependent on the frequency.
    • IR sensor will output 0V when a certain frequency is detected and output high when nothing is detected.
    • IR sensor has 3 pins, Vin, Ground, and Output.
    I was thinking of de-solder the IR sensor and put the photoresistor sensor in its place but something tells me to check with this forum first. Any help is greatly appreciated.


    Thanks!


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  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Wont work. The motion detector drives a high-gain ac-coupled amplifier. The ac is created when an object emitting heat moves past the shadow mask on the detector.
    The photo detector doesn't do that. It produces a very-slowly varying or DC output.
     
  3. wayneh

    Expert

    Sep 9, 2010
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    So your strategy is to detect occupancy based on whether the lights are on or not? Why not just get a signal off the light switch?
     
  4. dreamzboy

    Thread Starter New Member

    Nov 1, 2013
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    I thought about that but then I would have to tap into the light switch where it uses AC signal then convert it to DC for the wireless transmitter/receiver to work. It was just too much hassle which is why I wanted to use a photo sensor. I also have a wireless doorbell and it uses similar "switching" capability as a light switch. That seems like it might work better for this case.
     
  5. #12

    Expert

    Nov 30, 2010
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    How can I help...
    CdS cells are dreadfully slow. Almost a whole minute to change from "light" to "dark" resistance.
    They are all a bit different from each other. You will end up measuring the cell in the conditions present before you finalize your circuit.
    They are simply a resistor, no polarity, same for AC and/or DC.
    They have a break-over voltage limit and they have a wattage limit. These are generally not a problem.
    That's about all I have.
     
  6. wayneh

    Expert

    Sep 9, 2010
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    Not all of them. I was playing with a solar light this afternoon because its CdS cell no longer goes to a high enough resistance in the dark to turn on the light. Anyway, I was watching it with my meter, in and out of my work light. It changed from one value to the other within seconds - both directions - and just sat there.

    That said, it has "failed" so it may not be representative. Just for fun I put a 150k resistor in series with it and got the light working again as it used to. Not worth the effort to repair a 50¢ light, except for learning and fun.
     
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  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I agree with Wayneh, if CdS sensors are used in the better part of their current range (using the appropriate divider resistor) they can respond within less than a second giving a fairly large voltage change. They can be fast enough to detect someone walking in front of a light sensor.

    I'm not arguing with what #12 said because some sensors can be very slow to go from end to end of the complete range, especially near the slow end of the range (the dark end).
     
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  8. #12

    Expert

    Nov 30, 2010
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    Actually, I never tested a CdS in a short range of resistance. It seems obvious that changing by 1k instead of 1 meg would be quite a lot quicker, but some people come here thinking they are going to use a Cds cell for a fiber-optic receiver or a 38KHz remote control, and it just won't work.
     
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