Photocell On When No Light Off When Light?

Discussion in 'General Electronics Chat' started by XxAdvancingTechxX, May 28, 2013.

  1. XxAdvancingTechxX

    Thread Starter New Member

    May 28, 2013
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    Im trying to save like $30 on a battery powered motion night light. I really don't want it to be motion activated, just come on at night. I paid $2 for a click led button light which I disassembled and $0.87 for a photocell. I removed the on off switch and tried to connect the photocell to it, but it would not work. See attachments for images of what I have. One shows how the wires are connected and the bulb receives power.
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,033
    3,241
    You can't just throw a couple parts together and expect them to work, grasshopper. :rolleyes:

    Did you look at the characteristics of that photocell? It has a high resistance when in the dark and a lower resistance when illuminated. You need to add a transistor to amplify the resistance change from it to turn on the LED in the dark.

    What is the battery voltage and the LED current when it's on?
    What is the photocell resistance when it is illuminated?
     
  3. #12

    Expert

    Nov 30, 2010
    16,330
    6,818
    This is the basic shape of what you need. Give numbers about the voltage and current of the light bulb, and the resistance of the photocell, to get answers about the size of the transistor and the resistor.
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    Here is a circuit that has worked for me. You probably won't need a MOSFET for Q1. Depending on the current required by the light, you might use a cheap transistor, or you might be able to power it directly from the op-amp.


    ETA: There are probably simpler ways.
     
    Last edited: May 29, 2013
  5. XxAdvancingTechxX

    Thread Starter New Member

    May 28, 2013
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    Well it's just 2 AA batteries powering it. Don't know too much about all this.
     
  6. Shagas

    Active Member

    May 13, 2013
    802
    74
    You can do it the way #12 posted it .
    You will definately need a transistor . They cost pennies . Go to your local electronics shop and ask for a General purpose Npn signal transistor
     
  7. XxAdvancingTechxX

    Thread Starter New Member

    May 28, 2013
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    I don't understand schematics sorry. Im a nub at this. From what I do get, the wire leading from the light goes to a transistor then loops back around, from the middle of the transistor goes to a resistor back to the light and to the photocell. What I don't get is what is the lines to the left with the + and -?


    Also what sized resistor, transistor, and photocell should I use? Im currently using a photocell half the size of a pea.
     
    Last edited: May 31, 2013
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