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Photocell circuit

Discussion in 'Misc Electronics' started by J. Yazel, Mar 5, 2006.

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  1. J. Yazel

    J. Yazel Guest

    I would like to find a schematic for a photocell feeding a schmidt
    trigger (CMOS). This will input the light from a not-very-bright panel
    lamp. It is also very slow speed (once-a-half-second).

    Any URL available?

    Thanks for any help.

    Jack
     
  2. If it is a cadmium sulfide cell, the cell and a fixed resistor form a
    voltage divider with perhaps 5 or 12 volts applied. Then, one just sets up
    the trigger points (UTP and LTP) accordingly.
     
  3. Christopher

    Christopher Guest

    Hello Jack,

    I have a circuit and PDF PC board layout that could be modified into
    what you need, especially triggering off a low level lamp output. This
    will be using a photo-transistor which is more sensitive. Also using a
    low powered TLC555 gives you a CMOS Schmidt Trigger.

    http://www.oldtemecula.com/theremin/rs-opticaltheremin/rs-opticaltheremin.htm

    If you leave everything as I designed, it would pulse/buzz a small
    speaker every flash of the dim lamp.

    Modifications for what you indicated:

    Leave off C1 so it does not output a tone (oscillate), then adjust the
    value lower of R2 a 1Meg resistor to give you the trip point you need.

    Remove and "jump wire" C2 for a DC rail to rail voltage swing on the
    output. Leave Pin 7 of the TLC555 disconnected or cut the copper foil.
    Better yet use a IC socket and leave this IC pin folded upward.

    Pin 7 is just above the "1" of the U1 label on the IC graphic.

    Pin 1 has a black dot above it and the pins number counter clockwise.




    * * *
    Christopher

    Temecula CA.USA
    http://www.oldtemecula.com
     
  4. J. Yazel

    J. Yazel Guest

    Thanks very much. This looks very useful.

    After a quick scan, I didn't see the photocell number. Do you have
    it?

    I'm going to drive an LED counter with it. I need to see when the
    input lamp lights when I'm away and a rough count.

    Thanks again.

    Jack
     
  5. J. Yazel

    J. Yazel Guest

    =================

    I'm not familiar with the terms but if photoresistive indicates
    a cell that acts as a resistor, that's what I'm thinking that I need.

    If there is a generator cell that will trigger a CMOS Schmidt,
    then maybe I could use that. I don't think that I could adapt
    it to the input lamp since I don't have any way to measure
    the lamp intensity (not smart enough in optics).

    Thanks.

    And thanks to all other the other responders.

    Jack
     
  6. J. Yazel

    J. Yazel Guest

    I found it. Sorry.

    Jack
     
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