simple laser sensor help needed

Discussion in 'The Projects Forum' started by Gigantos, Sep 16, 2013.

  1. Gigantos

    Thread Starter New Member

    Mar 15, 2012
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    I would like to ask a fast question, I don't belive it is very complex, I made an extremely simple circuit, to use a photoresistor as a laser detector, this might be the wrong place to ask, but im giving it a shot, the "laser" im using is just a 3 dollar wal-mart laser pointer, I took a photoresistor, out of a grab bag from radio shack, and a blue LED, and a variable resistor, I put the variable resistor in line with the photoresistor, then to the positive side of the led, and the other side of the LED to ground, its using +12 volts, only because im using a +12 volt adapter, that was lying on my work bench already, I have no target voltage in mind, all its intended to do is latch an SCR, or maybe just a pnp-npn combo, I just wanted to ask if anyone has a better idea, it works but the main problems are, I cant get the photocell, acclimated to ambient indoor lighting, so it was always on, covering it with black electrical tape fixed that, the LED turns on, and then bleeds back to off, VERY slowly, so I think im getting current leakage thru the resisters, my first guess is that 12 volts is to high, im going to give 5 volts a try in the morning, but I was wondering if anyone might know of a simple and fast alternative, I have a IR emitter/receiver, and I tried using the receiver in place of the photocell, with no luck, I have done a lot of google research and all the circuits I come up with are either to complex for what I need, or use 2 photocells, im planning on only using one, for a pulsed on-off, anyways, just wondering if anyone might have any helpful hints, you don't necessarily need to draw/type out any schematics for me, just a helpful nudge in the right direction would be great, sorry for rambling so long lol =), also, this is for an extremely low tech project, so it doesn't have to be great, just so it stays off in ambient room lighting, no more intense than the lighting in a normal home lol, I also played with a comparator circuit, to just read the ambient light as a baseline, and the laser would be the trigger, but I need to build about 50 of these, so the more simple and the cheaper the better.
     
  2. Experimentonomen

    Member

    Feb 16, 2011
    331
    46
    I think a photodiode would make a better sensor than a LDR for laser stuffz.
     
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  3. #12

    Expert

    Nov 30, 2010
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    Cadmium Sulphide photo resistors are very, very slow. Almost a minute to go from full on to full off. It is not a leakage problem. You are using the wrong part.
     
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  4. Gigantos

    Thread Starter New Member

    Mar 15, 2012
    7
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    You are correct, my original intention was to use a photo-transistor, but my radio shack didn't have any, so I just grabbed the LDR, I ordered a 10 pack on ebay they should be here soon, on a side note, does anyone know if a IR receiver, would function as my trigger? I tried it, and no luck, im assuming the laser pointer isn't producing any IR or near IR light, but I only ask because a few of the diagrams I have came across, have shown them to be interchangeable, but when I tried it on my bread board, they didn't work.
     
  5. Gigantos

    Thread Starter New Member

    Mar 15, 2012
    7
    0

    your right, I hooked it up to my ohm meter, and it is VERY slow, about 50 seconds or so, lol, I intended to use a photo-transistor, but couldn't get one in the town im in, so I ordered some, my original thought of the voltage leakage, was prior to any further testing, thank you for the reply
     
  6. #12

    Expert

    Nov 30, 2010
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    Thank you for confirming that the parts available now are exactly the same as they were 30-40 years ago.

    CdS cells are slow, but they aren't useless. I find them good for detecting "night" and "daytime". When the "signal" is one cycle per day, one minute is (comparatively) so fast that I design in several more minutes before the circuit flops to be sure the cell didn't see a car headlight or a person standing too close and shading the sensor.
     
  7. Gigantos

    Thread Starter New Member

    Mar 15, 2012
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    0

    Meaning photo-transistors were also hard to come by 30-40 years ago? lol

    Im going to assume you know more about LDR's than me, in your opinion, if I hooked up one in a circuit that just used the ambient lighting as a baseline, like in a comparator type circuit, and used the laser to change the resistance level, as a trigger, though im still going to assume that a photo-transistor is a better choice all around for a laser circuit?
     
  8. #12

    Expert

    Nov 30, 2010
    16,337
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    Yes, a photo-transistor is a better choice for building a circuit that is faster than typical planetary motion.

    Gee whiz, you don't seem to have a grasp on how bleeping primitive it was only 50 years ago. A CdS cell is just a common chemical that was coated onto a piece of glass. It isn't even waterproof without a cover of some sort. The convoluted path in the etching was high tech in 1960. True, transistors had been invented by 1960, but they weren't available. Vacuum tube TV's were still being manufactured in 1970.
     
  9. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    Back in the day (50 years or more) we filed the top off of 2n3055 transistors and glued a piece of plastic to make our own photo sensors.
     
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  10. EVETS

    New Member

    Jan 29, 2011
    21
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    BillB I used to do that with 2n2222's. It was amazing how small the innards actually were.
     
  11. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Have you tried an IR receiver?
    It would be optimized to different wavelength, but the laser may be able to overpower it.

    IR~940nm?
    Red~671nm?

    At least you would have some ambient light immunity without having to modulate the source.

    http://www.everlight.com/datasheets/PD204-6C_datasheet.pdf
     
    Last edited: Nov 10, 2013
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