simple smoke detector

Discussion in 'The Projects Forum' started by lihle, Oct 14, 2010.

  1. lihle

    Thread Starter Active Member

    Apr 12, 2009
    83
    3
    guys i need you help. i am trying to make a simple smoke detector using a photo transistor and the LED. i have followed the instructions from the datasheet of how to test the photo transistor.

    what i have designed is the Photo transistor in direct line with the LED and then i measure the voltage when there is nothing in between LED and the Photo transistor. it gave me 5 V but when i put something like a paper it still give me 5v.

    what i want to do is that when i interrupt/place something between there should be no voltage.

    what i would like to bring up to you is that for me it is not easy to get components such as the Photo interrupter so thats why i am trying to develop one.

    hope you will help me out.

    lihle
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    Can you post a schematic of your test case?

    Bertus
     
  3. DickCappels

    Moderator

    Aug 21, 2008
    2,653
    632
    What this board needs is a clairvoyant engineer.
     
  4. lihle

    Thread Starter Active Member

    Apr 12, 2009
    83
    3
    sorry guys for not posting my design. i wont do it again.
     
  5. lihle

    Thread Starter Active Member

    Apr 12, 2009
    83
    3
    sorry guys for not posting my design. i wont do it again.here i have attached it.
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    The LM324 shows the - input at ground.
    The transistor will never reach ground level.
    Make a voltage divider of two resistors and put the - input in the middle of it.
    The resistors can be equal values between 1 K and 100 K.

    Bertus
     
  7. lihle

    Thread Starter Active Member

    Apr 12, 2009
    83
    3
    thanks for your recommendation. i have done that and it seem working .i measure the voltage at the input pin 2. but my problem is that i cant get the 5v when i put a paper between the LED and the Photo transistor. i tried to reduce the resistance that connect the connector of the photo transistor to 1k.

    i would like to ask if the problem might be because i m using an ordinary led.

    lihle
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,048
    The detector may have a visible light filter on it, and shouldn't respond to a visible LED. But it could be picking up all sorts of other things, like overhead lighting. Try pointing a TV remote control at it and see if you get a response. You could also monitor the receiver with an audio amplifier, so you can hear what's going on instead of relying on a meter. It should make quite a racket when you zap it with a remote.
     
  9. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    I have to agree, I think the common smoke detecotrs that used this technology operated with IR LEDs at a greatly reduced output level.

    Which brings on the question - what does smoke attenuate better? Visible or IR? And what kind of smoke?
     
  10. wayneh

    Expert

    Sep 9, 2010
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    3,048
    I believe smoke is mostly particulate, and the individual particles are much larger than the wavelengths of light or IR. So I don't think the choice of IR is related to wavelength per se. All wavelengths are scattered or blocked more-or-less equally.

    I speculate the choice of IR has more to do with other factors, such as the availability of higher powers IR diodes, or the signal-to-noise issues. Just a speculation.
     
  11. lihle

    Thread Starter Active Member

    Apr 12, 2009
    83
    3
    i have tried everything regarding my smoke detector circuit but its not working. what i have done is to salvage the IR led usually use in radio remote. i then substituted the ordinary led but still is not working.

    what can i do.
     
  12. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,048
    Take a deep breath, divide and conquer. You need to validate each function step by step, eg. make sure the LED is lit, make sure the receiver is responsive, and so on. Check connections for continuity and lack of shorts. Measure voltages at all nodes to verify they make sense, report them here if you have questions.
     
  13. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Do you have an ordinary opto-interrupter? An old mouse (optical or not) will have them. These are IR and ready to go.

    The IR led and the transistor are matched, so that will reduce the number of problems you are having.

    Changing the LED level will adjust the sensitivity to the smoke.
     
  14. lihle

    Thread Starter Active Member

    Apr 12, 2009
    83
    3
    Thanks i will try to get the optical mouse. as for the photo interruper i dont have it yet.

    lihle
     
  15. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Find a working smoke detector, take it apart and see what thy're using?
     
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