proximity sensor application

Discussion in 'The Projects Forum' started by varuntejavarma, Apr 16, 2014.

  1. varuntejavarma

    Thread Starter Member

    Apr 16, 2014
    36
    1
    Hello,

    I have to make a low cost proximity sensor for an automatic aroma diffuser. It should release the spray when someone passes it.

    My Idea: I dont want to use a microcontroller as it complexes the circuit. I want to let the circuit simple so I am thinking of using an Op amp by using its voltage difference option to detect the change in voltage output of the photodiode and activate the trigger. I also want it to stop releasing the spray for 15 min after the first release even if someone passes by. what can i use for this.

    Please let me know if my idea is even correct and suggest me some options.

    Cheers,
    Varun
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    The Completed Projects Forum is for Completed Projects only. It is meant to allow members to show plans for projects they built so other members can duplicate them if desired. New threads are also automatically moderated per Moderator review for this reason. Your thread does not belong in this forum, and was moved here.
     
  3. ScottWang

    Moderator

    Aug 23, 2012
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    Using IR sensor reflection or interruption to detect the human passes or you can also using PIR sensor to do the detection.
     
  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,399
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    Use flip flops for counter, 555 chip for timer.

    Diode output goes to comparator (op amp), that activates the spray and start the 15 minutes timer, flip flop counter counts to 15 at 1 minute intervals (need 4 bits, though you probably need to skip 0000, just do 0001 to 1111), at 15 you reset. Need some logic gate somewhere to ignore inputs from diode during the 15 minutes count.
     
    varuntejavarma likes this.
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Break down your project into functional blocks, work out a circuit for each, then work out the connecting logic so the parts interact to perform the function you want.

    A simple comparator with a light-dependent resistor (LDR) like a cadmium sulfide cell will work as a moving people detector. PIR is better, but more expensive.

    A 15 minute timer is tough to do with a simple RC stage, so consider a digital circuit for this. A CD4060 is an oscillator and counter all in one, and works well for this application. In operation, it counts up until one of its outputs inhibits itself. This inhibit signal can be used to enable (not activate) the spray circuit. The 4060 reset input is driven by the people detector. In this case, the reset is acting as the timer trigger.

    As I write this it is getting complex to describe. The overall logic goes something like this:

    Spray and timer are triggered by people detector.
    Timer inhibits people detector for 15 minutes.
    After timeout, people detector is enabled again.

    The messy part about this is that on power up the circuit will go into the 15 minute inhibit cycle, not be instantly ready to be triggered. Is this ok?

    ak
     
  6. ScottWang

    Moderator

    Aug 23, 2012
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    If a normal LDR should be considering the lightness of the environment, they have different brightness like day time and night time, except the sensor is putting in the closed environment, or you have a special method to solve the problem.
     
  7. varuntejavarma

    Thread Starter Member

    Apr 16, 2014
    36
    1
    Thanks a lot for the replies

    1. I think pir sensor would be expensive and less sensitive. I am thinking to use an ir led emitter and a photodiode to recieve the emitted ir rays.
    2. for a range of upto 1.5 metres. what specifications should i consider for the Ir led and photodiode
    3. to reject the background light and detect only infrared radiation, what measures can be taken

    I am a beginner in designing circuits. But i want to learn. Please excuse me for basic questions
     
  8. inwo

    Well-Known Member

    Nov 7, 2013
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    Two photo diodes connected to op amp, that would only respond to difference in levels.

    Then differentiate the output to respond only to a high rate of change.
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    If you are talking about the IR LED and photodiode as a reflection sensor, be aware that these are very difficult to tune for reliable operation over such a long distance. There are specialty assemblies for this that are used in production lines where the reflectance of the objects is known and repeatable, but not for general purpose walking around detection. An ambient light sensor that responds to passing shadows is reliable, and there are spray products like this already on the market.

    http://www.glade.com/en/products/sprays/sense-and-spray-automatic-freshener

    If you're talking about a light beam, where someone walks through the beam between the LED and diode, that is much more reliable.

    The usual way to make these things immune to background light is to pulse them at some frequency like 10 KHz, and have the photodiode circuit detect this with a frequency conversion circuit and a comparator. Nothing as complex as a radio (no tuning coils), but the same idea.

    ak
     
  10. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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  11. varuntejavarma

    Thread Starter Member

    Apr 16, 2014
    36
    1
    Thanks for the replies. I am making a prototype using PIC mcu at first.

    How can i exclude the surrounding light when using a IR photodiode.
    Does using a Band pass filter solve the problem or complex the circuit.
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Normally you don't; you modulate the IR as stated above. But you could put the diode and/or IR source inside a respective internally-blackened tube to make it somewhat directional.
     
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