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IR Detection

  1. Aug 17, 2010 #1
    Hey experts! I've designed an infrared detection circuit to drive a motor, but I need the circuit and my math checked. I tried 5Spice, but I can't get my head around it. Circuit image attached.

    Do you know about how far I would be able to detect someone crossing in front of the light? I need 10-20 feet. Even farther would be great!

    Edit: I should also mention that the TIP41C transistors are the correct components, but I am only using what Eagle had in its library for the other components, so they are not necessarily to spec.

    Attached Files:

    Last edited: Aug 17, 2010
  2. jcsd
  3. Aug 17, 2010 #2


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    Staff: Mentor

    You're not really adding any gain to the IR Rx diode -- it would be better to use a photocurrent-to-voltage converter stage with an opamp. Also, for best real-world IR TX-->Rx performance, you should modulate the IR transmission and demodulate at the RX stage, to help reject DC IR and AC Mains (60Hz, 120Hz) interference.

    You might be able to use Passive IR (PIR) sensing instead, if you are just looking for the motion of people / warm objects.
  4. Aug 17, 2010 #3
    Also, by modulating, you can typically drive the emitters at a higher current which can help increase the distance.

    One of the closest real-world examples I could give you for the detection of an object detection 10-20Ft away would be the circuitry associated with the safety beam on a garage door system.

    I'm with Berkeman - PIR sensors for motion usually have a pretty solid performance. Just make sure you select the appropriate device with the correct lens (distance and detection area).
  5. Aug 17, 2010 #4
    Alright, so I decided that PIR was indeed the option most suitable for my application. I have revised my circuit to be compatible with the AMN33111 PIR sensor

    However, this sensor is expensive! Does anybody know of a similar product that is quite a bit cheaper? xD

    Attached Files:

  6. Aug 17, 2010 #5


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    Staff: Mentor

    What is the function of Q1? Doesn't seem like it's needed anymore...
  7. Aug 17, 2010 #6
    I don't want to burn out the switch. In my experience, most switches I've used only handle about .5A, which is just about how much my load draws, so it was iffy.

    On the other hand, digikey search brings up small, cheap, effective slide switch that handles 6A @ 120VAC, (http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=CKN9891-ND), so I guess I don't really need it.

    Edit: Updated diagram.

    Attached Files:

    Last edited: Aug 18, 2010
  8. Aug 18, 2010 #7
    Some general notes:

    1. At power-up, the output of the PIR is typically not stable, so your motor may run unexpectedly at first.

    2. I'm pretty sure you will need some bypass capacitance to stabilize the output of the regulator. In addition, a bypass cap for the sensor wouldn't hurt.
  9. Aug 19, 2010 #8
    Yea, I was aware of this, and have corrected for it in the mechanical design rather than the electronic design. However, if there's an easy way to fix that in the electronics, I'd love to hear about it :)

    Added :) Any idea on the capacitor sizing I'll need?

    The sensor only outputs 4.5 volts and .1mV, so I had to switch to a darlington transistor with high (10k) hFe to get appropriate current to power the solenoid (load).

    Attached Files:

  10. Aug 20, 2010 #9
    The required bypass capacitance for the sensor is listed on the data sheet (application circuit).

    The bypass capacitance for the regulator would be listed on its data sheet as well.

    One other thing: the pull-down on the base of the transistor should go to ground. As it is drawn, (attached to the motor), its bias will vary between the motor on/motor off states.
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