Author Topic: Infrared Beam Sensor  (Read 2865 times)

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Offline krichTopic starter

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Infrared Beam Sensor
« on: September 08, 2008, 06:57:40 PM »
I'm trying to build a sensor which can detect when an object passes through an infrared beam.  I have the sensor built and it works well.  I have a QED222 IR LED on one side with a QSC112 IR Phototransistor on the other.  They are about 3" apart, currently.  My problem is that for my application, I need to extend the beam to about 8".  When I do this, the sensor stops working.  Checking the voltage through the transistor, it's not getting saturated by the IR from the LED.  It appears that I get about 1/2 Vcc at 5", 1/20 Vcc at 8".  Looks like a logorithmic decay there.  I'm already running the IR LED at the maximum 100mA rating, so I'm not sure where to go from here without getting into much more complicated modulation circuitry.  The LED can be run at max 1.5A w/ a 1% duty cycle.

I am using the QED222 and QSC112 because they are what I was able to find at my local surplus shop.  I'm concerned that these are not the most ideal components.  The QSC112 has a 4 degree detection angle.  That seems a bit small to me, but I don't have a lot of experience with such things, so it might be fine.  Also, the QED222 has a 40 degree illumination angle.  That seems a bit wide for a "beam" application.

The project is a beam sensor that, when interrupted, triggers a monostable 555.  The 555 Vout triggers a transistor which then turns on a series of white LEDs to illuminate an area.  The lights stay on an adjustable amount of time (via a pot on R1 of the 555).  I'll post a schematic, if possible.  I'm pretty sure I've got the right design, though.  I suspect my parts are letting me down now.

I'm hoping that swapping the LED out for something more...directional...will work.  Any suggestions?


Edit:  I got the schematic posted.  I just noticed that there's no resistor between the 555 and the 2n3904.  I added one on my current PCB.  It was a mistake and I haven't made it back to Eagle to fix it.
« Last Edit: September 08, 2008, 07:00:21 PM by krich »

Offline bulkhead

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Re: Infrared Beam Sensor
« Reply #1 on: September 09, 2008, 12:08:18 AM »
Use multiple IR LED's?  Exceed the max rating slightly?  Use a comparator to create a voltage trigger at 1/20 Vcc (or as high as you can get it)?  Search Digikey for narrow beam IR LEDs?

Offline izua

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Re: Infrared Beam Sensor
« Reply #2 on: September 09, 2008, 01:34:45 AM »
Remember one thing: the way visibile light from a led spreads, the same way IR light from an IR led spreads. So you don't really have a beam, but a spot with a lot of reflections.
But there are other sources of directional IR too. The really cheap pointer lasers on the market, for example, emit over 1/3 of their power as IR. You could take the electronics out of one, and focus them on your receiver/detector
« Last Edit: September 09, 2008, 01:36:14 AM by izua »
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Offline Admin

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Re: Infrared Beam Sensor
« Reply #3 on: September 15, 2008, 03:59:10 PM »
The datasheets will tell you the range and light angle.


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