poor man's photobeam break sensor

jeffjohnvol

I'm looking for a way to trigger someone crossing a path without using a PIR sensor since it has to cover a 30-40 foot range.· I've found units that do that, but they are somewhat pricy.

I was thinking about having a 40Khz IR emitter on one end and the sharp receiver on the other.· Does anyone see a flaw in this approach?

If I could do a reflector it owuld be better since I could keep all the electronics on one side, but I wasn't sure if the 40KHz emitter circuit I have would have enough power to reach there and back, or if reflectors actually work on IR.

Thoughts?

Thanks.
Jeff

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kwinn

Most reflectors should work with IR, but now the IR must travel 60'-80' so you need a source with a fairly narrow beam and/or higher power.

jeffjohnvol

I was thinking about getting a high power IR LED and push it through a small section (6 inches?) of PVC painted flat black inside. I know it would spread some, but if the receiver had 6 inches of the same stuff, it may be restrictive enough.

I figure I could get an IR camera to see if the reflector shines back. I built an IR emitter from a kit once (TV B Gone) and we were able to test the emitter by shining it on a webcam which showed on the screen as a regular light, so I figure I could do the same to get it pointed right.

Of course rain could be a concern, but I can handle a few false positives.

I will also need to develop a simple circuit that would go high when the IR signal is broken for half a second. I figure a simple RC circuit and a comparitor.

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End dependance on foreign oil.· www.pickensplan.com

Sandhuamarinder

Man use a laser beam and reciever on same side and reflector on other side.

Not sure what u are saying just had a quick reading so came up with this idea.


I am just a student. I might be wrong.

kwinn

The problem with using an IR emitter with a wide beam is that very little of the IR emitted reaches the sensor, so requires a higher power emitter or more sensitive detector. Use a led with a viewing angle of 20 degrees or less for best results. There are several available with a 20 degree or lower viewing angle.

jeffjohnvol

It (IR laser) would simplify the process by not having to build a ckt such as this: http://www.seattlerobotics.org/guide/images/irschem.gif


But I don't know if the natural IR in the environment from the sun during the day would throw it off.


Post Edited (jeffjohnvol) : 4/14/2010 12:54:17 PM GMT

Spiral_72

Sandhuamarinder said...
Man use a laser beam and reciever on same side and reflector on other side.

Not sure what u are saying just had a quick reading so came up with this idea.


I am just a student. I might be wrong.

Actually, that's a pretty good idea. A laser diode, tiny mirror and a photresistor (or photodiode)..... $10 Cool!

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"puff"...... Smile, there went another one.

jeffjohnvol

I did some googling on IR lasers. One from a DVD player will burn stuff at a distance of about 12 inches. It could make a nice stamp driven bug zapper, while being silent.

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End dependance on foreign oil.· www.pickensplan.com

Spiral_72

It doesn't have to be Ir does it? Just a plain ol' red laser would do and be invisible..... unless it was foggy I suppose, or raining.

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"puff"...... Smile, there went another one.

jeffjohnvol

Yeah, a red laser might become obvious after a while.

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End dependance on foreign oil.· www.pickensplan.com

kwinn

Be very careful when using a laser of any wavelength for this as it could lead to blindness. A narrow angle led and a retro reflector is a much simpler, safer, and less expensive approach. If you insist on using a laser I would recommend trying one of the inexpensive laser pointers. They are low power to reduce the blinding hazard and simple to work with.