Break Beam Detector Progress

Jaggermax

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I have been playing around with an alternative to the PIR for game detection in my cam.  In college, I worked with IR emitters and detectors a fair bit so I decided to give it a try to see if it would be feaseable.  

I installed an IR LED emitter in place of the bulb in one of the small Maglites.  I am pulsing the IR LED off and on using a simple 555 astable multivibrator circuit at 50 Khz at 5% duty cycle (to conserve battery life).

For my receiver, I am using a small 3" parabolic dish with the IR detector mounted in the focal point of the dish.  I am using a 555 timer again on the receiver which is tuned to the 50Khz signal.  The receiver has an adjustable sensitiviy on it so you can set how long the beam must be broken before it recognizes it.  I am basically using a circuit similar to one in the back of the Radio Shack sensor handbook.  

When the receiver circuit detects the 50Khz IR signal from the transmitter, the output is low.  If the beam is broken for the specified amount of time, the output from the receiver goes high. With the sensitivity adjustment all of the way up, anything breaking the beam will set it off.  At the lowest sensitivity setting, it takes a break of about 3 seconds to set the receiver off.  

I have been playing around with the transmitter and receiver and have found that I can set the two over 200 feet apart.  This is because I can use the focus on the maglite to get a tight beam.  However, I do not plan on placing it more that 50 feet away.  

The great benifet of this and the reason I am investigating it is that temperature, wind, and rain will not set off false alarms.  Of course the downfall is that now I have two seperate powered units, (the transmitter and the receiver/cam)  alignment is critical, and a broken limb that falls in between the trans and receiver will more or less shut it down.  I will continue to play around with this idea and let you all know how it turns out.  I may find that even with some of the temperature and humidity problems with the PIR, it still works better than a break beam detector.  
 



big tom

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Jaggermax- do you have any updates on the activeIR beam.  I am very interested in what you are up to, but from what I have heard, the power demand of the active beam scares me off.  
 

Archilochus

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Hi Jaggermax,
Can't remember, I might have mentioned this before.........
To really cut down on power you might try using "pulse bursts" from the IR emitter.
Have your 50kHz carrier turned 'on' in bursts lasting ~40 cycles about twice per second.
Demodulate at the reciever and then use a "missing pulse detector" to detect an object breaking the beam.  The missing pulse detector can be adjustable so you can set how long the beam needs to be broken before triggering the camera.  Increase "resolution" by having more bursts per second.
This is the way I've had my active IR detectors working.  With a 5% duty cycle and short pulse bursts the *average* power consumption is ~0.35mA.
>>>>>
One (of several) problem I had with active IR.... as the batteries in the transmitter weaken, the emitted IR intensity falls and the signal at the reciever becomes intermittent - causing repeated triggerings of the cam.  A few solutions I tried that worked:
1) IR Emitter low battery shut-down.  This worked, but didn't use up all the potential available from the batts.
2) IR emitter DC/DC step-up converter with low batt shut-down.  Boosts a low voltage to a regulated higher voltage.  Really gets every drop of available power out of the batts.  Also gives a stable IR output, as the voltage remains constant without the inherent waste of a linear regulator. The magnetics and ICs can be expensive to build an efficient circuit.  These circuits can be "noisy" when home-made - so keep them away from timers, sensors, uControllers, or any other sensitive circuitry.
>>
Another consideration.... These detectors will pick up !anything! - a beetle, leaf, twig, mouse, etc., etc.

Archilochus
 

Jaggermax

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Actually Arc, the circuit you described is exactly what mine does.  I got the circuit originally out of a Radio Shack IR project book by Forrest Mims (or is it Forrest Sims?).  I just tweaked it a little to get better performance and lower power consumptions.  Right now, I am modulating the signal at 50Khz at 10% duty cycle and am drawing about 2ma.  Not bad, but like you said I can get it down a little more (I will probably go down to a 5% duty cycle and CMOS circuitry).  And like you said the trick is to eliminate the bugs, birds and leaves problems.  

I played around with the sensitivity today on the thing and I got it to where falling leaves, a basketball thrown in front of it and heavy spray from a hose would not set it off.  There has to be a two to three second lapse in the signal to set it off. Now I have to figure out how to keep bugs from landing on the lenses and blocking the signal (may design some DEET lenses, lol).  Thanks for the input.  
 

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