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Low power, focusable dot

Shpaget

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Hi guys,
Sorry to do this this way and have my first post bother you with my problems, but I figured you'd know the stuff.

I'm looking for a cheapish (< $20), low power (< 5mW) red (or any other colour, really, as long as I can find a receiving phototransistor that is sensitive to that wavelength) module that can nicely focus on a target that is 10-15 meters away. I'm looking for sub 10 mm dispersion, preferably around 5 mm circle. I have some cheap modules from Deal Extreme, but after 5 m they are useless and throw the light around producing an ugly halo around an elliptical spot. I would like to reduce this halo as much as possible since it is illuminating my surrounding and it bothers me.
The purpose is to make a trip wire system that would protect a model railroad layout from sticky fingers. Since the layout simulates day and night cycle, during the "night" the laser halo is visible around the receiving phototransistor and doesn't look pretty. I am mitigating the issue with a duty cycle that basically keeps the laser turned off 90% of the time, but it's still not enough.
I've found this:
Buy Best 1225 1mW 635nm Red Collimator Laser Module - Black
But to be honest, the only reason I consider this to be any better than what I have is the price tag.
In specification they say:
Spot Mode (Spot size) Min. diameter: <= 1.0mm; 18mm at 15 meter
I'm not sure I understand what that means exactly, since it's supposed to be a focusable laser.
I would appreciate any input on the matter and would be grateful for a suggestion on what might fill my needs.

Thanks and best wishes from Croatia!
 



KrowBar

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Why don't you use IR if this is for a security system? No visible halo or dot in that case.
 

Shpaget

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Thanks for your reply.

There is an IR system controlling road vehicles in close proximity to the location of lasers. I wouldn't like the lasers to cause interference or completely overpower the control signals if there is some scattering or reflection.
The system works on 900 nm, so I suppose I could use a 800 nm laser without too much interference since the receivers are significantly less sensitive (roughly 50%) in that range.

All that being said, I don't mind visitors knowing there is a laser tripwire in place. I much prefer a deterrence method than catching them in the act. Also having a visible laser offers me the ability to easily check if it is working properly and indeed turns off when the beam is broken to protect the inquisitive eyes (regular working method is to have the laser turned on for a few microseconds and check the phototransistor if the beam made contact, if yes, turn it off and check again later; if not turn it off for couple of seconds and sound the alarm). IR requires a camera for checking and calibrating.

In case I decide on IR, do you have a recommendation for an 800 nm (or shorter) IR module that can focus that far?
 
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