laser based intruder detection

Hello friends
I'd like to have a basic optical intruder detection.
I saw this video that makes this using a cheap laser module and what looks like a LDR.
http://www.metacafe.com/watch/1133002/protect_your_home_with_laserbeams/

To avoid false triggering from daylight what can I use? I have thought about a phototransistor like this

or a plain LDR like this?

I suspect LDR is too slow to detect a quick light interruption.

I saw also photocells used for electric gates but seem quite costly. Any suggestions?

I think that if you use an LDR and put it inside a dark tube of some kind about an inch long (experiment !!) then the light from the laser wil be so much stronger than any ambient light reaching the LDR that you can easily tell the difference. Any change in ambient light will only cause a small change in your analogRead, but if the laser is interrupted a large change will occur.

To avoid false triggering from daylight what can I use?

Replace the LDR with a photodiode, and modulate the beam.
A/C components in daylight are fairly rare, in my experience.

I saw this video that makes this using a cheap laser module and what looks like a LDR.

Yes, that is, it is used in the dark though and would not be very good in daylight.

I am currently using a number of lasers in my project (a mini race car timer track system). I am just using standard (13mm?) LDRs and some 4mw (apparently) laser modules.

I found that if you use a red filter over the LDR (I am using 3mm red acrylic) then there is not enough red light from sunlight or any other light source to engage the circuit. Becuse the laser is all red, it all passes through. You might as well stick it in a tube as well but it is not really necessary.

I suspect LDR is too slow to detect a quick light interruption.

How quick a light interruption are you expecting, unless it is a very quick on-off-on then you should be fine.

If you would like any more info then feel free to ask away. Also if you would like some laser modules then I bought them in bulk so I would be happy to sell a couple to you.

Mowcius

As I already have a red laser keychain and a LDR I should try and see :slight_smile:

However I wonder what's the difference between a photodiode and a phototransistor.
The datasheet says the ones with daylight filter don't see under 700nm wavelength, however red laser is 650nm. I don't know if it emits infrared light in addition to red light. As it's a cheap laser it is I don't think they put any effective IR/UV filter.
Do you think I can buy one of these and try it?

to Mowcius: thank you for the tips. I let you know if I need more lasers

I doubt that the laser you have will emit enough IR light to be used for that purpose. I have tested some that I have for IR light and it is miniscule in comparison to the red light. I would still recommend a red filter...

Mowcius

As it's a cheap laser it is I don't think they put any effective IR/UV filter

A laser, pretty much by definition, is supposed to monochromatic.

hmm, the red ones I have, and the green one, all emitt some IR light

Mowcius

all emitt some IR light

Do they?

Or is it that they all excite the IR detector you are using? Detectors will generally have a wide frequency response where as emitters tend to be very narrow in the range of radiation they produce.

I have tested them with a filter, viewing purple though a camera...

Among other tests...

Mowcius

I have tested them with a filter,

But how do you know the response of the filter?
How do you know the response of the filters in the CCD sensor?

The way to isolate just one frequency from the emitter is to put it through a diffraction grating and isolate just a narrow group of wavelengths.

Only when you have done that can you say something about the response of filters and detectors because your input is known. Without nailing down the input or the detector (output) any result will be ambiguous.

Ok, i'm really not that bothered but just for fun, in january I will borrow a difraction grating to truly find out for you :smiley:

Mowcius

I read cheap Chinese lasers use infrared lasers, like 808nm, than pass it through some crystals to produce red and even green lasers. They are supposes to have a IR filter,but some infrared light manages to escape.

I'll buy a couple of visible light photodiodes, test them and share the results with you!

I think photocells from electric gates are easier to operate, just connect and just works, plus they should be more resilient to daylight. However I have to find a cheap couple of them

If you would like any more info then feel free to ask away. Also if you would like some laser modules then I bought them in bulk so I would be happy to sell a couple to you.

I PM'd you, mowcius.

Yesterday night I tried the red laser with the LDR I already have and has pretty fast response time.
Plus the resistance change was big even with all the lights on in the office, so if I put it into a black box with a hole it should be immune to daylight interference

Yes, as I said. I have found that if you get direct light from a torch/very close lamp etc then it can make a difference but with a red filter over, nothing seems to affect it.

Mowcius