Laser Perimiter Alarm

[dougy83]

Laser Perimeter Alarm

Hello all,

My mum would like a perimeter alarm system for her farmhouse to notify when someone enters via either driveway. I was thinking of using a remote unit housing a laser diode, oscillator and 3V lithium primary cell, with the laser being driven with 10us pulses every 50ms (average current 22uA for ~10 year operation). The detector will be a simple reverse-biased photodiode w/ AC amplifier.

So, the question is, does anyone know if (in general) cheap chinese laser pointers (<~1mW) can be switched on/off (and actually lase) in 10us, what their forward voltage is, and what current can they handle before destroying themselves? I know the diodes will vary between manufacturers, but was still curious if anyone has had any experience with any.

Total beam distance is ~100m, with a mirror somewhere around halfway to redirect it towards the house; hopefully the beam doesn't diverge too much.

 

[audioguru]

You need something very solid to mount the laser and mirror on because even a big tree sways in the wind.

 

[dougy83]

Thanks for the reply AG. Yes, you're right; I was going to use a steel pipe set in concrete as the mounting points; the house is wooden and may move a bit as the temperature changes, but hopefully a reflective funnel on the detector will account for this.

 

[RMMM]

This is not to tough a job..if it were indoor.

Keeping the optics clean in a lab is hard enough. Imagine rain, dust, bird crap etc.. outdoors

Using a colored laser diode to "aim" with will make things easier.

Using PROPER locking, adjustable, mirror mounts will also help more than you can imagine.

Laser tripwire is what you are going for, correct?

 

[dougy83]

Keeping the optics clean in a lab is hard enough. Imagine rain, dust, bird crap etc.. outdoors

Rain could be a big issue if the beam is deflected (or absorbed?) too much. As for knowing when to clean dust & cobwebs from it won't be an issue as it will cause an alarm. I think it'll be pretty safe from bird crap unless the birds spray horizontally?

Using a colored laser diode to "aim" with will make things easier.

I'll be using a standard red laser; during normal operation the duty cycle will be ~0.05% which will be impossible to see so I'll have a switch (reed) to increase the duty to between 50-100% for lining up.

Using PROPER locking, adjustable, mirror mounts will also help more than you can imagine. Laser tripwire is what you are going for, correct?

I was going to use a car mirror as it's adjustable (although easily moved by animals); or maybe something smaller (for that reason). Yes it's a 'tripwire' type arrangement.

 

[RMMM]

I was mentioning using the "visible" laser for aiming. For operating mode, as invisible as you can is best.

Laser diodes can be operated at 1uS pulses, no problem.

When throwing cheap and Chinese into the mix... It is a crap shoot.

And you car mirror will not help much. They are usually coated, and not the good kind

They reduce reflected light to try to prevent flash blinding the drivers.

If it is an older than 1985 or so.. You may be ok from the coating standpoint, but you will scatter much of your beam.

Getting 100m out of a sub 1mW diode will be tough without proper optics.

 

[dougy83]

I was mentioning using the "visible" laser for aiming. For operating mode, as invisible as you can is best.

Thanks. I think 0.05% of 1mW will be hard to see, except maybe in total darkness.

Laser diodes can be operated at 1uS pulses, no problem.

Good - that's what I was after... I just hope that generalisation extends to the cheap ones - I guess it should, it's just that I've read some can't be modulated (sam's Faq) - but maybe that's due to filter capacitors the driving circuit (the pointers I've looked at have only a series resistor).

throwing cheap and Chinese into the mix... It is a crap shoot.

Is that you quantitative opinion?

 

[RMMM]

Well, The last dozen I bought, 2 were DOA.

The were driven with one 9v and a resistor. With in the first hour, 8 more were dead.

The 2 that still work (two months later) are clearly different. Lead color, beam convergence, and casing.

The other 10 felt more like a soda can than a nice solid "heat-sink-ready" body.

I have had very good Chinese diodes that have lasted me a good long time.
Same with German and USA.

The only diodes I get that have had plastic lenses are the "what a good price" Chinese ones.

 

[ericgibbs]

hi Dougy,
You stated laser 'pointers' in your first posting, all of the 'pointers' I have used have been a 'soft' start and where not suitable for fast pulsing.

Obviously a laser diode can be pulsed on/off at tens of nanosecs rates if required.
My laser ranger used a 904nm laser diode pulsed on for 25nsec at 400pps and a matching 904nm receiver diode with inbuilt daylight filtering.

A ZTX651 used in avalanche mode was used to drive the laser diode.

Regarding the reflector, one problem outdoors is condensation on the mirror face.
Snow [if you get any] and 'mist' are bigger problems than rain.

For the beam path length I would ensure that you have enough height clearance for animals to pass underneath also the broken beam detector can cope with bird and moths flying thru the beam

 

[bryan1]

As you stated it's for the driveway whynot just use infrared led's like the ones used in shop doors. Proper sheilding and decent aiming will provide an invisible beam then when the beam does break a second infared beam can go to the house and a simple circuit could power a piezo for alerting of entry. No problems of trying to drive elcheapo lasers that won't last....

Regards Bryan



EDIT:- Eric if it ever starts snowing in Brisbane then one will know the new ice age has started.......

 

[dougy83]

The 2 that still work (two months later) are clearly different. Lead color, beam convergence, and casing.

Sounds like you don't have a lot of luck with lasers.. I was hoping to get a bit >2 months runtime out of mine.

all of the 'pointers' I have used have been a 'soft' start and where not suitable for fast pulsing.

Do these pointers have fancy current regulators? Or is it a function of the diode?

Obviously a laser diode can be pulsed on/off at tens of nanosecs rates if required.

1us turn on delay is better than required for my application, but if the pointers are no good I'll have a look at getting a real diode; they're available for around $10 from what I saw.

Regarding the reflector, one problem outdoors is condensation on the mirror face. Snow [if you get any] and 'mist' are bigger problems than rain.

Snow and mist are generally quite rare where it's to be installed (in the subtropics) and I'm thinking the use of a visible emitter will have less trouble with mist than IR - but yes, it's very possible it will interfere if mist does occur.

As you stated it's for the driveway whynot just use infrared led's like the ones used in shop doors.

Thanks for the reply. I would think this would use a fair bit more power than I've budgeted for; and my old eyes have trouble seeing IR either to focus it or to direct it (I know a camera can be used, but it's a little bit of a hassle).

 

[ericgibbs]

hi,
The laser pointers I used were potted and were able to operate over a range of supply voltages, so I suspect they had an internal regulator.

If you are choosing a particular radiation frequency for the laser diode there are graphs on the web showing the absorption and windows for transmission thru air.

Sam's Laser FAQ - Diode Lasers

I did find some info on pointers on the web, links attached, the last link is for hacking a green pointer.

Laser Pointer Forums - Discuss Lasers & Laser Pointers

laser pointer diode - Google Search

EDIT: added another link.
https://members.misty.com/don/laserdon.html

 

[dougy83]

Thanks Eric. Don's page seemed quite helpful, I wasn't aware that the margin between the lasing & diode destruction thresholds was so narrow.