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Help with beam sensor please ...

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Active Member

I wonder if anyone here has any experience with those
little hand held laser toys that project a thin beam,
for quite a distance.

I have one that is intended as a toy to play with cats
they chase the spot around. Its only small, it takes
three tiny cells, giving four and a half volts.

I'm thinking of trying to use it as a 'break the beam'
type intruder sensor.

But i don't know if it will run continuously or not.
Maybe i could reduce battery voltage to increase its
length of life?
Maybe running it on reduced voltage could de-crease
its life expectancy ?
Maybe i could pulse the supply with short on mark/space
time ratio high frequency pulses ?
Maybe i could expect 5 years as it stands ... ?

I have no experience of these at all,
any advice would be welcome.

Regards, John :)
I bought a few of those laser pointers awhile ago at a local hardware store for like $1.99 each. Heres a pic of the ones i have:

**broken link removed**

I tried taking one appart to see if it could be used in a project some how but i didnt go much farther than just sawing it in half with a hacksaw to get access to the tiny board inside. The board was soldered to the inside of the tube so it would be very difficult to work with.

There really isnt much on the board just a bare infrared emitter and a small pushbutton. So it appears the infrared diode is driven directly from 3 tiny watch type button cell batteries whenever the button is pressed.

But dont let their small size fool you, the laser pointers ive got have a range of about a quarter mile, so the beam is quite intense. So i must warn anyone working with these that this beam could easily cause serious eye damage. Dont do something stupid like look directly into the beam.

I was talking to someone who had also tried hacking one of these and he said the boards were too delicate to work with. I havent done any work with the board myself to confirm that, but looking at its small size that could be the case.

But to answer your question , i would say a pulsed laser beam would extend the life of both the batteries and the IR laser diode. To use them as part of a "break-the-beam" detector i beleive that would work , you could use an ordinary IR detector as the receiver. The hardest part would be getting access to the Laser pointer's board to solder some wires to it. But i think it can be done.
Hi nettron1000,

Yes they look just like the little units in the toy i have.
Even the batteries look the same.
I don't have a digicam, else i would send a pic, this one is
shaped like a plastic mouse, its a toy for a cat.

I was thinking of driving the unit with pulses, on for a
short time and off for about ten times as long.

Maybe at about 450Kc/s, i think thats typical I.F. frequency,
i should be able to use a row of I.F.s out of a small pocket
radio to pick out the signal.

Maybe that degree of sensitivity wont be needed, i dunno.

Maybe the majority of its life expectancy is used up in the
start up of the laser ? Maybe under running could shorten its
life ?

If they sell replacement heads as part of the package, should
that tell me something ? Maybe they fail often? or unexpectedly?

I don't know what to do for the best.

So i am going to run this at about 3 volts, which is a bit lower
than the three cells at 4.2 volts roughly, and i am going to see
if i need to modulate it.

Its intended to be run from inside inside the house, out through
a window, and to travel round the perimeter using three or four
mirrors back to a receiver, probably at the start point.

As you say the beam carries well, with not much attenuation for a
good distance, but i expect some deterioration from the mirrors.

It may also be possible to detect the presence of the beam, if
modulated, on the mirror surface with a sensor placed closely.
That way, an interruption could be tied to a particular section
of beam. This occurred to me after playing around with it, i
noticed that when it was aimed at a mirror, the beam did reflect
quite happily to the place the mirror put it, i noticed that
the spot from the beam was also clearly visible on the surface
of the mirror !

This obviously uses some of the strength from the beam, i assume
its just surface imperfections on the glass.

But whatever the reason, it might be possible to detect it with
a sensor placed closely to the spot on the mirror. That would give
me an indication of which section of beam was interrupted.

It may or may not need the extra gain of an I.F. strip, i don't
know yet, i have some experimenting to do !

Any suggestions you may have would be welcome,
Cheers, John :)

Just for you, heres a little 'aide memoir' to spell believe,
a lie
Cheers, John :)
Just in case i wasn't very clear about breaking
the beam into sections,

This is the sort of thing that i have in mind
as 'intermediate' sensors.

Don't know for sure if this arrangement will work,
or function as expected, but i think its workable.


  • sensor.jpg
    9.1 KB · Views: 467
Yep looks fine to me, clever idea. It doesnt take much reflected infrared light for detection by a IR sensor.

Have you experimented with this yet ?
not yet, but it wont be long, cos i want this fixed up.

I will let you know how i get on.

John :)
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