Author Topic: Ir Light Or laser for door alarm?  (Read 3739 times)

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Offline Ian

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Ir Light Or laser for door alarm?
« on: April 20, 2011, 01:34:08 pm »
Hi
So i wanted to build a door alarm. So i found this project online. http://www.robotroom.com/Comparator.html It does everything i need. Except i have one problem, do i use Ir light or a laser? I was going to use ir but i cant see IR so that complicates things. But if i use laser then they have to be perfectly lined up, it only has to be a little bit off and i have a false alarm. So i was thinking of using IR light but lining that up would be hard. The reason i was thinking Ir light was because it is not as directional. By the way i am going to be using the thing on my bedroom door, so i dont wont to use read switches. Any suggestions?
 

Offline XynxNet

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Re: Ir Light Or laser for door alarm?
« Reply #1 on: April 20, 2011, 01:59:32 pm »
Just use an IR-LED. Lining up IR is pretty simple because the LEDs don't have a tight fokus.
If you really want to check, you could use a webcam. Most of them are still a little bit sensitive for (near) IR, although they usually have IR filters.
 

Offline Psi

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Re: Ir Light Or laser for door alarm?
« Reply #2 on: April 20, 2011, 02:00:53 pm »
There are many ways to do it

Simplest is probably an IR led at the end of a tube (the tube insures there is only a small angle where the IR light can escape and reach the other side.

Pulse the led at a specific frequency using a 555 or similar

The receiver is just an LDR or photodiode which can also be mounted at the end of a tube to reduce interfearence if needed. You feed the signal through a filter to filter-out all but the frequency your led is pulsing at. Then you can use a diode and capacitor to convert that to a on/off signal for when the beam is interrupted.


« Last Edit: April 20, 2011, 02:03:07 pm by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline FreeThinker

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Re: Ir Light Or laser for door alarm?
« Reply #3 on: April 20, 2011, 02:10:19 pm »
Just use an IR-LED. Lining up IR is pretty simple because the LEDs don't have a tight fokus.
If you really want to check, you could use a webcam. Most of them are still a little bit sensitive for (near) IR, although they usually have IR filters.
You could try a digital camera or your camera on your phone, most will pick up  bit of IR as said above
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Offline Ian

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Re: Ir Light Or laser for door alarm?
« Reply #4 on: April 20, 2011, 03:33:53 pm »
how powerful of a IR led do i wont?
 

Offline Zero999

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Re: Ir Light Or laser for door alarm?
« Reply #5 on: April 20, 2011, 06:31:45 pm »
Hi
So i wanted to build a door alarm. So i found this project online. http://www.robotroom.com/Comparator.html
That article annoys me: there's no schematic and when comparators are unused their inputs should be connected to opposite supply rails, not both to 0V which can cause instability and I don't understand why he didn't just choose an MCU with built-in comparators.
 

Offline Neganur

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Re: Ir Light Or laser for door alarm?
« Reply #6 on: April 20, 2011, 07:55:20 pm »
I used an IR-LED with 180mW/sr in a similar project.

It has an angle of half intensity of +/- 10 degrees, you can see the corresponding light cone at a distance of about 5 inches in this video.

« Last Edit: April 20, 2011, 07:57:32 pm by Neganur »
 

Offline Ian

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Re: Ir Light Or laser for door alarm?
« Reply #7 on: April 20, 2011, 09:49:08 pm »
thanks
 


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