Ir Light Or laser for door alarm?

[Ian]

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?

[XynxNet]

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.

[Psi]

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.

[FreeThinker]

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

[Ian]

how powerful of a IR led do i wont?

[Zero999]

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.

[Neganur]

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.

[Ian]

thanks