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I'm thinking about making my own laser security system (think mission impossible). So I was thinking I'd use a laser diode to shine a beam across a doorway into a light dependent resistor that's on the high side of a voltage divider and look for that signal to go low and raise an alarm if it does.

So I've done some quick research for parts and I'm surprised at how expensive laser diodes seem to be: Do I really need to use a "laser diode" to satisfy my requirement to shine focused light across say a 3 foot gap, or is there some other component that is cheaper and gets the same effect?

Does my LDR concept on the "receiver" side of the gap make sense or is there a better way? I guess my biggest concern is that it won't respond (resistance won't rise) fast enough when the beam is broken and I'll fail to pick it up as a logic low. If that's the case I imagine what I'll want instead is something like a photo-transistor rather than an LDR.

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the first question is a shopping question. SE sites do not allow this. If you would like to ask how you should go about the problem, that would make a great question. – Kortuk♦ Mar 20 '11 at 8:18
@Kortuk, ok I've modified the question to remove the shopping aspect – vicatcu Mar 20 '11 at 14:09
I meant to tell you to just open a new one, only one answer so far, so no serious harm. – Kortuk♦ Mar 20 '11 at 16:08

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Hey, yes its possible and relatively easy, a simple red laser from a dollar store will work fine. Here is a great tutorial on how to do that: http://www.youtube.com/watch?v=t0FTzUhdg3w

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You don't need a laser at all, unless you actually want to have a visible beam. If you don't mind it being invisible, then just use an Infrared LED and a matching photodiode.

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i think having it be visible adds to the coolness-factor somehow though :) – vicatcu Mar 20 '11 at 23:54
@vicatcu but a red laser won't be visible (at least not the beam), unless you live in a foggy place :) – clabacchio♦ Mar 15 '12 at 8:30
@clabacchio true but you can see it when it strikes a surface, and you could create a fog to "reveal" it mission impossible style... – vicatcu Mar 15 '12 at 14:52

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