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Old 07-03-2017, 07:54 AM #1
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Default Laser Diode Security System

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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Old 07-03-2017, 08:13 AM #2
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Default Re: Laser Diode Security System

What about using a low powered diode (aka cheap pointer) and photodiode (better sensitivity and response than photoresistor) instead? If you put the photodiode in some thin inside black tube you can reduce other light and only laser will enter.

https://en.wikipedia.org/wiki/Photodiode

Edit: For working security system I would consider IR laser in this design - just because it cannot be seen by eye (or at least cannot be seen well). If someone uses fog or smoke, they might discover the beam. Still there might me much better solutions, but if you want or need laser specifically: If the device is hidden somewhere well, noone might notice it. In addition you might keep circuit working at some narrow light/IR intensity (corresponding to laser), what makes it potentially hard to hack (if someone for example discovers it and try to replace laser by own light/IR source "feeding" the photodiode or put mirored aparatus to bridge the beam, change - even fast - of intensity will activate the alarm).
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Old 07-03-2017, 08:23 PM #3
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Default Re: Laser Diode Security System

I agree with Radim's suggestions. If you're looking for a logic source, the photodiode is a better bet with simple amp circuitry.

If you're not set on DIY, you can source garage door sensors for pretty cheap online, and they've got a lot of the circuitry to help already. Those would work exactly as you want out of the box as well.
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Old 07-03-2017, 09:55 PM #4
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Default Re: Laser Diode Security System

If it was "Mission Impossible" typed, it should be modulated as any old light source will defeat your circuit the way it is designed. In fact if the photo diode is not encased in a tube, ambient light could keep it from working.
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Old 07-03-2017, 10:07 PM #5
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Default Re: Laser Diode Security System

To cover a doorway you can use a Dollar Store $1.00
<5mW Laser pointer. Then use a Photo transistor to
detect the beam.
Both Laser and Photo Transistor should be in a small
black interior tube. Without modulation this would
work quite well. Been there... Done that.

For longer distances a modulated Laser and
detector is required for reliability.

Another good place for Beam Break Detectors is
eBay and Google for DIY circuits.

Jerry
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Old 07-04-2017, 03:30 PM #6
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Default Re: Laser Diode Security System

Quote:
Originally Posted by lasersbee View Post
To cover a doorway you can use a Dollar Store $1.00
<5mW Laser pointer. Then use a Photo transistor to
detect the beam.
Both Laser and Photo Transistor should be in a small
black interior tube. Without modulation this would
work quite well. Been there... Done that.

For longer distances a modulated Laser and
detector is required for reliability.

Another good place for Beam Break Detectors is
eBay and Google for DIY circuits.

Jerry

I'd go with this too... Not sure what LDs OP is looking at, but the diodes required are not expensive...
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