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Let's say I have a square. I also have a device that has an emitter of some sort. I need to detect when my device is pointing at my square.

Here's my question: What emitter and receivers can I use to detect like this without anything visible?

Things I've thought about trying:

  • Lasers - No. Clearly visible on object
  • IR - No. Not accurate enough.

Any suggestions would be appreciated.

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  • \$\begingroup\$ Not nearly enough information. How big is the square, what is the range from the pointer to the square, and how far away from the square is the detector? Plus, and this is really important, what exactly do you mean by "pointing at my square" in terms of accuracy? Just exactly how close to exact does the pointer have to be? 1 degree? 2 degrees? 5? And how does this impact the range questions? For instance, if you're 10 feet from a 1 foot square, a 1 mm laser beam (and IR lasers are cheap and accurate) can be anywhere up to +/- 2 1/2 degrees from center and still hit the square. \$\endgroup\$ Commented Nov 24, 2014 at 4:26
  • \$\begingroup\$ The range and size of the square is arbitrary. The accuracy isn't so important, either. \$\endgroup\$
    – user54268
    Commented Nov 24, 2014 at 4:29
  • \$\begingroup\$ You mean, a ten foot square at 400 miles is required? I use those numbers because they are a rough idea of the accuracy required for optical satellite links (~ 10 microradians). I assume you don't need this accuracy. However, since you say IR is not accurate enough, and IR lasers are easy to find, I can't tell just what accuracy you do need. And it's important. When you say IR, do you mean a TV remote? Those have very wide beams, and do so intentionally. \$\endgroup\$ Commented Nov 24, 2014 at 4:35
  • \$\begingroup\$ Yes, I meant an TV remote. Also, the range will be about 5-20 metres. \$\endgroup\$
    – user54268
    Commented Nov 24, 2014 at 12:43
  • \$\begingroup\$ By not so important, I meant that it doesn't need to be very accurate at all. (I need to detect not if it's hitting the center of the square, just if its hitting the square.) \$\endgroup\$
    – user54268
    Commented Nov 24, 2014 at 13:19

1 Answer 1

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What you want to do is possible, but it will take some work.

Start with the emitter. Go on eBay and search for "focusable 980 nm laser". This will get you a compact unit which can be defocussed to produce a roughly 6" spot. You'll need either IR viewing gear to determine the size of the spot at the distance you'll be working at, or you can fix the emitter and use a detector to map the spot size. At any rate, the spot size must be well under the minimum separation distance (which you have specified as 12 inches). What you'll get is a cheap unit, and you may need to add heatsinking to it to allow it to operate for long periods of time without overheating. You will also need to educate yourself on the subject of eye-safe calculations (start here, for instance http://www.laserpointersafety.com/safetyinfo/safetycalcs/ Note that when you defocus the pointer you increase the divergence).

The detectors can be an array of photodiodes connected in parallel. BP104's are cheap (~ 50 cents a pop), and you'll need an array of about 24, connected in a 4 x 6 array on 6 inch spacing. Make sure you get photodiodes which are responsive at 980 nm - not all are. Connect the diodes in parallel and you'll only need one amplifier per array. The diode array can be covered with a piece of IR-transmissive plastic sheet, which will appear glossy black. If you decide you need a smaller dead band around your "square", you can just sharpen the focus of the laser module, but then you need to decrease the spacing of your photodiodes accordingly, which will increase the number you need.

With IR-transmissive plastic in place, you may be in luck, and be able to just do a DC-level detection to detect the beam. If you've got variable background lighting, particularly sunlight, you'll need to do more work. In this case, make sure the laser diode module you get has TTL modulation capability. Run the module at some frequency such as 1 kHz, and use a 1 kHz bandpass filter on the output of your detector amplifier.

If you're still having problems, you need to do something like make a phase-locked loop to pick up the modulated signal, and in extreme cases you may need to make a lock-in amplifier.

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