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I have the following setup... A laser-pointer pointing downwards at an angled 45 degree mirror which is rotating at about 600-2200rpm on a high speed electric motor. As the mirror rotates the laser dot is reflected outwards around the room and moves sideways along the walls of the room in a large circle. I want to be able to detect this laser dot when it passes a point with an optical sensor on the wall but the speed at which the laser moves means it doesn't focus on a photodiode with a circuit long enough to saturate it and turn it on.

The main solutions I've researched so far have been avalanche photodiodes and transimpedance amplifiers to increase the sensitivity and response time however I haven't found many reference circuits built for similar applications; I'm also concerned about other light setting it off but one problem at a time I suppose!

Can anyone recommend an area to research or material that would be worth reading or if possible a circuit solution to the issue that might be of use?

Many thanks for any help offered

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  • \$\begingroup\$ Unfocus the laser a little bit, so the dot will become larger. From my calculation it follows that a dot 1cm in diameter will pass over a point at 2m far away for a duration about 21 millisec at maximum speed. Which is pretty much. \$\endgroup\$ – Eugene Sh. Sep 14 '15 at 19:57
  • \$\begingroup\$ BTW, what is the purpose of such a setup? I am smelling an XY-problem here... \$\endgroup\$ – Eugene Sh. Sep 14 '15 at 20:18
  • \$\begingroup\$ Cheers, the un-focusing the laser idea is good but I'd want to keep that laser precision otherwise I could just use a powerful flash-light instead; the actual purpose started out as few experiments messing around with putting my own tachometer together but now it's kind of expanded into a simple "can this be done" escapade with intentions of using it as a basis perhaps for a high speed laser pulse communication experiment I'd like to try. \$\endgroup\$ – Moilergy Sep 14 '15 at 20:28
  • \$\begingroup\$ Even 1 ms is plenty of time for an amplifier to respond. If you are able to keep the spot size below 1 mm, you're doing pretty good. (Also, you haven't said the distance from the source to the detector, so I'm not sure how Eugene worked out the pulse width vs spot size) \$\endgroup\$ – The Photon Sep 14 '15 at 20:31
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    \$\begingroup\$ See this TI app note for a prototype op-amp transimpedance amplifier circuit. \$\endgroup\$ – The Photon Sep 14 '15 at 20:31
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While I don't know all the design or budgetary constraints of your problem, from the technical point of view it should be very doable. I would find a narrow bandpass optical filter (with the center of the band at the wavelength of the laser) that I would place in front of my optical detector to filter out all the stray light. While APD (or even a PMT) would give you the best results, optical PIN diode (check Hamamatsu diodes) would suffice for your application. You just need to follow it with a good quality amplifier.

As to the research material, I would look up stuff on the design of flow cytometers or other flouresence detection systems. These instruments are able to detect much weaker optical signals at a rate of 10k events/second.

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