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• Project Introduction

  Nathan Kerns • 12/14/2020 at 01:37 • 1 comment

  I started this project only a week or so ago, after seeing this video about the concept. I started designing a circuit to replicate the one in the video, using a transimpedance amplifier to process the signal and maybe another op amp to amplify the signal a bit. The problem I had was trying to find an appropriate laser diode, one with the photodiode built in. There were a few on Digikey, but mostly in IR, which wasn't a great option since the beam wouldn't be visible. That's a problem for safety, makes it harder to work with, and simply doesn't look as cool. Thorlabs had a lot more options, and this one looked best to me. Still, it's about $15, which is not expensive but also not cheap for a single component. I just happened to be using some cheap laser diodes for another project, and noticed that they had a little gap in the back that allowed some of the light through. I could attach a photosensor there and have something that's functionally the same, though probably not as precise.

  I started off with a photoresistor, since that's all I had on hand, and hot glued it to the back. The entire assembly was then wrapped in electrical tape to try to prevent any light from getting in and affecting the results. This prototype worked to an extent, producing a signal that was mostly noise, but appeared to have some evidence of interference. The signal was way too noisy to be of any use, though. The problem seemed to be with the choice of sensor. The standard CdS based photoresistor was extremely slow, having a rise time measured in tens of milliseconds when the signal I was trying to measure was in the tens of microseconds. It would not be good enough for anything beyond a proof of concept.

  The first prototype I made. A little too much hot glue was used, causing the tape to not create a good seal against light.
  

  The test setup. The laser points at a piece of reflective tape, attached to a speaker outputing a low frequency sine wave at a very low amplitude (imperceptible by sound or touch).
  

  Two samples of the signal from the first prototype. It's not very clean, but the interference is definitely there, especially in the second image.

  Two other types of sensors were available to replace the photoresistor: phototransistors, and photodiodes. There's a few differences between the two, the most important being that photodiodes have a faster response time and more resources available online detailing their use. But the both work in about the same way, turning photons that hit them into current. I'll admit, I didn't consider any of this when I bought some. I just got the cheapest pack on amazon that had 2 day shipping, which turned out to be phototransistors (but where advertised as photodiodes - one of the product reviews pointed this out). When those arrived, I made pretty much the same thing as before, but I had to file down a notch in the phototransistor to get it to fit better. I used a lot less hot glue this time, and managed to create a...

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