I thought the walkway lights were ready to install. On the test bench, everything worked fine. But every time we assembled the lights, the motion sensors stopped reacting to motion. For the past few days, we assumed something was getting screwed up in the sensitivity tuning of the PIR sensor. But pommm pondered on Monday whether it could be the acrylic (plexiglass) filtering out the medium infrared light that the PIR sensor uses to sense motion.

So pommm and I ran a quick test with a PIR motion sensor and light controller board mounted in a “test bench” layout on a flat piece of acrylic. (Shown in the image above.)

Testing it in the normal exposed configuration worked pretty well. Moving around caused the normally 1.8V signal to drop dramatically, which is how the microcontroller reads the motion event.

So we dropped the acrylic tube that sits around the lights over the motion sensor and tried again:

You can see on the oscilloscope that despite us moving around, there was no noticeable dip in the signal.

Conclusion: we can’t have the motion sensors inside the acrylic light compartment. Now we are looking to source some small black plastic boxes to drill a hole in and mount the PIR motion sensors so that the white plastic lens bulb can stick out. (Probably with a little bit of silicone sealant to weatherproof the edge of the hole.)

When a material is clear for visible light, you sometimes have to remind yourself that it can be completely opaque to other wavelengths.