Inside a pedestrian crossing human-detector

Not what I was expecting. This is the PCB from inside a pedestrian detector as used on pedestrian crossings.
It uses two fresnel lenses behind a sheet of black infrared filter material to fire out a matrix of 30 infrared dots in the same way as a disco LED moonflower light. It then measures the amount of IR reflected back from each beam and uses a PIC microcontroller to analyse any fluctuations caused by someone moving in front of the unit.

On further investigation it looks as though the pairs of 1 ohm resistors in parallel may actually be current sense resistors to regulate the current through each set of LEDs to around 1A. It also looks as though there is a resistor and capacitor in series with each microcontroller control line for the LEDs to make sure that a glitch or crash can't leave the LEDs powered continuously. Although each set of three LEDs has its own current regulator (for wider voltage-range use?) the microcontroller only uses five lines to switch them as groups of six.

I'm not sure what condition causes the relay to operate as I didn't get it to click a single time while I was playing with the unit. It may be faulty or just require a specific sequence of events to indicate a level of activity or if there's been a gap in the traffic and someone has crossed early.

The software in the PIC microcontroller must be quite complex to analyse fluctuations in ambient light and when something is causing false triggering of one beam repeatedly. I'd guess it also processes the input to make a relay trigger decision based on pedestrian activity and time.

The unit is designed to run on 24V AC or DC and the relay looks like it may be providing a volt free contact to the control system. The other relay position is probably for another function like fault indication or maybe to indicate other detected conditions like a high number of pedestrians.

The inside of the case had a barrier and sealing foam to prevent light from bouncing across internally and affecting sensitivity. The same barrier technique is used in infrared distance and obstacle sensors.

Oddly, the lower LEDs had a film of translucent residue on them that looked like it had precipitated out from the adhesive light-blocking foam or from moisture condensation. With this type of optical system the clarity of the lens is important. It wiped off easily with isopropanol restoring the clarity to the lenses. The fact it only affected a specific layer of LEDs made me wonder if it was a deliberate dulling spray for the closer range beams. But I think it was just residue.

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