Initially, the sensor was powered from a bulky and expensive power supply. However, now that the device is working, I was trying to move to using a cheap wall-mounted power adapter - smaller, cheaper, and easier to use. However, the sensor is sensitive to power supply noise, so I made an extra stabilizer and added plenty of LC filters.

I didn’t put much effort into it, to be honest. It was just an additional PCB made quickly before placing a PCB order. I used components that I already had, so... the voltage stabilizer I used was... NCP1117. Not the best low-noise chip on the market.

Surprisingly, the sensor was  working - it’s just that the signal was modulated. Instead of a nice sharp spike when a particle was detected, a series of spikes appeared. Good enough, I was thinking; temporarily, this could be filtered in software. These aren’t super fast signals anyway.

Nevertheless, I tried to make it better, so I soldered more 10µF caps in parallel to those already on the PCB. Not better, not worse.

Here I’ll note that different types of capacitors are better suited for filtering different frequencies. Electrolytic capacitors are good for the lowest frequencies. 100nF ceramics are the go-to choice for filtering power lines in digital circuits, and sometimes they are accompanied by 10nF ceramics to filter higher frequencies.

So then I was thinking: maybe add one of them? The 100µF didn’t make a difference, but randomly adding a 470nF at the output solved the problem. Why? I don’t know.

Now I think that I could have just used an oscilloscope, connected it in AC-coupling mode, and tried to see what was wrong — instead of randomly trying to fix things.

In the next revision, instead of NCP1117, I will use TPS7A8701 (much better, superior RMS noise and PSRR characteristics).

To summarize: the power supply part works, but it needs a rework and better chip selection.

The second feature of this auxiliary PCB is a logic level shifter, which is just an open-collector, common-emitter transistor. This was also not the best design.

Most dev boards or cheap logic analyzers require 3.3V as a high state. If they have a dedicated Vcc pin available — great, it will work. But I wanted to use it with cheap clones of logic analyzers (the kind popular on Aliexpress). They don’t have a Vcc pin exposed — just signal pins and GND — so it won’t work out of the box. A 3.3V voltage source is needed. I missed the opportunity to add some kind of cheap, adjustable power supply for this on the PCB.

One more thing I usually hate — but would actually be useful here — is an LED to indicate that the device is on. It would have been really helpful during testing and assembly. In the next revision, I’ll add it, but I’ll also add a jumper in parallel so it can be disabled in production mode. I hate when LEDs stay on at night (even worse — when they blink like on a router! I always put black tape over those).

So, to summarize: this is a weird project log — everything goes wrong, but it kind of works in the end.