DIY Humidity Sensor - Nov 2023

What if I could make a RH sensor from scratch ?

DESCRIPTION

Since making my spin coater was the first step, now I can put it to better use than applying inks and photoresists.

A friend of mine very kindly offered me a small vial of “expired” PEDOT:PSS, thanks Autumn :)
I do not know much about this substance but she told me that the electrical conductivity of the material changes according to ambiant humidity, and yeah given this for free I was forced to try making my own air moisture sensor.

I am volontarily not reading anything before getting started, I want to explore on my own.
The vial looks bluish, is almost empty, has long been stored at room temp, the viscosity is quite high and the mfg date is 02/2021 so here we go…

Right, I have made a sensor pcb with digits in Eagle, take note of my heat-isolated pads, not the best but I guess that’ll do.
I decided to go with digits in order to get a better surface area near the center, I am not even sure I have enough product to coat the full gap anyways.

The actual PCB after etching

demo_photo

Yeah, it has strong pitting due to my pcb manufacturing technique, this is going to be bad…

The actual PCB after trimming

demo_photo

I trimmed the PCB edges, it now resembles a CPU :)

WARNING: HEAVY PITTING

demo_photo

I checked the gap under a magnifying glass and I see no bridges, the resistance between the two pads at this stage measures as open loop, thus no bridges. The gap itself is not very as consistent as I would like but I really struggled to toner transfer this board. I’ll do better next time.

Masking the pads for spin-coat

demo_photo

I washed the board with acetone to remove any grease or debris before applying the masking tape.
I do not need the pads to be coated but I do not want to have a puddle of product just next to the sensor area, thus I only masked the pads.

Spin coating

demo_photo

After emptying the vial I only got around 0.7ml so I used all of it. Consistency is a bit viscous, like a light oil so I set a low speed, 2k rpm spin coat 0.7ml of solution.
The spin coater top plate was improperly seated and sagged a lot during the process, you can see it in the video. Gone are my dreams of an even coat.
To my surprise, the whole board seems to be coated, but I have a lot of doubts as for the uniformity.
I have no second chance so that will do for now.

Removal of the tape

demo_photo

When looking at this area, we can see the separation between what has been coated and what has not.
I was told that the solution needs to be cured at 130C for 15min.
I put the board into on old oven which has thermostat settings and no precise temp control.
I set th4 for 20min, the oven was preheated beforehand. This oven is in pain, this is the workshop oven, everytime it gets used I feel bad for it, last time it was used I made HDPE plastic sheets from bottle caps…
This process did not alter the color of the product but now it feels dry and does not stick to the glove when touched.

Probing

demo_photo

I have soldered wires to the board using the lowest melting point solder for the shortest amount of time possible(200C, 60Sn/40Pb, rosin flux core). According to my very inaccurate desktop air moisture sensor, here we are at 62%RH which means my sensor has a resistance of 116.8k at 62%RH +/-10% :)
I did not know what to expect in terms of resistance, but seeing a resistance where I had an open loop before is already a big win considering the conditions.

Better probing

demo_photo

I made a basic resistor divider, hooked it up to 5v DC and started testing.

I could not resist testing to see if I would see an effect by breathing near the sensor and saw a small voltage drop, but at some point the measured voltage started increasing. I was unable to get decent measurements because after the increase in output voltage all I got was a much weaker voltage sag and it was increasing !!!
I do not understand fully what happened but the sensor material might be saturated in water ?
What is puzzling is that from my initial testing the resistance of the material was supposed to increase with moisture, here it increased up to a certain point then decreased.

EDIT: I looked it up, turns out PEDOT:PSS film will react to both moisture and heat, although this test would not actually measure moisture exclusively I was definately going to see an effect ^^

I thought that I would wait for a few hours and let it rest, not knowing if heat drying would destroy the sensor.

EDIT: This true to a certain degree, the film could crack but low heat would have sufficed and perhaps fixed my issue ?

AAAANNNNDDDD

I totally forgot about it until a week later, the copper is absolutely destroyed. I knew the PEDOT was acidic (the bottle label says PH 1.5 to 2.5), perhaps the material reacted with the copper and the air moisture ?
EDIT: I have checked, from what think I now understand, PSS is the part that makes the solution acidic, PSS is also the part that is hydrophobic and is the whole reason why this works. PEDOT is the electrically conductive part. The PSS part absorbs water and swells, this physically rearanges the structure spacing the PEDOT making it less conductive in the presence of moisture? Nevertheless, the cured film should not have degraded the sensor this much, I am now left without explaination.

What makes this more mysterious is the fact that the solder which was added after the PEDOT and not touching the film also seems to be damaged for some reason ?
Perhaps some weird reaction with the flux from the solder, perhaps leftover acid from the board etching (for future reference I have used FeCl3 rinsed with demineralized water), perhaps the oven process (the oven has seen all kinds of stuff), idk.
The board was left on the desk in my office, the room humidity is moderate and I have never seen anything like this happen to any of my boards, this is odd.
I guess we’ll never know, and I have now spent much more time than I had hoped for on this side project, perhaps someday I’ll revisit the idea.
I almost thought it was working when I saw the voltage change, I would have loved to see how long the moisture stayed in the sensor, the initial measurement delay and how long the sensor would work in open air, would it eventually fail ?

TLDR: I got impatient and ruined the (badly manufactured) sensor, breathing on the sensor was a bad idea, I had low hopes anyways, oh well.

MANY MANY MANY things are done improperly here, if I had to do it again:

The dead board

demo_photo

“7 days later - PCBs arise from the dead”

The damage does not show uniformly, this means that it is very likely that the coat was not uniform in the first place or perhaps the suspension had started to separate ?