Martin Uhlmann Finds an Epidemic of Faulty Soil Moisture Sensors — and Comes Up with a Quick Fix

82% of the sensors Uhlmann purchased for his project turned out to be faulty — but, thankfully, there's a quick fix for most.

Gareth Halfacree
11 days agoSensors / Debugging / Garden

Martin Uhlmann, who is hard at work on a smart plant pot project dubbed Flaura, has discovered faulty capacitive soil moisture sensors in the market — but, thankfully, has come up with an easy fix: soldering a single wire or resistor into place.

"I recently ordered a bunch of capacitive soil moisture sensors for a plant care project," Uhlmann explains. "During testing, I noticed that unfortunately some of them were very unresponsive and responded extremely slowly to changes in soil moisture."

Digging into the problem, and comparing the good models with the seemingly faulty versions, Uhlmann quickly honed in on the difference: A change in the board design which added a single via — right in the middle of, and interrupting, a copper trace, leaving part of the sensor entirely disconnected.

If you're using capacitive soil sensors in your project, this video could save you a lot of troubleshooting. (📹: Martin Uhlmann)

"It can be fixed by soldering a new 1M Ohm resistor between the analog-out pin and the ground pin of the sensor," Uhlmann explains of the thankfully relatively-simple fix, "or by soldering a cable between the ground pin and the disconnected side of the existing resistor."

In a supporting video, Uhlmann also details the issue with resistive soil moisture sensors — which rapidly corrode if left energised - and two less common faults: A missing voltage regulator and the use of an incorrect timer chip.

More information is available on Uhlmann's Reddit post.

Gareth Halfacree
Freelance journalist, technical author, hacker, tinkerer, erstwhile sysadmin. For hire: freelance@halfacree.co.uk.
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