Fixed a dead Blumat Digital Soil Moisture Sensor

[u/hanswurstgmbh]

A friend uses several Blumat Digital sensors in his garden. While his older units lasted years, a few new ones failed after just two weeks. The manufacturer replaced them, and he gave me the defective units to see if they could be fixed.

Initially, I assumed it was just a dead battery, but replacing it didn't work. Here is how the repair went:

1. Disassembly & Diagnosis

I opened the outer housing by removing the 3 back screws.

Even with a fresh battery, the display remained completely blank. I removed the remaining 4 screws on the inner frame to access the circuit board.

2. Identifying the Defective Part

Lifting the foam cover on the PCB revealed a blown SMD component. It turned out to be a failed capacitor. A standard 104 ceramic capacitor (100nF) works as a suitable replacement (a least that's what Ai said).

I picked up a few at a local electronics shop for a couple of Euros.

3. Repair

1. Scraped off the protective coating around the damaged SMD component and desoldered it.
2. Traced the circuit with a multimeter to verify the connections.
3. The original traces tied directly to GND and RST, providing solid pads to solder the new through-hole ceramic capacitor into place.

Result

After reassembling the housing and reinserting the battery, the screen turned back on immediately.

If you have a broken Blumat Digital unit with a blank screen, it might just be this capacitor rather than a total loss. A fix is also a lot cheaper than a new unit (50 cents vs 45€)

TL;DR: Blumat sensor died after 2 weeks due to a blown SMD capacitor. Replaced it with a 100nF (104) ceramic capacitor and it works again.

[u/crush-stone]

How did you know which capacitor to be used as replacement? Where to solder it?

[u/hanswurstgmbh → u/crush-stone]

I asked Gemini with a picture and the answer was that: C3 Capacitance: On microcontrollers (in this case, likely a low-power MCU in a QFP32 package), a pair like C3/C4 located right next to the power pins almost always serves as a decoupling/decoupling capacitor for the supply voltage (V_{DD} to GND) or as a filter for the internal voltage regulator/VREF. Standard values here are 100 nF (VCC decoupling) or 1 µF to 4.7 µF (voltage regulator buffer, ceramic X7R/X5R, rated for at least 6.3V/10V).

And my question was "can I use something bigger to solder in?" Answer:
If you don't have an SMD capacitor on hand, you are better off using a leaded ceramic capacitor (multilayer ceramic capacitor / ceramic disc capacitor) with 100 nF (usually labeled 104).

So I bought exactly that in my electronics supply shop and found by accident, that the big solder spots (test pads) are connected directly to the condensators. I just continuity tested everything first.

[u/Adventurous_Test_202]

The hero we don't deserve