After getting good readings from the 2nd PCB version, we decided to make a third iteration that would solve all its problems at once!

The list of changes is quite significant:

1. Power source. While LiPo batteries are good and more eco-friendly than disposable ones, they complicate production and shipping. We decided that if one battery can last for 2 years, then a disposable one wouldn’t be such a bad choice - and so we changed it to CR2032.

2. Connectivity. IR LED proved to be unreliable and inconvenient - so we switched to BLE, and it looks like the only way to keep the cost low with it is to use nRF52 chip - namely, nRF52810, so that it would also handle all calculations (thus replacing atmega328). In order to keep power consumption even lower, it works as a non-connectable device, only advertising data once in a while.

3. We reduced the size of the upper part of the board as much as possible - basically it’s the size of the battery plus a small PCB antenna (34*30.2 mm).

4. We made the probe thinner and more rounded at the lower edge to reduce risk of damage to the plant’s roots when uPlant is inserted into the soil. They're now 6 mm wide and 1.2 mm thick (same thickness as the main PCB).

5. We made probes removable and of several lengths for different sizes of pots (so far we have made two lengths to test them - 60 mm and 120 mm).

Here’s some fre$h green v3 PCBs - we got them just a few days ago, and made some green 3D-printed cases to match the look: