Building a digital plant watering reminder

A soil moisture sensor and a single LED or buzzer. The sensor sits in the pot, and when the soil dries past a threshold the light comes on to nag you. That is a genuinely useful project buildable in an evening with an Arduino, a sensor, and about ten lines of code. Start there before adding Wi-Fi, apps, or automatic watering, because the basic version already solves the actual problem.

They measure how well the soil conducts, which tracks moisture. Cheap resistive sensors (two prongs) work but corrode within months because current runs through the soil. Capacitive sensors cost a euro or two more, don't corrode, and last far longer, so skip the resistive ones despite their lower price. Even good sensors drift, so calibrate yours by reading it in dry and wet soil and setting your threshold between.

Yes, with an ESP32 instead of a basic Arduino. The ESP32 has Wi-Fi built in, so it can send a notification, post to Home Assistant, or message you when a plant gets thirsty. This is the natural upgrade once the local version works. Build the standalone reminder first, confirm the sensor readings make sense, then add the networking layer on top of a foundation you already trust.

You can, but add it carefully. A small pump and a relay let the system water on its own, which is brilliant for holidays but risky if it misjudges and floods or empties the reservoir into a dry pot. I would run a notification-only version for a few weeks first to confirm the moisture readings are sensible, then add watering with a strict limit on how long the pump can run per cycle as a safeguard.