I design and build many sensors of my own. Depending on what I “suddenly” need to monitor I will first scour AliExpress or BangGood and if there is not something already rolled, I will build my own.
Using WeMos, custom Arduino code, a number of DS18B20 and a water flow meter I monitor the cold inlet temp, hot water outlet, heating return and heating flow temps as well as cold-water flow. Using MQTT that reports back which OH2 picks up and graphs.
No pics of the board and sensors but here is the result:
Solar Controller - Shed
Using a number of hall-effect current sensors (ACS712 and ACS758) along with a Wemos , a number of relays, a ADC (ADS1115) I measure the PV panel output voltage and current, the battery voltage and charge current as well as the drain current (gives me a net current).
It can also control the inverter, dummy loads and switch over from street supply and batteries when conditions are right.
It’s further connected into the rest of the smart home and activates a “low power mode” when no one is home and running off batteries.
Same flow meters and temp meters on all the plumbing in the bathrooms to monitor and log that.
I have a 2KW set of panels on the house. I want to now measure the voltage (Vmax 500V :D) and the current they put out. From those, and with logging, I can predict better when to run different appliances and how to manage the battery charging to try and ensure they get charge priority over the rest of the house.
I’ve got more ideas than I have time to do. Some practical ones are:
House electrical circuit metering
For fun - lighting detector using a basic AM radio. When the forecast is for a storm and the detector detects a strike it will flicker all the hue lamps. It’ll drive my wife mad
I’ve always enjoyed building my own sensors and measuring everything but never had a single point to use and display it. OH has helped, nay, fueled my addiction