When I started this project, it was critical for me to find a good way to detect the state of the seat. Several similar products got bad reviews and were frustrating to users because their sensing was not reliable. I tried many different types of sensors and am still testing various options trying to come up with even better sensing.
For a while my preferred method was a capacitive proximity sensor made of one sheet of conductive material (aluminum foil). Inspired by “Sit.Up”, this design measures the change in capacitance of the sheet caused by a nearby person.
This sensor is easy to build and needs only a single resistor to connect to a micro-controller. Unfortunately, it has some noise which is significant relative to the difference between idle and active states and in some cases it may pose a problem.
Currently, I switched to using a similar, but different design. My preferred method now is a capacitive pressure sensor. Made of two sheets of conductive material (like a complete capacitor). The latest sensor is more stable, requires only one digital pin and one resistor. It is a bit more complex to build as it needs two sheets of foil instead of one, but IMO it is worth it. I wrote a detailed article explaining how to build and use such a sensor, as well on the theory behind its operation. Please take a look. I am happy to get your feedback.