One of the first projects on my old blog is a simple posture sensor based on an Attiny85 and an ultrasonic distance sensor. The way it works is very similar to the webcam software. It is attached to a chair and measure the distance to the back of a person's head. If the person slouches, the distance increases and piezo buzzer starts beeping.

http://coretechrobotics.blogspot.de/2014/02/a-very-simple-posture-sensor.html