I made a sleeve for a weightlifting belt, that contains an infrared proximity sensor on the outside, and a motor on the inside. The motor is triggered to vibrate when the proximity sensor senses the safety bars, which would be set at a an appropriate height to match the right squat depth for the lifter. For the demonstration, the green LED on the front is triggered at the same time as the motor.

Like everyone else, if I had more time I would work to make the belt more aesthetically pleasing, however I do like the way the wires look. I also would want to make it more reliable, and work out a way for the sensor to look for the ground, instead of the safety bars. Finally, I would think about incorporating a different sensor, such as an accelerator or flex sensor.