Stewy uses this to see what's in front of it. The HC-SR04 sends out a 40 kHz ultrasonic pulse and times the echo to calculate distance. It's accurate to about 3mm and works from 2cm out to 400cm.

It mounts inside the head housing using two printed stops, and connects to the head PCB with a 4-pin JST-XH cable. Runs on 5V, draws less than 15mA.