I played about with making a robot face. I combined a Ultrasonic Sensor (HC-SR04) and NeoPixel leds. The leds reflect the distance – one pixel per 10 of cm.
I’m wondering if I can do SLAM – Simultaneous Location And Mapping with a single (or a few) very-cheap ultrasonic sensor. Well, the answer is almost certainly no, but I’m very curious to see how far I can get.