Today I learned that one of the first wireless remote controls was ULTRASONIC and BATTERY-FREE. A tiny hammer inside the remote would strike one of four aluminum plates, generating ultrasound that was picked up by sensors in the television.
They couldn’t use radio frequency because they hadn’t figured out how to pair devices with specific televisions. Integrated circuits and microprocessors did not appear until more than a decade later. The limited range of ultrasound addressed this issue, a problem that was solved in later products using infrared radiation.
Hi, I'm Rauf Aliev. 