Human

This is a very basic infrared detector/emitter circuit. One major downside of this circuit, is that ambient infrared light will interfere with its detecting obstacles...

This Design Idea involves a low-power motion-detector circuit that operates from battery power for extended times. Part of the design includes a solenoid-operated trip mechanism that triggers whenever it detects motion. The drive circuit for the solenoid in the original design worked fine as long as the battery was fresh but failed as the battery got into the middle of its life, even though sufficient energy remained in the battery to operate the solenoid...

These ultrasonic circuits are all quite old: my notes date them at mid-70s so they don`t use ICs. Nevertheless there are several places where an op-amp would possibly simplify things. Despite their age I hope they are of interest: certainly basic principles don`t change...