I recently had to build an infrared close proximity sensor that could tell if something was dropped in a parcel delivery drop box. After some searching, I ended up using ... object to detect its close ...

In the real world, missile systems use advanced radars, infrared sensors ... targeting data for the launcher, an Arduino Uno uses an ultrasonic sensor pair mounted atop a servo.

Typically, reflective-type infrared sensors are used for this task. But the circuit in the figure shows an infrared close-object alert ... ripplecarrying binary counter/divider and oscillator.