Here’s a design circuit for a ultrasonic motion detector (or movement detector) circuit. The circuit claimed has a high movement sensitivity. Even air moving (hot air rising, wind blowing) will trigger it when the trimpot is set near the most sensitive position. This is the figure of the circuit;
The transmitter sends out a steady ultrasonic tone at 40 kHz. At this frequency the wavelength is about 6 mm. Any reflected sound is detected by ultrasonic receiver. The signal is then amplified by IC1: A and IC1:B. IC1:A is self biasing via C2 & R5. The time constant of the first amplifier is set to let the 40 kHz signal through. Between the first & second amplifier there is a negative peak detector (diode D1 & R8) which follows the envelope of the 40 kHz signal. If there is no movement the envelope is just a straight line. The time constant of IC1: B is much slower so that it will follow this envelope. All the amplifiers are AC coupled to prevent DC bias problems.
