Ultrasonic Proximity Sensors

Ultrasonic waves are reflected by objects and can detect approaching objects. Compared with infrared sensors, ultrasonic sensors are less likely to malfunction in rainy days and dusty environments, and can pass through transparent plastic films.

The ultrasonic sensor uses MA40S4S (transmitter) and MA40S4R (receiver) produced by Murata Manufacturing of Japan. The transmitting circuit is connected to an oscillator with a NOT gate. The oscillation frequency is adjusted by VR1, f=1/2.2C1R1v, and the transmitting frequency is 40kHz. The receiving circuit uses a phase-locked loop low-frequency decoding integrated circuit LM567. If the required frequency is detected, it will output a low level, and the output end is an open collector output. The detection frequency (40kHz) is adjusted by VR2. When the reflected signal is detected, the LED lights up.

The detection distance is 1-2m. The disadvantage of ultrasonic sensors is that they fail to detect soft sound-absorbing materials because the reflected waves are too weak. Therefore, they are only suitable for detecting hard objects.