This is the ultrasonic distance detector or sonar ranging system which can be used for general purpose. There are kit version available on the market, but of course the price is 3x more expensive than if you build your own sonar ranging system.
Some application may use this circuit such as robot applications, body height measurement, blind shoes, etc.
How the circuit works:
The transmitter emits an ultrasonic signal (40kHz). The 555 timer chip of the transmitter provides the driving 40kHz signal. Every time the reset pin (pin4) of the 555 timer goes high, a resulting signal of 40kHz on pin 3 is used to drive the ultrasonic transducer. Then, the receiver simply listens for the return echo after it bounces off an object. The small echo signal, when detected, is amplified 1000 times using a standard operational amplifier (LM741 op-amp). The signal is then fed into a tone decoder (LM567) set to lock onto a 40kHz signal. The output of the tone decoder is HIGH when no echo is heard and swings LOW when an echo is detected. The output from the tone decoder can now be fed into a microcontroller or some other type of IC to determine when an echo was received. To help minimize false triggering, the output is fed into a voltage comparator set to trigger at the appropriate level. The LED at the output of the comparator acts as a visual indicator when an echo is detected (very useful when debugging). The typical range of this system is from a few inches to 5-6 feet, depending on the quality of the components, shielding, and most important, tuning.
Detailed explanation about this ultrasonic distance detector circuit: visit this page
Or download this documentation: