Here’s a nice recipe for building your own ultrasonic distance sensor – similar to the Parallax Ping, though it requires a bit of software coding to handle timing and measurement –
Everybody knows the speed of the sound in the dry air is around 340 m/s. Send a short ultrasonic pulse at 40 Khz in the air, and try to listen to the echo. Of course you won’t hear anything, but with an ultrasonic sensor the back pulse can be detected. If you know the time of the forth & back travel of the ultrasonic wave, you know the distance, divide the distance by two and you know the range from the ultrasonic sensor to the first obstacle in front of it.
Here we use an ultrasonic piezzo transmitter with its receiver, they are very efficient, easy to find and quite cheap.