Do you really have to built it yourself?

Look at the http://www.robot-electronics.co.uk/a...c_Rangers.html

Once I had to built that ranger in 1990 but then I could not find ready made modules.

After a few year I tested the modules of the above link and was surprised.

First of the idea to drive the Tx US transducer by the well known MAX232 chip (it can produce fast +/- 10 volts of pulses!) and second by its simplicity and performance.

Ioannis