Here is how I use 40kHz Tx/Rx trancducers
Assembled Rx units
Tx unit
How I get my 40khz signal, only because I had a 30Mhz clock module handy
I use the clock circuit to provide a dual range "count clock"
This is a single "channel" counter I use
The 3 channel unit assembled
This is pretty easy to get working and you will learn a lot more!