Hallo Arif,

ultrasonic transducers (UST) mainly consist of some piezo-ceramic crystal showing a certain resonance frequency, mostly at 40kHz. So, it works similar to an quartz crystal. Big difference is, that resonance is not as sharp and frequency response covers a much broader range.

It's interesting to note, that these transducers were indeed driven like quartz crystals, when they were fabricated for the first time. In my 'archive' I found some older designs, where the UST was in the feedback loop of a high gain amplifier. Disadvantage of this apporach is, that start-up of oscillation is much harder to achieve, because of the rather poor resonance. Also, this badly influences frequency stability.
Today, most designs produce a square wave signal of 40kHz by the help of an external oscillator and drive the UST low ohmicly, means they provide a low impedant voltage output, like it's observed for piezo beepers. So, resonance of UST is no longer used for the fabrication of oscillation.

Of course, there are different topologies, which are also suited to produce an ultrasonic wave, but USTs consisting of piezo-ceramic crystal dominate.

Here is a datasheet of some USTs from Murata:

http://www.murata.com/catalog/p19/el0560.pdf

It's interesting to note, that most transmitters can withstand very high driving voltages of up to about 100Vpp! So, when wanting to achieve very high distances very high amplitudes should be applied. Power supply current can then be saved by the use of pulsing technique.

In the following I show two very simple schematics, a transmitter and a reciever, which can be used for first experiments:



In the transmitter circuit 'enable' input supports pulsing technique. Here, a microcontroller can be connected, if transmitting of information is wished. Frequency of oscillation should be adjusted by the help of trimmer in order to achieve maximum output power of UST. Then UST is working at it's resonance frequency. Because frequency response covers a rather broad range, adjusting of oscillation frequency is uncritical. In order to produce high output power a bridge configuration was chosen, which produces double signal level at UST.
For best perfomance use 25kOhm cermet-trimmer and 150pF NP0 ceramics.

Circuit of reciever provides a rather high impedance input. LM358, which is thought to be powered by unsymmetrical supply voltage of +5V, gets a well filtered bias voltage of about 1.6V at non-inverting input. Feedback loop to inverting input provides maximum gain for 40kHz, means circuit works like a bandpass filter. This frequency selective performance is very advantageous, because all kind of interference like johnson noise, hum (and it's harmonics, think of neon lamps!) and unwanted acoustic noise can be highly rejected!!
For best performance, 390pF NP0 ceramics should be used. Also, I would highly recommend the use of standard metal film resistors with +-1% tolerance and +-50ppm/K tempco for the whole circuit.

With the help of second operational amplifier in LM358 a simple level detector can be built, which follows the bandpass stage. If output of this level detector is connected to microcontroller, absence of ultrasonic signal can be detected, or even information can be transmitted by using pulsing technique.

Good luck,
Kai