ultrasonic transducer arduino circuit- plz plz plz help!

Using Arduino General Electronics
2 
const int pingPin = 9;

void setup() 
    { tone(9, 40000); }

void loop() { }

If you use a digital output the Peak to Peak voltage is 5 volts (or very close to it).

I don't know how you would detect the 40 kHz signal without a oscilloscope. If you had a second Arduino and a second transducer you could try playing a 41000 Hz tone and listen for the 1 kHz beat frequency.