Hi, sorry for the missing infos, I thought 16 mm 40 kHz transducers were some sort of a standard.
Here are the ones i bought: https://it.aliexpress.com/item/Split-ultrasonic-transceiver-ultrasonic-sensor-ultrasonic-probe-to-2-2-yuan/32668187552.html?spm=a2g0s.9042311.0.0.op3wIa
There are some specs listed, actually I could not find a Voltage or Current. 
Anyway, the link to the driver circuit seems complicated, I mean, I don't quite figure it out, I wouldn't even know which one of the circuits shown I should go for - sorry, I'm such a fool at electronics.
Hope thedata above might be some help.
Thank you!