I think what he wanted (maybe, its as clear as mud), is one of these.... (<£3 on ebay!)

View attachment 272981

Hi Everyone
Level sensor Sorry my mistake
& thanks for pointing it out

I will replace it with the (HC-SR04) & i can use it as a level sensor
(without branching away from the original question)

I just focused on my original question
(what is the best suitable transistor?)
Keeping in mind my parameters
40khz switching speed +
maximum base voltage of 5v +
connecting the transistor to the ground lead

Thanks for the advice everyone

 

I will replace it with the (HC-SR04)

See here: https://gustavobultaco.com/hc-sr04-ultrasonic-sensor-datasheet-89/