Need help hacking ultrasonic modules

[kdharbert]

I would like to be able to recover the 40khz signal from a commonplace SR-04 ultrasonic ranging unit. These units are made to simply trigger a logic pin when a pulse train is received and do not output the pulses themselves. It seems like I might be able to tap a pin on the board to recover this signal.

Seems like somebody might know enough about this unit it to tell me.
Any help?

 

[MarkT]

Just watch out for the high voltages if its using a resonant transformer to drive the transducer, stick to the primary side!