Long story short I'm looking to build an ultrasonic sensor that can also be used to communicate with a seperate sensor. (So it must be able to send out a pitch around ~42kHz, but also alter the pitch so that a second sensor could listen to the output and determine an FSK signal, so I could change the output signal to 43kHz if I wanted). Optimally it'd have an operating range of around 10kHz or more (so 38kHz to 48kHz), but I'm not especially set on an particular range.
At first I was thinking I'd just buy a piezo electric speaker such as this one, but I'm unable to find a microphone that can listen to a 42kHz signal.
Next I was thinking I could just buy a prebuilt ultrasonic sensor, but the only ones I can find don't allow you to modulate the output frequency.
I know things like this already exist as the ones on cars can modulate the output frequency, however I'm unsure what the best route for me would be. (I haven't had tremendous success finding an OEM sensor that I could modulate from a car but I'm open to that possibility).
Thanks in advance!