Yep the project is just a hobby . Band pass filter ? Don't I need an LC tank for that or a crystal? Can I make one using a 555 timer ?

Here is one receiver solution with 555:
http://tubetime.us/index.php/2011/02/23/555-contest-entry/

 

Does the 555 chip also tune the circuit to a specific frequency ? Is this possible . Also I still dont understand what happens with the piezo element sensor that detects the ultrasonic waves . How does the tuning to this cirquit happen ? Adding weight increases only sensitivity or also tunes the reciever ?

 

You can regard the piezo elements as speaker/microphone. They just convert the electrical signal to ultrasonic signal or vice verse. For the receiver, you need to tune the capacitor or inductor to let the LC resonance frequency to the the same as the carrier frequency, just like the legend AM/FM radios.