Hi Nikos,
You should get the book called "Experimenting with the PICBASIC PRO compiler" by Les Johnson. He has an example program called SON_PROX.bas complete with schematics and explanation. It uses 2 ultrasonic transducers modulated at 40 kHz. The circuit uses an op-amp to amplify and convert the received sound into a TTL voltage and a transistor to adjust the sensitivity back to the pic. That's a pretty low parts count. He's using a PIC16f84 micro so a 628 will work. He reports that his set-up will give a sensitivity of approximately two feet. Ultrasonic transducers are what most of the commercial units use for back-up sensors if I'm not mistaken. Let me know how you make out.
Good Luck
jessey
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