The TS601 Ultrasonic Range Finder is a common and cheap sonar based system, at about $15.00 its a pretty good bargain. It's interface is simple, a single pin. You apply a digital pulse for a little bit more than 5us, then wait for a pulse who's width represents the time it takes the sound to hit an object and bounce back.

The initial implementation is complete and reports the response in millimetres.

A sample Arduino test program looks like this:

#include "Arduino.h"
void setup (){}
void loop ()
{
    while(1){
        pinMode(7, OUTPUT);
        digitalWrite(7, 1);
        unsigned long us = micros();
        while(1){
            unsigned long now = micros();
            if( (now-us) > 20)
                break;
         }
         digitalWrite(7, 0);
         pinMode(7, INPUT);
         unsigned long ms = millis();
         while(1){
             unsigned long now = millis();
             if( (now-ms) > 30)
                 break;
        }
    }
}

Updates to come....