You can choose between the one or two pin approach with the HY-SRF05 Ultrasonic sensors which cost roughly the same as the HC-SR04, so it seems a little odd to hack the SR04 instead of picking up ...

The HC-SR04 sonar modules are available for a mere pittance and, with some coaxing, can do a pretty decent job of helping your robot measure the distance to the nearest wall. But when sellers on eB… ...