But with his long term goal in mind, he also modified the HC-SR04 sensor by removing the on-board pull-up resistor and adding a new one on the connector side to combine the signals.

The HC-SR04 appeared on the market a few years back as a low-cost alternative of the Ping. [Emil] found that the HC-SR04 would work reliably on hard surfaces as far as 4 meters away from the sensor.