Last time I used an HC-SR04 with a Teensy, I just wired the TRIG and ECHO pins together and told the NewPing Arduino library to use the same pin for both functions. I’m not sure where I read ...

[Emil] got his hands on a dozen HC-SR04 ultrasonic sensors, but wasn’t too happy with their performance. Rather than give up, he reverse engineered the sensor and built an improved version. H… ...