Integrator for HC-SR04 UltraSonic Range Finder PUBLIC

Integrator for simple DtoA conversion from a particular digital PWM device.

This is an integrator circuit I'm setting up for doing a DtoA conversion for an HC-SR04 ultrasonic range finder. It would be better to have the pull down be a JFET but the PNP transistors were more available at my local retailers and I was impatient. I hope to use this in place of the more expensive MaxSonar brand devices that are officially supported by the DIYDrones Ardupilot project.

The HC-SR04 outputs a digital pulse with width equal to the round trip time of its sonic ping sequence. This integrator is tuned so that there is a near linear relationship between the distance (pulse width) and the voltage between approximately .3V and 3.3V of about 7.5mV/cm . Reading from an analog pin on the arduino this should give me an effective resolution of ~ 2.6 cm.

I've tied the base of the transistor to the signal generator for convenient analysis, but I'm most likely going to drive it with a digital 5V signal so that the output can be cleared between readings.