Devantech SRF04 Sonar Rangefinder

This ultrasonic rangefinder uses a short blanking interval (100 microseconds) to cut down on noise from the initial ping. It’s also designed to use low amounts of power, helping reduce energy costs and lengthen battery life, and has a compact design that leaves room for other components. This Devantech sonar ranging module is a good option for projects requiring accurate ranging information.

There are a couple of requirements for the input trigger and output pulse generated by the ranger. The input line should be held low (logic 0) and then brought high for a minimum of 10 microseconds. The pulse is generated on the falling edge of the input trigger. The ranger's receive circuitry is held in a short blanking interval of 100 microseconds to avoid noise from the initial ping and then it is enabled to listen for the echo. The echo line is low until the receive circuitry is enabled. Once the receive circuitry is enabled, the falling edge of the echo line signals either an echo detection or the timeout (if no object echo is detected).

The SRF04 was designed to be just as easy to use as the Polaroid sonar, requiring a short trigger pulse and providing an echo pulse. Your controller only has to time the length of this pulse to find the range. The connections to the SRF04 are shown below:

The SRF04 Timing diagram is shown below. You only need to supply a short 10uS pulse to the trigger input to start the ranging. The SRF04 will send out an 8 cycle burst of ultrasound at 40khz and raise its echo line high. It then listens for an echo, and as soon as it detects one it lowers the echo line again. The echo line is therefore a pulse whose width is proportional to the distance to the object. By timing the pulse it is possible to calculate the range in inches/centimeters or anything else. If nothing is detected then the SRF04 will lower its echo line anyway after about 36mS.