I'm currently building a robot using Lego Mindstorm with an ev3 brick that will follow a pre-determined path (marked by black tape), maintain a certain distance behind another robot, and coming to a stop at the end of its journey. The ev3 will be connected to 2 DC motors, 3 light sensors, and an ultrasonic sensor. The specifics of the test are irrelevant because I just need some help with build an interface circuit for the ultrasonic sensor (SRF04). I have the trigger circuit completed. I'm using a 555 timer in a-stable operation with parameters: Rb = 220 kOhms Ra = 1 kOhms C = 0.1 uF. Parameters are defined by the TI document (http://www.ti.com/lit/ds/symlink/ne555.pdf) on page 11. This gives me a period of about 31ms and duty cycle of about 50%. Now I need to convert the echo output of the sensor to be readable for the ev3 brick. I was thinking average value but I'm not sure if that's correct or how to go about it. Any ideas? Available lab circuit elements: capacitors, resistors, op-amps, 555 timers, voltage regulators, and diodes.