Need help with an ultrasonic sensor interface circuit to ev3

[btpolk]

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.

 

[meBigGuy]

Sound travels at 1ms per foot, so you need to be able to sense the reflection delay with that in mind. The ev3 only samples at 1000Kz, so you will need some external circuit. You can create a voltage that is proportional to the echo return time.

Or, possibly you can create a trigger signal that interrupts the processor and have it figure out the delay.