Hello:

This is the ultrasonic signal. Received singal will be preamp, and feed to ADC (TLC548). MPU is ATMEL89S52 or 89C2051. This is the amplified signal:


I'm trying Ultrasonic ranging. I don't mean to calculate exactly the distance in "cm" or "m". I just want to have a number from 0 to 255, represents a range that I design, for example, 255 = 2m, 0 = 0m.

I can sucessfully convert the echo into a number between 0 and 255. But my question is, when the object is near, the signal is usually significant but when the the object is far away from the sensor the signal is very weak, which means that there is no absolute level of "signal higher then 'this' is the echo signal". How to write a program to detect the greatest echo within the "2 meter time", beside using a comparator?

I've written a program like this:
Transmitt the ultrasonic pulses, and start a timer which INC a variable every 70uS.
The MPU keeps comparing the incoming signal from the ADC with the previous result. If the new one is greater then the previous one, I keep the new one as the "greatest" and save the copy of the "variable INCing by the timer", until the end of "2 meter time".

After the "2m time", the "variable" should be containing the time of which the largest echo comes.

But the program doesn't work well. LED keeps flashing as if the distance is changing frequently.

For these "time dependent" program I don't know how to debug...

How would you work out the time between "Transmit" and "Echo @ 2m"? I mean the "program flow"? "the Logic"

Sorry, after resize the notes becomes barely visible... and for my poor english which I can't use it well to express my meaning.

Just ask my questions if it is not clear, please.
Thanks~

Stanley