can any one give some idea about how to design a circuit for range finder using at89c2051 and 40khz ultrasonic sensors.... i have given a sample circuit, http://www.kerrywong.com/2011/01/22/a-sensitive-diy-ultrasonic-range-sensor/
which i found in google. they used atmega... can i use at89c2051 instead of atmega.
Is it a serious question? We all know, that no Doppler effect is involved as long as sensor, medium and object don't move. The nice animated picture in post #2 isn't related to basic echo ranging.
Regarding the original question, you didn't supply a vaild link, so I can't refer to the circuit. Generally, you can surely build some kind of echo sensor with AT89C51, or any other µC of your choice. If the circuit under discussion uses any special features of the ATmega, it may require a redesign.
here is answer of your question ---------that no Doppler effect is involved as long as sensor, medium and object don't move......................
using Doppler effect u can find distance easily whether objects are moving or not because you have to set a reference point which is known to you and with respect this one you can calculate change in phase angle of the reflected waves....
As an additional remark. Combining echo ranging (time of flight measurement) with doppler speed detection has some prerequisites in time and frequency resolution that are most likely hard to achieve with the parameters of 40 to maximum 200 kHz frequency usable for ultrasonic in air.