Hi Bruce,

When you want to use the ultrasonic, maybe you have to calculate the temperature(using LM35 or the other sensor) too. Because the ultrasonic is depend on temperature and humidity.
It can measure the distance precisely if you have already compensate it with calculating the temp. Because, when the object is detected and the timer is stop, You have to divide the (timer value) with ultrasonic constanta. And the const value is depend on the temp.

To count the short distance, you can seek what Mehdi said in this forum. (search=ULTRASONIC)


Pradono