Hi Dana,

Thanks a million for your help. I haven't had a chance to get to check the code yet but I'll let you know how it goes.

I'm using an analog devices chip (ADuC812) to generate the 40kHz square wave pulse and to initiate the counter when that happens. I intend to use the chip to stop the timer on the return of the reflected wave to the analog input port of the aduc812. It will then carry out a few calculations for me, and write the result to a suitable LED/LCD display(haven't decided which one to use yet). My electronic transmitter circuit manipulates the square wave produced by the chip by filtering to form a near sine wave at the ultrasonic transmitter itself. The reflected wave is picked up by my receiver unit, amplified, rectified and sent to the analog input of the chip, where the counter will be stopped,(not sure how to detect the analog dc voltage at this point so as to stop the timer yet). There are a number of shady parts of it so I'd appreciate it if you could pass on any improvements/tips you might sufggest considering you have tackled a similar problem before!! Once again thanks a lot for your help.