SRF04 - some things i dont understand

[Malachi]

Hi i was wondering if you could help.

I based my rangefinder for a project on your SRF04 and i have had loads of
problems with my design because I dont fully understand it..

http://www.robot-electronics.co.uk/files/srf1.pdf

Basically i was wondering, how do you come to the calculations of c6 and
r7 values for the delay RC for the comparator?

Its not a simple R*C - gives time delay is it?

Also I was wondering in the circuit, what kind of voltages would you expect
GP4, gp5 - from the PIC and TX1?

P.s am i right in saying that the input impedance R12 is a bit small
(transducer impedance not taking into account?) also C9 is a little
excessive for 40khz? It could be smaller to still block the required DC and pass AC

[Malachi]

Also i was wondering - im interested in forming a large part of my report on a road profile measuring system which would require a small range (i.e the sensors are ounted on the underside of the car) so could i do this by having a similar circuitary but by removing the MAX232E? Would the resultant reduction in range (not sure how much this resultant would be) be useful for this?

Thanks for any help

[Malachi]

At the least could someone explain to me how i could vary the range that the receiver accepts? Before my deadline tmoro haha