Hi, for my ultrasonic sensor project i want to go with a design based on the link shown, but before I do I want to understand a few things


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

I was wondering - in the receiver -what design decisions are made with C6, C9 and R7 (in terms of values and why they are there!).

Im pretty sure i know C9, its just to remove residual DC

The way to calculate C9 i assume would be just to say....

1 / (2π f RIN)
with f being 40000 (40khz ultrasound) and Rin being 10000ohms (as I assumed the input impedance of the transducer to be 10000ohms)

I do not know what C6 and R7 do, and how effect the circuit and how there values are chosen?

Thanks for any help, im really stuck.

Originally Posted by Callo1234

Hi, for my ultrasonic sensor project i want to go with a design based on the link shown, but before I do I want to understand a few things


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

I was wondering - in the receiver -what design decisions are made with C6, C9 and R7 (in terms of values and why they are there!).

Im pretty sure i know C9, its just to remove residual DC

It's to pass AC, and to block any potential DC.

The way to calculate C9 i assume would be just to say....

1 / (2π f RIN)
with f being 40000 (40khz ultrasound) and Rin being 10000ohms (as I assumed the input impedance of the transducer to be 10000ohms)

It's not calculated at all, just a random value, massively higher than it needs to be - it's only got to pass 40KHz.

The impedance of the sensor is a lot higher than that, and it's a poor design because it's presenting far too low an impedance to the sensor (only 2K2).

I do not know what C6 and R7 do, and how effect the circuit and how there values are chosen?

It's a timing circuit, to slow down the comparator been enabled after the pulse has been sent.

Originally Posted by Nigel Goodwin

It's to pass AC, and to block any potential DC.



It's not calculated at all, just a random value, massively higher than it needs to be - it's only got to pass 40KHz.

The impedance of the sensor is a lot higher than that, and it's a poor design because it's presenting far too low an impedance to the sensor (only 2K2).



It's a timing circuit, to slow down the comparator been enabled after the pulse has been sent.

Thanks a lot, yeh I was told by somone else to stick another 10000ohms on the impedance in - its like they havent taken into acount the transducer input impedance - eventhough that schematic is from a on sale sensor!

Didnt have a clue about the timing circuit! Cheers

Originally Posted by Callo1234

Thanks a lot, yeh I was told by somone else to stick another 1000ohms on the impedance in - its like they havent taken into acount the transducer input impedance - eventhough that schematic is from a on sale sensor!

It's not a question of an extra 1000 ohms, you really need in the 100's of thousands, or the millions. That's why you would use non-inverting mode rather than inverting mode.

Oh ok, will check that out...

Also for the time constant in a parallel RC circuit (in the circuit c6 and R7) is the same as if it were a series RC

sooo T=RC

And therefore the time delay is just C6 x R7 (the time delay of the comparator after the pulse is sent)?