# Could someone explain a few things for me about a ultrasonic receiver?

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#### Callo1234

##### New Member
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.electro-tech-online.com/custompdfs/2009/04/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.

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#### Nigel Goodwin

##### Super Moderator
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.electro-tech-online.com/custompdfs/2009/04/srf1-1.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.

#### Callo1234

##### New Member
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

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#### Nigel Goodwin

##### Super Moderator
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.

#### Callo1234

##### New Member
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)?

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