Thickness detection circuit

[Revathysrini]

Hi,
Im tracing out thickness detector circuit (attached herewith) in which first stage of op amp is used to convert photo diode current to voltage. Could nt understand the op amp configuration in second stage. is it a comparator?

Kindly suggest me.

 

[andre_luis]

Apparently a comparator with an adjustable threshold.
The signal from uC could be some output configured as a timer/PWM.

 

[KlausST]

Hi,

I don't see a comparator.

The first stage generates 270mV per uA of photodiode current,
The second stage has gain of 2.2 inverting (-2.2) but may add a variable offset to the output.

***
I wonder how and how precise you want to measure thckness..and thickness of what?
The output will be influenced by ambient light. = error

Klaus

 

[Revathysrini]

Hi,
Thank u both for reply. This circuit is for detecting currency thickness. whenever double note(currency) is passing through, the light received by photo diode is partially interrupted (depends on note thickness) which is sensed by microcontroller & drive corresponding motor to reject this note.

 

[KlausST]

Hi,

Currency...
This means it should be reliable and fail safe.

Ambient light might be one problem, easy to solve.
Think about dust, spiders or anyother thing that may change the flow of light.

But it should reliably work with different types of currency. Old, new, dirty, different colors prinred on, different types.
At least you need an analog value and ADC for the software to adjust on these parameters.
A digital 1/0 signal won't be flexible enough.

Klaus