2 color detector using opamp

[Michael_N]

Hi All,

I want to build a circuit which can detect 2 different colors. I want to use whatever I have in my inventory if possible

I was thinking of using a reversed LED as sensor. I want to detect white and black.
I have amplified the voltage from each color.
Black is 2,5V
White is 3V


I was hoping to use LM358 op amps as comparators to have 2 outputs.
The first output is dedicated to white color
and the second output is dedicated to black color.

My problem is that one of the outputs will always be on regardless og the color in front of the sensor.

What I need is the dot function I have with LM3914. How does that work?
LM3914 is just a series of comparators, so no mistery there
But how can they make the "dot" function?

Actually I have the LM3914, but it is a bit of a waste I think, when I only need 2 outputs

Kind regards
Michael

 

[chuckey]

Black is 2,5V
White is 3V, very interesting, but what is 2.49 V, 2.75V and 3.01 volts. Once you have sorted out exactly what voltages correspond to white and black (which are not colours, but just luminance levels), then you need to build a window detector for each voltage. Which is what the LM 3914 does. It is easier to use the correct chip, but you will still have to fiddle about to get the correct effect.
Frank

 

[BradtheRad]

But how can they make the "dot" function?

Pin 9 controls whether the 3914 is in dot mode or bar mode. From the datasheet:

"The comparator is designed so that pin 9 can be left open circuit for dot mode."

Actually I have the LM3914, but it is a bit of a waste I think, when I only need 2 outputs

You can use a stack of 2 or 3 op amps as window comparators.
One of each set of inputs will go to a multiple resistor divider.

It may or may not be easier than using a 3914.

 

[Michael_N]

Thanks for the replies
Tomorrow I will try the window detector option. I will let you know about the result.

I know how you activate or deactivate the dot mode on LM3914, But how does it work, internally in the chip?

- - - Updated - - -

Thanks for the replies
Tomorrow I will try the window detector option. I will let you know about the result.

I know how you activate or deactivate the dot mode on LM3914, But how does it work, internally in the chip?