How do design a circuit that tells the difference of 2 Voltages is within range

Differential amplifier + window comparator, combining two simple comparators.

yes - lookup window comparator

As other have suggested, an opamp can do all those operations. One integrated circuit that has that function can be the old timer 555.

fgavin said:

One integrated circuit that has that function can be the old timer 555.

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Don't see how(?).

Henry, what's the input maximum voltage and polarity?

Or just use low-offset-voltage comparators, no need for the amp.

fgavin said:

As other have suggested, an opamp can do all those operations. One integrated circuit that has that function can be the old timer 555.

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Don’t know how you’re going to do this with one opamp or a timer.

@ Barry, 555 is effectively a window comparator - which window can be varied - within limits .....

Easy peasy said:

@ Barry, 555 is effectively a window comparator - which window can be varied - within limits .....

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I had the impression that I had access to the resistors on the 555 that set the voltage limits, but I guess not, only the control voltage pin is available.

I was told in school that the opamp can perform any mathematical function, be it a comparator, integrator, differentiator, sum, difference(-sum), amplifier.
So far we might only need the comparator and amplifier

We need a bit more info from henry:
henry, can you clarify if for input A and input B you want this condition, if abs(A-B) < 0.05V then output = 1V? and 0 otherwise?

Hi,

More detailed, more pedantic:

"The difference between two inputs" just means "A-B".
So mathematically the task is: if (A - B) < 0.05V
So if A = 3 and B = 4 ... the result is FALSE.
If you don't want this then you probably mean: if the absolute value of the difference is < 0.05 V
This mathematically means: if |A - B | < 0.05 V
Or in hardware: difference --> rectifier --> comparator
But this is equal to: if --0.05 < (A-B) < 0.05
Or... if (A - B) < 0.05 AND (A-B) > -0.05
This could be done with the above mentioned window comparator, by using thresholds of -0.05V and +0.05V

Things you additionally need to consider/ define:
* what supply voltage do you have? (Do you have limits/ targets for the supply current?
* how accurate do you want the threshold?
* hysteresis to avoid jittery output (chattering)
* timing...how long do you allow the output to get the expectated state? Nanoseconds, milliseconds?
* define the output levels more clear. 0V as well as 1V are absolute values that allow no tolerance. This never can be realized. --> Please define the valid range for each output level.

Lets say: (example)
* LOW: -0.1V .... +0.25V
* HIGH: 0.75V ... 1.25V

Klaus

Easy peasy said:

555 is effectively a window comparator

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Not really.
A window comparator generates an output when the input voltage is within the window voltage (no latch).
The 555 can be configured as a level sensitive latch, whose output goes high with the input is below 1/3 the supply voltage and low when the input goes above 2/3 the supply voltage.
Not the same thing.

the 555 operates when the input goes outside the window, hence window comparator is a short description, also the limits can be modified away from 1/3 and 2/3 for an experienced designer.

Easy peasy said:

the 555 operates when the input goes outside the window,

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Yes, but it does nothing when the input moves back inside the window, which a normal widow comparator does.
There is a difference between a level sensitive latch and a window comparator.

I feel that the discussion is a kind of theoretical. You won't actually use LM555 as windows comparator for this
application, e.g. due to lack of voltage accuracy. I'd suggest LM393 for a first try.