Circuit Design - Comparators to light an LED

1. Oct 22, 2013

hayniesk

Circuit Design -- Comparators to light an LED

1. The problem statement, all variables and given/known data
I need help designing a circuit that takes an input voltage and then lights a single LED if and only if V in is greater than or equal to 2V and less than or equal to 1 V.
It can only use 1 Power Supply unit, +/-5V
Circuit must not draw more than 50mA

2. Relevant equations
voltage divider = vtotal*R1/(R1+R2)

3. The attempt at a solution
I know that there definitely needs to be a voltage divider of some sort (potentiometer?) range the voltage between 0V and 5V, and I probably will need 2 op amps. But I don't know how to design the circuit such that the LED will only light up if the voltage is within the given range.

Help appreciated!

2. Oct 22, 2013

Staff: Mentor

I think there is a typo in your problem definition where I bolded it. Can you check to see if the wording of the voltage range shoule be fixed?

And I think you should initially try to do this using 2 comparators.... Does that help?

3. Oct 22, 2013

hayniesk

The part that you bolded is in fact correct. Those are indeed the instructions. That is where my problem lies.

I am thinking about setting up two comparators, and i have a sketch of it. I just don't know how to manipulate it to turn off only when vIN is between 1 and 2V.

4. Oct 22, 2013

Staff: Mentor

Sorry, but that makes no sense. The two voltage ranges are mutually exclusive, so you cannot AND them.

"if and only if V in is greater than or equal to 2V and less than or equal to 1 V"

5. Oct 23, 2013

CWatters

It makes sense to me although I agree the wording could be better. The word "and" is not being used to mean the logical AND.

The LED should be ON for Vin is <1V OR Vin >2V and OFF when Vin is between 1V and 2V.

Two comparators can be used to test Vin. Consider making an OR gate from diodes.

6. Oct 23, 2013

CWatters

7. Oct 23, 2013

Or the same class from two years back? I'm actually working on this same problem now and can't really figure it out...

8. Oct 24, 2013

CWatters

Wany posted a circuit on that other thread. Works like this..

One comparator generates a logic 1 (5V) output if the input is below 1V
Second comparator generates a logic 1 (5V) output if the input is above 2V.

A Diode/Resistor Logic OR gate is used to OR the two outputs of the comparators (google Diode OR gate).

Such an OR gate is built from two diodes and a resistor. The resistor is also used to limit the LED current.

If you don't know how to set up the comparators there are plenty of guides on the web.

Otherwise tell us where you are stuck.

9. Oct 24, 2013

Staff: Mentor

Isn't there a car commercial running now that points out how important AND versus OR is?

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