I'm looking to get some help in designing a board that takes an input voltage and confirms if the that voltage resides between V = 27.5V and V = 29.5V without an external power source. When the input voltage is between this range I would like an LED to light up. So far I've been exploring using TL431s as an attempt to create this voltage window.
![1712730942076.png 1712730942076.png](https://forum.allaboutcircuits.com/data/attachments/307/307246-d628f9389e50afedbbd7244f69556881.jpg)
I've split the circuit into two for lower and upper limits I would like to achieve.
The first I believe should switch on the resistor when the voltage at Vref of the TL431 = 2.5V, which should be the case from the voltage divider at Vin = 27.5V.
The second therefore is incorrect and instead of switching it on at 29.5V I would like to shut it off when it reaches that value or shortly after exceeding it.
How could I achieve turning off the LED in the second circuit and how would I go about combining the two to make the LED turn on only when Vin is in the range of 27.5V-29.5V?
![1712730942076.png 1712730942076.png](https://forum.allaboutcircuits.com/data/attachments/307/307246-d628f9389e50afedbbd7244f69556881.jpg)
I've split the circuit into two for lower and upper limits I would like to achieve.
The first I believe should switch on the resistor when the voltage at Vref of the TL431 = 2.5V, which should be the case from the voltage divider at Vin = 27.5V.
The second therefore is incorrect and instead of switching it on at 29.5V I would like to shut it off when it reaches that value or shortly after exceeding it.
How could I achieve turning off the LED in the second circuit and how would I go about combining the two to make the LED turn on only when Vin is in the range of 27.5V-29.5V?
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