Question

Design a 'bar graph driver' using open-drain output comparators for following conditions: Vin<2V Vin<3V Vin<4V Vin > 4V One LED light up Two LEDs light up Three LEDs light up Four LEDs light up Five LEDs light up Number of LEDs used must be minimum and Vec-5V

Answer 1

Solution : Open drain output means we need to use pull up load loads.

Let's the output of the comparator is given by -

Vinput > Vref output = 0 V

Vinput > Vref output = 0 V

As the resistor values are same, so voltage will divide equally.

Working :

Vin < 1V :

In this condition for

Comparator 1 Vinput > Vref output = 0V ------ LED-1 ON

Comparator 2 Vinput < Vref output = 5V -------LED-2 OFF

Comparator 3 Vinput < Vref output = 5V ------ LED-3 OFF

Comparator4 Vinput < Vref output = 5V -------LED-4 OFF

Comparator 5 Vinput < Vref output = 5V ------ LED-5 OFF

Vin < 2V :

In this condition for

Comparator 1 Vinput > Vref output = 0V ------- LED-1 ON

Comparator 2 Vinput > Vref output = 0V -------LED-2 ON

Comparator 3 Vinput < Vref output = 5V ------ LED-3 OFF

Comparator4 Vinput < Vref output = 5V -------LED-4 OFF

Comparator 5 Vinput < Vref output = 5V ------ LED-5 OFF

For Vin < 3V :

Comparator 1 Vinput > Vref output = 0V ------- LED-1 ON

Comparator 2 Vinput > Vref output = 0V -------LED-2 ON

Comparator 3 Vinput > Vref output = 0V ------ LED-3 ON

Comparator4 Vinput < Vref output = 5V -------LED-4 OFF

Comparator 5 Vinput < Vref output = 5V ------ LED-5 OFF

For Vin < 4V :

Comparator 1 Vinput > Vref output = 0V ------- LED-1 ON

Comparator 2 Vinput > Vref output = 0V -------LED-2 ON

Comparator 3 Vinput > Vref output = 0V ------ LED-3 ON

Comparator4 Vinput > Vref output = 5V -------LED-4 ON

Comparator 5 Vinput < Vref output = 5V ------ LED-5 OFF

For Vin > 4V :

Comparator 1 Vinput > Vref output = 0V ------- LED-1 ON

Comparator 2 Vinput > Vref output = 0V -------LED-2 ON

Comparator 3 Vinput > Vref output = 0V ------ LED-3 ON

Comparator4 Vinput > Vref output = 0V -------LED-4 ON

Comparator 5 Vinput > Vref output = 0V ------ LED-5 ON

Thus required condition is achieved by this circuit,with only 5 led's