This circuit indicates
which of three voltages in the range from about about -4V to about +4V –
at A, B and C – is the highest by lighting one of three indicator LEDs.
Alternatively, it can be wired to indicate the lowest of three voltages
or to indicate both the highest and lowest voltages. Op amps IC1a, IC1b
& IC1c are wired as comparators, while the three indicator LEDs
and their series 1kO current limiting resistors are strung across the
op amp outputs to implement the appropriate logic functions.
For example, LED A will light only when pin 8 of IC1c is low (ie, A greater B) and pin 7 of IC1b is high (ie, A greater C). Similarly, LED B will light only when pin 8 of IC1c is high (ie, B greater A) and pin 1 of IC1a is low (ie, B greater C). LED C works in similar fashion if the voltage at C is the highest. Note that if all the LEDs
and their parallel 1N4148 diodes are reversed, the circuit will
indicate the lowest of the three input voltages. And if each 1N4148
diode is replaced by a LED, the circuit will indicate both the highest and lowest inputs.
Author: Andrew Partridge – Copyright: Silicon Chip