Signal Presence Detectors?

[Grant Richter]

Thomas Henry has a nice design in EN#134 page 17

It uses a single quad amp. Two sections to form a triangle LFO, one for a
peak detector and one for a PWM comparator. The comparator drives a bi-color
LED and the raising voltage levels change the colors from off - green -
yellow - red.

You can use all the comparators for displays you want, just remember to run
all the power and ground lines for the display and comparators to the
incoming DC power lines BEFORE the decoupling filter. That way display noise
gets filtered from the power lines with the other garbage.

Always try to put LEDs and such high current loads on a separate ground
system from any ground terminal going to the + input of an op-amp. That is
the Quiet/Noisy ground approach.