This circuit with a bargraph scale was found in a 90s Popular Electronics. The circuit measures the intensity of a sound in three scales. R14 works as a sensitivity adjustment.
This is an old test circuit for integrated TTL counters, feeding a Nixie tube, common at the time....
I found this circuit in an Electronics Handbook from 1989. The circuit can be assembled with a...
This circuit makes use of a 2N301 germanium power transistor. We found it in a documentation from...