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.
Two operational are used on this triangular-rectangular oscillator suggested by Motorola in its...
This circuit from a Popular Electronics from the 90's can generate signals from 0.03 Hz to 3 kHz...
With 8 to 30 V power supply, the output power depends on both the voltage and the load. The...