This circuit can be used in a signal level indicator with LEDs. The circuit is from an Electronics Now dated January 1996. The power supply must be dual.
This circuit was found in a Popular Electronics from 1991. The transistor could be the BC548. The...
This circuit activates a relay with a frequency signal determined by the adjustment components. The...
This circuit for frequencies from 1Hz to 100 kHz was found in the June 972 Radio Electronics...