Found in American documentation from the 1980s, this circuit has a diode feedback. At the exit we have an indicator LED. The LM3900 is a transconductance operational amplifier.
This circuit is from a Popular Electronics from April 1991. The magazine no longer exists, but the...
This two-color LED indicator was found in a Popular Electronics from the 90's. The transistor...
This circuit causes an LED to flash at a frequency determined by the capacitor. The circuit has an...