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 small converter using Only one transistor was published in the Radio Electronics magazine from...
This circuit was found in a Popular Electronics of the 1990s. Its purpose is to expand the scale of...
This receiver was found in old English documentation and any general purpose NPN transistor can be...