This Japanese commercial radio with 5 transistors is from the 60s and is powered by 6 V. It is a conventional superheterodyne configuration with good performance.
This circuit indicates whether the voltage to be measured is higher or lower than the zener diode...
Obtained from an English publication, this circuit actually needs a current limiting resistor,...
Constant current sources are used in applications such as battery chargers, electrolytic tanks, etc....