This circuit was obtained in a Hands-On Electronics from the 90's. The circuit turns on one LED at the end of the timer, erasing the other. The timing can be increased by increasing C1.
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....