The indicator for this circuit is a 0 to 1 mA milliammeter. The operational is from the 70s, when the circuit was published. Modern equivalent can be used.
If the battery is good, the LED flashes. This simple tester uses an LM3909 integrated circuit. We found...
This circuit was found in a 1926 German publication. It is a crystal or galena (D) receiver. The...
In the figure we have a typical configuration with operational, which can be different from the...