If the battery is good, the LED flashes. This simple tester uses an LM3909 integrated circuit. We found this configuration in an old documentation.
This transistorized circuit is powered by a voltage of 6 to 12 V providing high voltage for a neon...
This circuit is from an Electronics World of 1968. The transistors are of the time and the power...
This circuit is from a 1972 German magazine Funkshau. The circuit can use BC548 transistors...