This circuit was found in a Popular Electronics from the 90's. The CMOS integrated circuit used is not very common nowadays. The range depends on the infrared LEDs. R7 can be changed.
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...