The circuit of this device is perhaps the simplest assembly of all circuits of this type. This scheme is suitable even for beginner hams.
The circuit is constructed of 2 transistors (kt315), they can also be replaced with more powerful ones (kt817 or kt815) or you can choose similar ones, such as s9016, s9014, etc.
The resistance of a variable resistor ranges from 1 to 2.2 kOhm. The LED is selected standard, with voltages from 2.5 to 3V, the choice of color depends on personal preference.
To configure the indicator, you need to connect it to the power supply and put on the desired voltage, then rotate the AC resistor.
In that case, if the LED is on, then the battery must be charged, if not, then everything is fine and recharging is not required. This scheme is highly accurate and also quite simple. The indicator lights up immediately and without any warning.
It is used for 12v battery, although you can also configure it for 6-3v if desired. If you assemble this device in several copies, then you can almost always know the state of the battery.
