This project is useful for model planes, because you don't have to guess the battery charge anymore. The battery charge is shown using two high power leds: a red and a green one. These leds are connected via a current limiting circuit to an attiny45 which controls everything. 

The board connects to a servo output of the flight controller. When the lever of the remote is in the bottom position, nothing happens. When the lever is in the middle, the leds blink to show the battery charge and in the top position the leds blink and a buzzer sounds. The buzzer is to help find the plane when it is landed/crashed somewhere in a field.

The led show the battery charge. They blink trice when the battery is fully charged, twice when the it is half full and once when the battery is empty and it's time to land. The values used for this are as following: above 3,7v is full, between 3,4 and 3,7v is half full and lower than 3,4 is empty. 

The board is powered via the balance charge port of the battery. It uses two cells. This way, the attiny45 is powered with 3,7v and the leds with 7,4v. The battery is connected via regular header pins, so any battery with two or more cells can be plugged in. The voltage of the battery is measured by the attiny45 itself. The microcontroller has the ability to measure the voltage on its vcc pin.