Clear plastic XT60 that lights up red when your battery voltage is low and it's time to land.
The circuit must be altered specifically to your battery cell count. For a 4S battery it's good to use a 13V zener (D1 in schematic). This triggers the LEDs at around 13.5V which is right below 3.4V per cell. This is lower than many drone racers like to discharge their batteries but it is not uncommon and therefore a warning at this level is well suited.
It is not an ideal circuit for 5S batteries because there is no common zener values around 5x3.4V.
4S = 13V zener. Triggers at 13.5V battery voltage, 3.375V cell voltage.
5S = 18V zener (pref. 17V if you can find one). Triggers at 18.5V battery voltage, 3.7V cell voltage.
6S = 20V zener. Triggers at 20.5V battery voltage, 3.41V cell voltage.
Taking the 4S version as an example, the zener diode D1 starts conducting when battery voltage reverse biases the diode with more than 13V. Q1 will not turn fully on until Vbe of ~1.2V is reached. When Q1 is turned on, the drive signal must be inverted, since we want the transistor to conduct when the battery voltage is low. This requires another transistor and gives the ability to add a large low pass RC filter to Q2 gate. The filter hinders the LEDs to constantly flicker during flight. Battery voltage fluctuates much more than you would normally imagine.