The 10:1 voltage divider can be used with the optional dialog part for the ATmega328 withoutthe activated DC-DC converter for the zener diode measurement. Without the pressed key thevoltage converter is not powered. For that the external voltage (for example battery voltage) can bemeasured at the zener diode port. You can only measure positiv DC voltages up to 50V . You havealso to respect the correct polarity.