Arduino nixie v41 Operating Instructions Manual page 15

Once all the components are on the board, hook up the power, and check that the power
LED comes on.
Check also that the voltage is 5V between the "GND" test point and the "VCC" test point
Test Step
and at the power connector.
If the LED does not come on, turn off immediately to avoid damage to the components.
Check your soldering and the polarity of the components.
If the components are in the right way, connect the power again, and check that the 7805
Trouble
voltage regulator does not get hot. If it does not, measure the voltages in the low voltage
shooting
circuit.
Measure the voltage at the input ("Vin") and at the cathode side of D1 (nearest the centre
of the board). This should measure 0.7V less than the input voltage.
If all is well, proceed to the next step. If not, check carefully the orientation of the components and the
power leads. Diode D1 protects the board from having the power connected inverted.
If the LED comes on, check for a few seconds that the 7805 does not heat up. It should stay almost cold.
Hint: The 220 uF capacitor
The electrolytic capacitor has a stripe on
it to denote the negative side of the
capacitor. The positive side of the
capacitor (which goes into the "+" on the
board) is the other one!
Hint: The LED orientation
The LED has one lead longer than the
other, and a flat on one side. The side
with the shorter lead (the cathode) goes
into the hole on the board nearest the
diode.
The LED should look something like this:
220uF capacitor "stripe"
The LED