Arduino nixie v41 Operating Instructions Manual page 11

FRONT These are the controls that go on the front panel: The input button
and the Light Dependent Resistor to detect ambient light.
GND: The "ground". One lead of the button and one lead of the LDR
and one lead of the button are connected to this.
BTN1: The other lead of the button is connected to this input
DLS: "Dimming LDR Sense": The other lead of the LDR is connected
to this
VCC: Regulated 5V output to drive any LEDs or lighting. Note that
you can also connect the LEDs to the VIN if you want to reduce the
load of the regulator.
LED The LEDs are connected to these sink (they consume current, not
source it) terminals. To connect up you take the positive sides of the
LEDs to either VIN or VCC and connect the negative sides of the
LEDs to these terminals.
R: PWM cathode connection for the back light RED channel.
G: PWM cathode connection for the back light GREEN channel.
B: PWM cathode connection for the back light BLUE channel.
T: PWM Cathode connection for the blinking "tick" LED.
RTC The connection for the RTC (Real Time Clock) module. Connect this
to the appropriately marked terminals on the RTC module.
CATHODES The terminals to the cathodes (individual digits "0" - "9") for each
tube. Terminal "0" drives the cathode "0" and so on.
ANODES The terminals to the anodes for each tube. The allocation of anode
to tube is:
1. Tens of hours
2. Hours
3. Tens of Minutes
4. Minutes
5. Tens of seconds
6. Seconds