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How I Made: Time-to-go-clock

FEATURE

• Red Turns off red LEDs


• Green Turns off green LEDs


• Blue Turns off blue LEDs


• Edit Mode Puts clock in edit mode
  (effectively, this changes the operation
  of the rotary encoder so it can be
  used to change the future event
  being monitored)

Frankly, if there were
fewer cut-outs, I could
have dispensed with
some of these or assigned
them to the encoder. But
their convenience means
they do get used more,
especially changing the
LED backlight colours.
Finally, I installed the rotary encoder,
LCD dimming potentiometer, and motion
detector in various other existing or newly
created holes.

STAYING INFORMED
You can see the code at hsmag.cc/nSTnRR.
A not inconsiderable advantage of buying
the GRA and AFCH driver board is that

they offer some sample code to run a clock

using the board. That said, since I needed to

make some fairly significant modifications to

this code, it was important to start with an

understanding of how their code works.

First, some basics. Nixie tubes work in a

similar way to an array of ten LEDs with a

common anode (at least, for a tube that can

display the digits 0–9). Each LED represents

one digit, so you can light any LED/digit by

applying the requisite voltage to the anode,

then grounding the cathode of the LED you

want to display. In this arrangement, you can

just connect each LED cathode to an Arduino

pin (through a resistor, of course) and drive

the appropriate pin low to turn on that LED.

Since the Arduino keeps pretty good track

of elapsed time, switching on each LED

sequentially in one-second intervals is not

much more complicated. Already we are well

on the way to making a clock! To make a

working Nixie-based clock, however, we still

need to overcome three obstacles:

• Nixie tubes require around 180 V DC to
  light up; an Arduino cannot source or
  switch anything like that.


• A clock displaying hours, minutes, and
  seconds requires control of six tubes. If
  we persist with the approach
  above, we’ll need 60 pins just
  for switching the Nixies; even
  an Arduino Mega has only 54
  I/O pins.
  
  
  • While the Arduino keeps
    pretty good track of elapsed
    time, it doesn’t actually know
    what time it is.
  
  
  
  

Since we’ve already moved on to

discussing the code, let’s just note that the

driver board solves obstacle one by stepping

up the 12 V from the Arduino’s Vin pin to the

required voltage.

The second obstacle is overcome by a

mixture of hardware and software. On the

hardware side, the driver board has three

shift registers, each with 20 high-voltage

outputs. Connected in series, these registers

can therefore do the necessary switching of

all six tubes without multiplexing.

On the software side, we need to tell the

registers which of their pins to switch. This

is done by converting the digits to display

on the clock (held in the stringToDisplay

variable) to a 64-bit variable, then sending

that data via SPI in eight byte-sized chunks to

the registers. This is handled by some clever

bit manipulation in the doIndication function

(courtesy of GRA and AFCH).

Last, we can crack the third problem

by using a library of time-related functions

(TimeLib) and getting the current time from an

internet time server.

Computers, microcontrollers, and the like

establish what time it is by counting the

number of seconds from a reference point, for

example 1 January 1970. The TimeLib library

helps deal with the tedious mental arithmetic

needed to figure out what the time and date

is if, for example, it is 1 516 365 708 seconds

The clock’s defining feature, broadly hinted at

in the name, is that it can display the time to

any future event

Above

The LCD screen

can display any

messages you want