garage, all displaying the exact same correct time.
You can expect to spend around $35-$75 or more to
build this project depending on your junk box, building
your own shield and power supply, or where you buy your
Arduino. The impulse secondary driver circuit shown in
Schematic 2 uses the IRFZ44N — an IGBT device. This
transistor is perfect here and for other control projects
because the gates of these devices can be connected
directly to the Arduino's output pin and driven with 4. 5 to
10 volt logic levels. The LEDs and their resistors are
actually optional, but they let you know if and when an
Arduino's output is active.
Either the Arduino UNO or Mega 2560 model can be
used for a master clock, along with an Ethernet shield.
Both Arduino and SainSmart Ethernet shields are
even after the
the use of the
Protocol (NTP) to
timer (system clock)
once each hour.
Neither of the
clocks are accurate
enough to be a clock time base (not even close).
The IBM master clock with an Invar steel pendulum
and compensating mercury pendulum weight (like mine)
had a guaranteed accuracy of 10 seconds per month back
in the 1930s. Because the Arduino's system timers gain or
lose about a second per hour with current code (around
12 minutes per month), an external time base source is
required. In fact, my first attempt on the UNO was
17 seconds slow per hour. So, after looking at GPS,
temperature controlled crystal oscillators, real time clocks
(RTC), WWV (radio) modules, and an Internet Time Server
as possible sources, I chose the cheapest and most
maintenance free over time: the time server.
The Ethernet shield accesses the Wide Area Network
(WAN) with an NTP time source request from the official
US government's time keepers: the National Institutes of
Standards and Technology's (NIST) Internet time server.
This is an accurate and reliable clock time base
choice for the Arduino master. The Arduino's
system timer (clock) is still used, but its timer is
corrected every hour by the NTP time request two
minutes before synchronizing all the clock
movements at the top of each hour.
In other words, the master clock will never be
off more than about a half second, regardless of
temperature, voltage, crystal aging, and other
variations. Bear in mind the impulse secondary has
a resolution of one minute, not one second. Only
the master clock has a resolution of one second.
The impulse secondary will only be on time for
one second, then will become up to 59 seconds
slow until it is brought forward at the next impulse,
at zero seconds of the next minute.
There is useful information at http://tf.nist
.gov/tf-cgi/ servers.cgi. Be sure to heed their
warning that: "All users should ensure that their
software NEVER queries a server more frequently
than once every four seconds. Systems that
34 January 2015
■ PHOTO 9. Arduino Mega 2650 with
■ SCHEMATIC 2.
IRFZ44N, IGBT line
■ PHOTO 8.Arduino UNO with fabricated driver shield.