exceed this rate will be refused service ... "
The Arduino-based version of the IBM minute impulse
self-regulating clock system works by outputting two
signals. Starting at the top of the hour, an A and B pulse
are presented for a duration of 0.4 seconds on, then off
once each minute, with the following exception. The A
signal is raised once per minute at zero seconds, and on
every odd second between 10 and 50 during the 59th
minute. Secondaries that are fast (due to mechanical
failure) will receive the A pulse once per minute until their
59th minute. Then, they switch from their N/C (A) contact
to their N/O (B) contact, and will only advance to the top
of the hour on the next B signal.
This secondary will switch back to the A contact
between four to 14 minutes after the hour, depending on
the model of the secondary. The B signal is raised once
per minute at zero seconds for each minute, except for
minutes 50 to 59. Secondaries that are late (due to
mechanical failure) will receive the A pulse once per
minute until the system is at the 59th minute, then receive
up to 21 rapid A pulses every other second until the late
secondary reaches its 59th minute. The secondary then
switches its contacts to the B contact and waits for the
next B pulse.
The circuit I use to drive the clocks is very simple and
uses an IGBT transistor to apply a ground to the
secondary's coil. I connect one side of the secondary's
coil through line C to + 24 volts (for 24 volt coils) and take
lines A and B to ground to operate the electromagnets.
This is the same logic as the wiring of the relay technology
wiring design shown in the 1938 IBM diagram in
Schematic 1. This allows me to use a common and cheap
high current IGBT transistor whose gate is turned on by
the Arduino's output pin (refer again to Schematic 2).
Perusing the IRFZ44N datasheet from jameco.com, you
will find the Source to Drain Current vs. Gate to Source
Voltage graph in the datasheet's Figure 3. Notice that 4. 5
volts at the gate should turn on the IGBT to about nine
amps — enough to run about 375 model 561-2 secondary
movements (Photo 4). This is more than the two amp limit
of IBM's electrical relay system could handle.
The IGBT source terminals are connected to the
ground terminal (the - 24 volt and - 5 volt power supply
return), and the drain terminals are connected to the line
A and line B terminals. I placed a jumper from the + 24
volts terminal to the line C terminal on the driver shield,
and included the LEDs and their limiting resistors at each
IGBT gate to show me the state of the lines. You will see
the A and B LEDs light each minute, the rapid A pulses
beginning at the 59th minute: 10 seconds, and the B LED
not illuminate between the 50-59th minute. The impulse
secondaries are all wired in parallel to complete the
system wiring: A to A, B to B, and C to C.
The Arduino's maximum sourced current per output
is 20 mA. Maximum total current for all outputs combined
is 150 mA, or 7-1/2 outputs. I believe the IGBT's gate
current load presented to an Arduino I/O pin by itself is so
small that it is negligible. It is the electromagnetic field that
turns the transistor on, and practically all of the I/O pin's
load in this circuit is from the LED-resister pair. Go to
Limitations for more information. The SparkFun
prototyping shield schematic shows a 330 ohm resistor
that limits the LED current to about 15 mA.
January 2015 35
ITEM PART# COST** SOURCE SOURCE
RS405L P/N 1541466 $0.89 ea Bridge Rectifier, 1,000V PIV, 600V 4A www.jameco.com/Jameco/Products/
L7805CV P/N 924633 $0.45 ea 5V Regulator, 1.5A www.jameco.com/Jameco/Products/
LM7824T P/N 51414 $0.35 ea 24V Regulator, 2.2A www.jameco.com/Jameco/Products/
IRFZ44N P/N 669951 $1.25 ea N Channel Power MOSFET, 55V, 49A, TO-220AB www.jameco.com/Jameco/Products/
OSTTA024163 P/N 152347 $0.69 ea Term Block 2 Pos, 5.08 mm Solder Thru-Hole 15A www.jameco.com/Jameco/Products/
LED see note P/N 1554161 $3.80 20pk Red LED, low current, 7 ma forward current. LED
and resistor optional. Use 680 ohm resistor to
limit max current to 7ma. www.jameco.com/Jameco/Products/
Resistor, LED P/N 690822 $0.99 10pk 680 ohm resistor to limit LED current to 7 ma www.jameco.com/Jameco/catalogs/
Perfboard P/N 616690 $5.95 ea 6" x 4. 5", 0.1" pitch, 0.042" dia holes www.jameco.com/Jameco/Products/
Arduino Ether P/N 2152375 $37.95 Arduino Ethernet shield w/o PoE* www.jameco.com/Jameco/Products/
SainSmart EthP/N W5100 ($19.99)/SainSmart Ethernet shield for UNO and Mega 2560
Proto shield P/N Dev-07914 $9.95 ea
Modification required to plug into Ethernet shield; refer to Photo 13.
LIST *Power over Ethernet Prices subject to change without notice.