■ FIGURE 7. A timing diagram of all six
Hall effect sensors and the starting bit.
two-input NOR gates, U15, is not needed, so it is not installed. This leaves two
of the multiplexer channels floating, so
they are tied low through two additional pull-down resistors, R18 and R19.
■ FIGURE 8. A timing diagram of all six
Hall effect sensors and the starting bit
showing a missing pawn in Position #3.
or low. The fifth, seventh, ninth, 11th,
13th, and 15th channels connect to
the NOR gate’s outputs. The inputs to
the NOR gates are the 60 Hz
oscillator and one each of the Hall
effect sensors. With no magnetic field
present, the sensor is inactive and the
output is pulled high. The high state
blocks the 60 Hz oscillator signal from
appearing at the multiplexer input.
When the sensors are in the presence of a sufficiently strong magnetic
field, the output switches low. The low
state allows the 60 Hz oscillation to
pass through to the multiplexer input.
gate is used to invert the most significant output of the counter to the disable input of the first 8-to-1 multiplexer. Thus, the first 8-to-1 multiplexer is
only selected for the first eight counts.
The noninverted most significant
output is routed to the disable input of
the second 8-to-1 multiplexer. The
second multiplexer is therefore selected for the remaining ninth through
16th counts. The two multiplexer’s
inverted outputs are connected to a
NAND gate. Inverting the inputs of a
NAND is equivalent to an OR gate, so
the NAND gate passes the output signal of whichever multiplexer is active.
Quick Summary
To summarize the differences in
the two versions: Install resistors R18,
R19, R20, and R21 for the four-sensor
version, but omit them in the six-sensor version. For the six-sensor version only, you install Hall effect sensors
U13 and U14, associated pull-ups, and
capacitors (R22, R23, C13, C14, C15,
and C16), U15 (the second package of
two-input NOR gates), R23, and R24.
The Assembly and
Testing Phase
The Electronic Detection Process
The resultant effect is one pulse of
60 Hz (which appears to your eye as
a continuous illumination) about three
seconds long. (Again, refer to the
sidebar on CFF for a more detailed
discussion of flicker rates). This marks
the start of the sequence. There are six
distinct times for pulses of one second
duration; one for each of the sensor
circuits (see Figure 7).
If one of the pawns is missing (for
example, number three), there will be
a missing pulse (see Figure 8).
If you have purchased a kit, you
begin this phase by removing the
parts from the box and taking a
cursory and then more detailed parts
inventory. Next, you will assemble the
wooden cradle, pawns, top panel,
acrylic sheet, and install the felt and
paper pads. You then assemble the
PCB and finish by testing the project.
Multiplexing
We created the 16-to-1 multiplexer by combining two 8-to-1 multiplexers with two NAND gates. One NAND
The Four-Sensor Version
This version is slightly different. The
counter is reset after 12 states, so the
multiplexer, in effect, becomes a 12-to-
1 multiplexer. The circuit remains a 16-
to-1 multiplexer, however, four channels
are never selected. The counter reset
occurs when the two highest significant
bits (the “eight” and “four” outputs) go
high at the same time. These two
outputs are gated through a NAND to
create the counter reset signal.
A common PCB can cleverly accommodate both of these two circuit’s
requirements. The counter reset for the
four-sensor system is used when the two
most significant outputs are individually
jumpered (the jumpers are labeled R20
and R21) to the counter reset NAND
circuit. The counter reset NAND circuit
is disabled for the six-sensor system by
leaving out the jumpers (R20 and
R21) for the inputs of the NAND
and installing pull-down resistors
R24 and R25 instead.
When there are only four
sensors, the second package of
■ FIGURE 9. The stripped and
pre-tinned wire and optical cable.
Ease of Kit Assembly
To facilitate assembly and testing,
we pre-cut the optical fiber cable and
wires, then pre-stripped and tinned
(solder dipped) one end of each of the
two wires and attached a #6 lug on
the other (see Figure 9). We pre-cut
all wooden pieces and pre-drilled
the pawns’ bases for easier magnet
insertion.
There is an LED built in for test
purposes on both versions. This test
LED does double-duty on the
enhanced version. It is an LED with a
shroud and optical fiber that also
serves as the missing pawn visual
detector. The LED on the standard
four-pawn version is a stand-alone
purplish-blue LED that you observe
during the check-out procedure. It is
not visible to the outside world as part
of visual detection scheme.
50
May 2007
