The Mysterious Magic Box — Part 2
oscillator to LED driver to verify its
operation. It will look like it is continuously on with the LED driver; it should
be flashing at about 60 Hz.
Complete the assembly of the
sensor circuits by adding 0.1 µF, 0.01 µF,
or 0.02 µF, the sensors, NOR gates (U7,
and U15 for the six pawn version), and
pull-up resistors (R1-R4, R22, and R23). It
works best to install the sensors last. First
bend the sensor leads at a 90-degree
angle (see Figure 10 again). Insert the
sensor leads into a 1/4” spacer.
This is an absolute must and is vitally crucial to ensure good reliable operation. Insert a Hall effect sensor into the
appropriate white plastic spacer and
bend the leads over with the sensor’s
nomenclature side face up so that the
sensor’s body just barely overlaps the
edge of the spacer. This ensures a proper length that the sensor will jut out to
coincide with the maximum magnetic
field from the magnet directly above it.
Next insert the leads into the corresponding pads on the underside of
the PCB (the side without a silkscreen).
The spacers will be near the capacitor
and the pull-up resistor leads solder
pads, so it is easier to solder these
components in place before installing
the sensor. If you install each sensor
correctly, it will “point” to the center of
the PCB (see Figure 15). The silkscreen
outline for the sensor location has a
chamfered corner to denote pin 1 (see
Figure 16). When you look at the face
of the sensor with printing and with
the leads pointed down, pin 1 is on the
left, pin 2 is the center pin, and pin 3
is the pin on the right (see Figure 17).
Test each sensor circuit at the
output (pin 3) with a DMM or use a test
lead to jumper the output (pin 3) to the
LED driver. The presence of a sufficiently strong magnetic field with the close
proximity of one of your assembled
pawns makes the output of the sensor
short to ground. The LED remains on
until the sensor detects the magnet.
When the sensor detects the magnet,
the output goes low, turning off the LED.
Assemble logic (U1, U2, U3, and
U4), pull-up resistors (R11), and pull-down resistors (R15, R24, R25). Test
this by connecting the 9 VDC power
■ FIGURE 15. The Hall
effect sensors all properly
pointing inward toward the
supply and observing the
LED. It should be on for
three counts at the start of
the sequence and one
count for each sensor
You can accomplish
this by holding one magnet
close to each sensor and observing
the pulse timing change. There should
be one long pulse for the marker pulse
and one short pulse for the detected
magnet. The short pulse time relative
to the long pulse will change depending upon which sensor you activate.
the ends of wires) to improve their
soldering ability during assembly. Most
often you do this using a solder pot (as
we have done for you in the kit). The terminal block is a Eurostyle 5 mm (.197”)
pin-to-pin spacing model. It uses 0.05”
round pins instead of 0.025” square pins
like wire-wrapping terminals have.
Adding the Tingling
Circuit (T1 and TB1)
Take the two wires (as in the kit)
and unscrew the terminal block’s terminals. Now slide the pre-tinned bare
ends of the wires underneath each
terminal. Tighten the screws so the terminal block securely holds the wires.
There are two brass screws on the two
back hinges. Unscrew these and temporarily remove their nuts to slip the
lug over the screw. Take the other end
of each wire with the #6 lugs. Attach
the nuts back onto these two screws.
These are for the transformer’s
output, and provide the tingling
sensation for missing pawn
detection by the tactile method.
Now you can push-fit the top
of the terminal block onto the
two pins that you soldered onto
the PC board (see Figure 18).
The transformer’s output
attaches to the terminal block. It
holds two conductors or wires.
Each wire needs a #6 lug on one
end and a pre-tinned bare wire
on the other end to prevent
corrosion. Pre-tinning is applying
solder to metallic parts (usually
A Two-Part Terminal Block
The terminal block’s upper
portion slides over these two pins.
This conveniently allows you to slide
off the two wires which — after you
unscrew the collet of the LED on the
enhanced version — allow you to
temporarily remove the PCB from the
base. This might prove handy when
you change the nine volt batteries.
More Foolproof Instructions
It does not matter which way you
attach the upper portion of the termi-
■ FIGURE 16. Here is the PCB
component outline drawing/
May 2007 53