The Mysterious Magic Box — Part 2
Power Conserving CMOS ICs
This project’s ICs — including Hall
effect sensors — are all CMOS. The two
555 timer ICs can also operate up to 2
MHz. The CMOS high input impedance
allows you to use smaller timing capacitors than you would normally use on
the non-CMOS NE555. This results in
greater accuracy in time delays and
oscillations. Lower power consumption
across the full range of power supply
voltages is the best feature for battery-operated applications like this.
NOTE: Most instructions assume
the six pawn version with the LED
holder and optical fiber cable. For
the four pawn version, substitute the
regular LED and ignore instructions
related to the optical fiber option.
Insert the magnets in the pre-drilled base of each pawn. If you drill
the pawn holes too deeply, fill them in
with wood putty — until the magnets
are flush with the outside surface.
Place a felt pad over each magnet
to conceal it to help preserve the trick.
Temporarily remove the brass
screws and nuts from the rear hinges.
Assemble the three wooden
pieces with the six Escutcheon pins,
small brads, or nails provided. Ensure
that the piece with the recessed
drilled hole faces to the outside and
appears on the right.
Attach and shrink the 1/8” shrink
tubing at one end of the optical cable.
Cut at 90 degrees the combined
optical cable and shrink tubing with a
blade as close to the end as practical.
Insert the shrink tubing end through
the narrow side of the (Nylon)
shoulder washer (see Figure 10) and
make it flush with the wider side.
Insert the narrow side of the
plastic (Nylon) shoulder washer (
molded Nylon insulator) into the piece with
the 1/4” recessed hole (to the right)
until the washer is flush with the wooden surface. This completes the cradle.
Insert the cradle into the box,
again, with the recessed hole or shoulder washer to the right. The optical
cable and hole should line up with the
hole in the side of the box. You can
test this by pointing the free end of the
optical cable at a light source and
observing the light in the hole from
the outside of the box.
Replace the brass screws and nuts
you initially removed from the back
hinges. It is best to hold the nut and turn
the screw from the back of the box with
a screwdriver, advancing the screw
within and securing the nut. If holding
the nut proves difficult, you may wish to
allow the shrink tubing to hold it as you
start the nut and get a turn or so going.
Remove the paper Alignment
pattern from the box’s lid if you have
a kit. (Align this pattern with the
acrylic sheet using the four corner
holes as reference points and
temporarily tape into place.)
The Alignment pattern inside the
box’s lid has three sets of intersecting
circles. The largest represents the peel-off adhesive backed paper patterns
used to conceal the electronics below.
The next two sized holes represent the
plastic hardware spacers through which
you pass the sensors so their respective
magnets will be able to activate only
that sensor. There are some crosshairs
that indicate where the active sensing
area of the Hall effect sensors’ facades
will reside once bent over. Bend all
sensors toward the middle in all
instances to intersect the crosshairs.
Precautionary Assembly Tips
The following tips are from our
empirical observations after assembling
this DIY project’s PCB numerous times.
• Tilt Sensor Leads. Leave these leads
long so that you can purposely short
them during test. This alleviates your
concern about tilting the PCB in just
the right way to power the project.
• Save Your Spent Leads. Some of the
resistors serve as bridges where resistors R10, R20, and R21 should have
gone. As the design evolved, it used a
different voltage regulator of just 3. 3
volts, so there was no longer a need
for a series limiting resistor (R10). Now
■ FIGURE 10. The nylon extended shaft
you bridge it with a spent clipped
resistor lead. Components R20 and
R21 are labeled as resistors, but are
actually jumpers that are used for the
four pawn version. These two jumpers
are not included in the six pawn version. You use two pull-down resistors,
R24 and R25, to disable the circuit
required in the four pawn version.
• Power Component Leads. Leave
these long. They will later serve as
convenient “hooks” for a probe when
you test the project’s power supply.
• Install the Battery Holder and
Transformer Last. We suggest you do
this, which will require you to use clip
leads and an external DC power supply
during testing of the project’s power
supply section. If you don’t have a DC
power supply on your bench, use clip
leads to your nine volt battery.
• Installing Capacitors. Exercise care
here by bending them over affording
clearance when you solder the Hall
effect sensor leads. This assumes you
install the capacitors first.
• Installing Resistors. We highly recommend you check all resistors with a
DMM since there are some 1% resistors
with five stripes; the extra stripe is a reliability code and this can be confusing.
• Look for Shorted Adjacent Pins.
Double-check the adjacent pins on
the Hall effect sensors and voltage
regulator for solder bridges. We
suggest using the continuity beeper
found on most DMMs to check their
very closely spaced 50 mil leads.
• Hall effect Sensors’ Crucial Lead
May 2007 51