Nuts & Volts - May 2007

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.

Mechanical Assembly

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 shoulder washer.

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.