Nuts & Volts - May 2007

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 PCB’s center.

supply and observing the LED. It should be on for three counts at the start of the sequence and one count for each sensor location activated.

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.