Step 1: Gather parts!
15 circuit boards
15 LM324 quad op amp
15 14-pin sockets
60 white LEDs
60 induction coils*
* Currently I am using coils from Digikey part number M10013-ND. The inductance is 4.7mH. They do not look like the ones pictured in this instructable.
60 10 ohm resistors
60 10M ohm resistors
60 1K ohm resistors
30 100K ohm resistors
15 10 μF capacitors
60 0.1 μF capacitors
You could change these numbers if you want to make a smaller table. There will be 1 quad op amp, 4 LEDs and 4 induction coils for each board.
For the table:
1/4 inch translucent white acrylic 33" x 21"
1/4 inch clear acrylic 33" x 21"
For op amps I am using a quad op amp LM324, but you can use any op amp chip.
Step 2: Plan circuit
You can do this on a perfboard or a printed circuit board if you want to be fancier. I am selling printed circuit boards , please see my website for details on ordering boards.
Essentially the coil picks up a voltage, which is amplified by the op amp, which drives an LED.
I have placed the coil next to the LED so there is more of a one-to-one correspondence between detected field strength and LED brightness.
See a legible version of the circuit diagram here: http://andydoro.com/table/images/circuit.gif
Step 3: Assemble circuit
You can also use perf board.
The board should run on between 3 to 5 V DC current. Use a voltage regulator (7805 or 3940) in order to get a steady voltage.