1) Get eight decimal thumbwheel switches.
2) Collect nine identical resistors for each switch, from 1 ohm through 10 megohms. (E.g., nine 1-ohm resistors for the first, nine 10-ohm resistors for the next, and so on.)
3) Set aside one resistor in each set. Bend the leads of the other eight resistors to fit across sequentially numbered pins (e.g., from pins 1 to 2), solder them in place, and trim the leads (see inset).
4) Solder the unbent resistor onto pins 4 and 5 and a piece of wire from pin 9 to pin C. Mark each switch’s magnitude.
5) Stack each switch in order (from high to low), slip a wire through each remaining pin, and solder all of the connections.
6) Secure one wire to the last open pin at the stack’s top and another wire to its bottom. Connect each wire to a banana jack.
7) Wire the box into your circuit, dial in the value you need, and say good-bye to futile resistance.
Time: 5 hours
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.