A small box is constructed and fitted with four light emitting diodes (LED) and four toggle switches. The way in which the LEDs react to the switches depends on how the components are wired. Ideally, several of these devices would be constructed. While all of the devices would look identical, they could be wired differently. Students could then deduce the underlying circuitry by watching the behavior of the LEDs.
Three sample circuits can be found in Appendix B and Appendix C. The LEDs used in our devices have a forward voltage drop of 2.2 Volts and an operating current of 20 milliamps. If the LEDs you obtain vary greatly from these specifications, the resistors will need to be resized.
Don't be afraid to design your own circuits or even create 'expert' versions of this device using more than four switches and/or LEDs. Just remember that using a single 9 Volt battery limits the number of LEDs that can be wired in series on a single circuit branch to four, assuming each LED has a forward voltage drop of 2.2 Volts. Your mileage may vary, depending on the components you obtain.