It consists of four buttons "left" "right" "input (ENTER)"and "EXIT". The circuit is independent of the input button only, because this is also the role of the power switch.

I will introduce about the switch itself. The photo is to disassemble the switch of type "tact switch".

I tried to create a diagram of the mechanism of the tact switch. A dome shaped disk on top of the terminal was placed at regular intervals. When the pin does not touch anything is the state of the "off" switch. A constant force is applied to the disk, is connected two contacts and the electricity will flow. This is the state of the "on" switch. The disk has an elasticity, in the absence of focus will return to the original. In case of NXT, does not tick when you press the button. So, it is likely to hit the pin material and the conductive rubber is being used instead of metal.

Consider the circuit for reading the state of the switch.
"Schematics" I thought about is here in my own style.

Use the tact switch and "bread board", I tried to reproduce this circuit.
The breadboard is a foundation that can be inserted into the part. You can have led to a hole in a given direction, there electrified.

Press the switch and view, there was a reaction to the voltmeter. Voltmeter has shown "5.31V (read bolt) ".

I tried to create a diagram of the state switch is turned on. Circuit is connected; the battery has been connected to the voltmeter. The value of the voltmeter showed "5.31V" will be called "battery voltage". Then, has always been a critical, voltmeter has very large "resistance". Therefore, even if the switch is turned on, "the electricity will not flow" almost completely. Unlike the Voltmeter, light bulb or motor is a low-resistance part. Since it is less resistance, large amounts of electricity will flow. The "resistance" is introduced in the next page.