Quick questions on Switches and variable resistors explained: O-Level Electronics Sensors and Transducers

A switch is the simplest input device: it makes or breaks a connection. Common types include the push-to-make switch (closed only while pressed), the toggle switch (stays in position), and reed or tilt switches that respond to a magnet or to movement. A switch gives a two-state input, ideal for digital circuits, but it must be wired so the circuit sees a definite voltage in both states.

If a logic gate input is connected only to a switch, then when the switch is open the input is connected to nothing. This is called a floating input: its voltage is undefined and can drift or pick up electrical noise, so the gate may read an unpredictable 0 or 1. A floating input is a fault, not a logic 0.

The cure is a pull-down resistor connecting the gate input to $0\ \text{V}$, with the switch connecting the input to the supply:

A variable resistor lets a user adjust an input by hand. Used as a rheostat (two terminals), it changes the current in a series circuit, for example to dim a lamp. Used as a potentiometer (three terminals), it is an adjustable potential divider that taps off a variable voltage, for example a volume or brightness control feeding an amplifier. The potentiometer is the usual way to provide an adjustable analogue input.