Describe switches and variable resistors as input devices and explain how a switch with a pull-down resistor gives a logic input

What this dot point is asking

SEAB wants you to describe switches and variable resistors as input devices, and to explain how a switch combined with a pull-down resistor produces a clean logic input. The central insight is that a switch on its own leaves a logic input floating and undefined, so a pull-down (or pull-up) resistor is added to force a definite logic 0 or logic 1, and a variable resistor lets a user set an analogue input by hand.

The answer

The switch as an input

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.

The floating-input problem

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 pull-down resistor

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

• Switch open: the pull-down resistor holds the input at $0\ \text{V}$, a clean logic 0. (Only a tiny current would flow, so almost no power is wasted.)
• Switch closed: the input is connected to the supply through the switch, giving logic 1. The pull-down resistor limits the current that flows to $0\ \text{V}$.

A pull-up resistor does the opposite: it holds the input at logic 1 when the switch is open and the switch pulls it to logic 0 when pressed. Either way, the resistor guarantees a defined level in both states.

The variable resistor as an input

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.

Examples in context

Example 1. A reset button on a logic board. A push switch with a $10\ \text{k}\Omega$ pull-down feeds the reset input of a counter. While unpressed, the pull-down holds the input at logic 0 so the counter runs; pressing the button takes the input to logic 1 and resets it. The resistor is what makes the unpressed state reliable rather than random.

Example 2. A joystick or slider control. A potentiometer turned by a slider provides a smoothly varying voltage that tells a circuit a position, such as a fader on a sound desk. As the slider moves, the tap point changes and the output voltage tracks it, giving an analogue input set entirely by the user's hand.

Try this

• Cue. State what is meant by a floating logic input. An input connected to nothing, so its voltage is undefined and may pick up noise, giving an unpredictable logic level.

• Cue. With a pull-up resistor and a push switch to $0\ \text{V}$, state the logic level when the switch is open. The pull-up holds the input at logic 1 when the switch is open; pressing the switch pulls it to logic 0.

• Cue. Name the two ways a variable resistor can be connected and what each adjusts. As a rheostat it adjusts current; as a potentiometer it adjusts a tapped-off voltage.