Input and output sub-systems: sensors and input transducers (LDR, thermistor, switches) in potential dividers, and output transducers (LED, buzzer, relay, motor) with their driver and interfacing requirements.

What this dot point is asking

A system is only as good as the way it senses the world and acts on it. The specification expects you to know the common input transducers, how they sit in a potential divider to produce a usable voltage, and the common output transducers with the driver and protection each one needs. Examiners pair this with the potential divider and the transistor switch constantly, because the input and output blocks are where most marks for "name a suitable sub-system" live.

The answer

Input transducers

On their own these devices give a changing resistance, which a circuit cannot act on directly. The fix is to put the transducer in a potential divider so the resistance change becomes a voltage change. Which way the output moves depends on whether the transducer is the top or bottom resistor.

Sensors in a potential divider

A variable (preset) resistor is often used as the fixed element so the switching threshold can be calibrated.

Output transducers and drivers

Why interfacing is essential

The output of a processing block is typically a clean logic level or comparator output at a few milliamps. A motor or lamp may need hundreds of milliamps. The transistor switch bridges this gap: a small base current controls a large collector current, so the small signal commands the large load.

Examples in context

Example 1. A frost alarm

A thermistor placed as the bottom resistor of a divider makes the output rise as temperature falls (its resistance rises in the cold). A comparator switches when the output crosses a preset, driving a transistor and buzzer. The whole alarm is three blocks, and the input sub-system is just the thermistor divider.

Example 2. A motorised blind

A light sensor divider feeds a comparator that, through a transistor and relay, switches a motor to lower the blind in bright sun. The relay both supplies the heavy motor current and isolates the low-voltage electronics, and a flyback diode protects the transistor from the motor's inductive kick.

Example 3. A door-entry indicator

A push switch pulls a logic input low when pressed (input), a logic block latches the state (process), and an LED with its series resistor shows the door is unlocked (output). Here the input transducer is simply a switch, but it still needs a pull-up resistor so the logic sees a definite high when the switch is open.

Try this

Q1. An LDR is the top resistor of a potential divider. State whether the output voltage rises or falls as the surroundings get darker, and explain why. [2 marks]

• Cue. Darker means the LDR resistance rises; with the LDR on top, a larger top resistor passes less voltage to the output, so the output falls.

Q2. Give two reasons a transistor is used between a logic gate and a small DC motor. [2 marks]

• Cue. The motor needs more current than the gate can supply (the transistor amplifies the current), and the transistor isolates the delicate gate from the motor's larger, noisier load.