Electronics is the branch of physics that uses small components to control electric current, especially to process information and make decisions. In CSEC Physics you meet the difference between conductors and semiconductors, the diode, simple logic gates and a few input components used in control circuits.
Conductors, insulators and semiconductors
- Conductors (metals, graphite) allow current to flow easily because they have free electrons.
- Insulators (plastic, glass, rubber) do not let current flow.
- Semiconductors (such as silicon) lie in between: they conduct only a little, but their conductivity can be controlled — which is what makes them the basis of all modern electronics.
The diode
A diode is a component that allows current to flow in one direction only. When connected the right way round (forward biased) it conducts; the wrong way round (reverse biased) it blocks the current. This one-way behaviour is used in rectification — converting alternating current (a.c.), which keeps reversing, into direct current (d.c.), which flows one way. A single diode gives half-wave rectification; an arrangement of four diodes (a bridge) gives full-wave rectification.
A light-emitting diode (LED) is a diode that gives out light when current flows through it. LEDs are used as indicator lamps and in displays because they are small, robust and use very little energy.
Input transducers (sensors)
Control circuits respond to the surroundings using input components whose resistance changes:
- A thermistor has a resistance that decreases as temperature rises — used as a temperature sensor (e.g. in fire alarms, thermostats).
- A light-dependent resistor (LDR) has a resistance that decreases as light intensity rises — used as a light sensor (e.g. switching on street lights at dusk).
These sensors are used in a potential divider so that a change in light or temperature produces a change in voltage that the rest of the circuit can act on.
The transistor as a switch
A transistor is a semiconductor component that can act as an electronic switch: a small current (or voltage) at its base controls a much larger current through the rest of it. Combined with a thermistor or LDR, a transistor can switch a device (a lamp, a bell, a fan) on or off automatically when it gets too hot, too cold, too dark or too bright — the basis of automatic control systems.
Logic gates
Logic gates are switching circuits that make simple decisions. They work with two states, usually written 1 (high/on) and 0 (low/off). The main gates and their rules:
- NOT gate — one input; the output is the opposite of the input.
- AND gate — output is 1 only when both inputs are 1.
- OR gate — output is 1 when either (or both) input is 1.
- NAND gate — an AND followed by NOT (output 0 only when both inputs are 1).
- NOR gate — an OR followed by NOT (output 1 only when both inputs are 0).
The behaviour of a gate is summarised in a truth table, which lists the output for every possible combination of inputs. For example, an AND gate's truth table gives output 1 only on the row where both inputs are 1.
Logic gates are combined to build useful systems — for instance, an AND gate fed by a "dark" sensor and a "movement" sensor could switch on a security light only when it is both dark and someone is present.
The potential divider — how sensors are used
A sensor only becomes useful when its change in resistance is turned into a change in voltage that a circuit can act on. This is done with a potential divider: two resistors in series across a supply, with the output voltage taken across one of them. The supply voltage is shared between the two resistors in proportion to their resistances, so:
- if one resistor is an LDR, then in the dark its resistance is high and it takes a large share of the voltage; in bright light its resistance falls and its share drops;
- if one resistor is a thermistor, the output voltage changes as the temperature changes in the same way.
That changing voltage is then fed to a transistor or logic gate, which switches a device on or off. This is the heart of an automatic system such as a street light that comes on at dusk, or a fan that switches on when a room gets too warm.
A worked control circuit
Consider a fire alarm. A thermistor and a fixed resistor form a potential divider connected to a transistor. At normal temperatures the thermistor's resistance is high, the voltage fed to the transistor is low, and the transistor is off, so the alarm is silent. When a fire raises the temperature, the thermistor's resistance falls, the voltage across the other resistor rises, and once it is high enough the transistor switches on, allowing current to flow through the alarm bell. Tracing the chain — sensor → potential divider → transistor → output — is exactly the kind of reasoning CSEC control-systems questions reward, and it shows how the separate components in this topic work together.
Common exam mistakes
- Saying a diode lets current flow both ways — it conducts in one direction only.
- Mixing up the sensors — both the thermistor and LDR have resistance that falls as the condition (heat/light) increases.
- Errors in truth tables, especially confusing AND (needs both) with OR (needs either).
- Forgetting that a transistor uses a small current to control a large one.
Key terms to remember
- Semiconductor — a material (e.g. silicon) whose conductivity is between a conductor and an insulator and can be controlled.
- Diode — a component that allows current to flow in one direction only.
- Rectification — converting alternating current (a.c.) to direct current (d.c.) using diodes.
- LED — a light-emitting diode; gives out light when current flows.
- Thermistor — a sensor whose resistance falls as temperature rises.
- LDR — a light-dependent resistor; resistance falls as light increases.
- Transistor — a component that acts as a switch; a small current controls a large one.
- Potential divider — two series resistors that share the voltage, used to give a sensor an output voltage.
- Logic gate — a switching circuit (NOT, AND, OR, NAND, NOR) described by a truth table.
Quick recap
- Semiconductors (e.g. silicon) have controllable conductivity and underpin electronics.
- A diode conducts one way only — used to rectify a.c. to d.c.; an LED emits light.
- Thermistor (resistance falls as temperature rises) and LDR (resistance falls as light rises) are input sensors.
- A transistor acts as a switch — a small input current controls a large output current — enabling automatic control.
- Logic gates (NOT, AND, OR, NAND, NOR) make decisions; describe each with a truth table.