Electronic Systems: Sensors, Logic and Components
Analogue and Digital Signals
Electronic systems process information as signals. There are two types: analogue signals vary continuously — like the voltage from a thermistor that rises and falls as temperature changes. Digital signals have only two states: HIGH (logic 1) or LOW (logic 0). A button that is either pressed or not pressed produces a digital signal.
Most sensors produce analogue outputs. To use an analogue sensor with a digital circuit (such as a microcontroller), an analogue-to-digital converter (ADC) is needed. The ADC samples the voltage at regular intervals and converts it into a binary number. Without an ADC, the microcontroller cannot read the sensor. ADC questions are often left blank — memorising this definition is an easy mark.
An ADC (analogue-to-digital converter) converts a continuous analogue voltage into a digital binary number that a microcontroller or processor can read. Sensors that produce analogue outputs (LDR, thermistor, microphone) all need an ADC to interface with digital control systems.
Sensors
Sensors are input devices that convert a physical quantity into an electrical signal. The two most common in engineering exams are:
- LDR (light-dependent resistor) — its resistance decreases as light intensity increases. In bright light: low resistance → high current. In the dark: high resistance → low current. Used in automatic street lighting, security systems and photocopiers.
- Thermistor — its resistance decreases as temperature increases (NTC type — negative temperature coefficient). Used in thermostats, engine management systems and fire alarms. A thermistor in a potential divider circuit produces an analogue voltage that changes with temperature.
Other sensors include pressure sensors (transducers), proximity sensors (inductive or capacitive), and sound/vibration sensors (microphones, piezo sensors). All produce an electrical signal proportional to the physical quantity they measure.
Logic Gates
Logic gates are the fundamental building blocks of digital electronic circuits. Each gate has one or more binary inputs (1 or 0) and produces a single binary output determined by a logical rule. You must be able to name the gate, draw its symbol and complete its truth table.
AND gate: output is HIGH only when ALL inputs are HIGH. Rule: “both A AND B must be 1 for output to be 1.”
| A | B | Output |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
OR gate: output is HIGH if ANY input is HIGH. Rule: “A OR B (or both) is 1 for output to be 1.”
| A | B | Output |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 1 |
NOT gate (inverter): single input, single output — output is always the opposite of the input.
| Input | Output |
|---|---|
| 0 | 1 |
| 1 | 0 |
Extension: the specification requires AND, OR and NOT logic only. NAND, NOR and XOR are included here as background because they appear in real circuits.
NAND gate = AND + NOT. Output is LOW only when ALL inputs are HIGH; otherwise HIGH. It is the inverse of AND. NAND gates are the most common gate in integrated circuits — any logic function can be built from NAND gates alone.
NOR gate = OR + NOT. Output is HIGH only when ALL inputs are LOW; otherwise LOW. The inverse of OR. Like NAND, any logic function can be built from NOR gates alone.
XOR gate (exclusive OR): output is HIGH only when inputs are DIFFERENT (one is 0 and the other is 1). When both inputs are the same (both 0 or both 1), output is LOW. Used in comparator circuits and error-detection systems.
Passive Components: Resistors, Capacitors and Diodes
You must be able to identify these components by their circuit symbols and describe their function:
- Fixed resistor — limits current in a circuit. Symbol: rectangular box (BS 3939 UK) or zigzag. Value in ohms (Ω). Used in series with LEDs to prevent burnout.
- Variable resistor (rheostat) — a resistor whose value can be adjusted mechanically. Used in dimmer switches, volume controls and sensor potential dividers. Symbol: rectangular box with an arrow through it.
- Capacitor — stores electrical charge. Polarised capacitors (electrolytic) must be connected the correct way round; non-polarised capacitors can be connected either way. Used for smoothing rectified DC and as timing components.
- Diode — allows current to flow in one direction only. The triangle symbol points in the direction of conventional current flow; the bar is the cathode (negative terminal). Signal diodes are small and fast; rectifying diodes handle larger currents in PSU circuits.
- LED (light-emitting diode) — a diode that emits light when current flows through it. Must be connected the correct way round (longer leg = anode = positive). Always used with a series resistor to limit current. Output devices in control panels and status indicators.
Common exam error: students identify a transistor symbol as a resistor. A resistor symbol is a box (or zigzag). A transistor symbol has three connections — collector, base and emitter — with an arrow on the emitter. A diode symbol is a triangle pointing to a bar. Memorise these three symbols and you eliminate a common source of lost marks.