Electronic systems

Input — a device that senses a change and produces a signal. Process — the thing that decides. Output — a device that acts on the world. Feedback — a path carrying a measurement of the output back to the process.

Because a circuit cannot act on a quantity. Name the device that turns it into a signal — a thermistor, an LDR, a microphone, a push switch.

Ask whether anything in the circuit measures the OUTPUT. A timer counts whatever happened; a thermistor inside an oven measures the result of the heating.

Input: a thermistor in the cavity. Process: a controller comparing with the dial setting. Output: the heating element. Feedback: the thermistor measures the air the element is heating.

By the sense they reach: seen (LED, lamp, LCD, braille display), heard (buzzer, speaker, headphones), felt (haptic), done (motor, relay), kept (printer, plotter).

Right when the user is not looking — every alarm depends on it. Wrong in a shared or quiet place, because it reaches everyone, and useless to a deaf user.

A pin supplies a few tens of milliamps; a motor needs hundreds and a mains load needs amps. The pin switches a transistor or relay, which carries the load current from the supply.

A flyback diode. When the current stops, the collapsing magnetic field produces a reverse voltage spike that would destroy the switching transistor.

An open-loop system acts on its input and nothing measures the result. A closed-loop system measures its output, feeds it back, compares it with what was asked and corrects the difference.

To let the system correct for everything the designer could not predict — a cold room, a heavy load, a slipping wheel, a worn part. Accuracy is the consequence, not the purpose.

Open: a toaster on a timer, a doorbell, fixed-cycle traffic lights. Closed: an oven with a thermistor, a cruise control, a lift, a powered wheelchair with wheel encoders.

A loop that corrects too hard overshoots, corrects back and oscillates. A dead band and gentler correction are the usual fixes.

A high-gain voltage amplifier with a non-inverting input (+), an inverting input (−) and a single output. It amplifies the difference between its two inputs.

Its open-loop gain is about 100,000, so a millivolt of difference saturates the output. Two feedback resistors return part of the output to the inverting input, and their ratio sets a usable gain.

Interference picked up along a cable and drift with temperature appear on both inputs equally, so amplifying the difference rejects them. That is why sensor leads run as pairs.

Between the sensor and the microcontroller in a smart thermostat, connected doorbell, fitness band or kitchen scale — amplifying and filtering a millivolt signal so the converter can read it.

A microcontroller and its software built into a larger product to carry out one specific task — one program for the life of the product, invisible to the user.

Functionality — things the product could not do before. Efficiency — using only what the job needs. Automation — making decisions the user used to make.

The useful behaviour often exists only between them. A thermostat, a boiler controller and radiator valves each do one small job; the heating system lives in the messages they exchange.

Bugs, security updates for the product's whole life, repairs needing software nobody else has, and features that die when a server is switched off. Keep the core function working without the network.

So a circuit drawn in one country can be read in another by someone who does not speak the language. The diagram shows what is connected to what, not what the board looks like.

A variable resistor is a resistor box with an arrow across it. An LED is a diode triangle-and-bar with arrows leaving it. A relay is a coil drawn beside a separate pair of contacts.

The wires are connected. Wires crossing without a dot are not connected — missing this is the commonest way a circuit is misread.

As input, process, output. Find the supply rails, then the sensing chain, follow the signal to whatever decides, and end at the load the current finally does work in.

Safe in use and in failure; energy-efficient for the job; low standby power; and repairable or recoverable at end of life.

A switched-mode supply instead of a linear one, LEDs instead of filament lamps, and a processor that sleeps between events with a sensor waking it only when needed.

Because it is drawn every hour of every year in billions of devices. It is capped across the EU at 0.5 W per device precisely because the per-product figure is small and the total is not.

By shortening its life. A control board lasts a fraction of the mechanical parts, is rarely repairable and is not stocked for long, so a working machine is scrapped for a cheap part.

Analogue: any value between its limits, changing continuously. Digital: only a fixed set of discrete values, usually two — on and off, 1 and 0.

Everything between the samples, and the difference between each reading and the nearest available level. More bits and faster sampling reduce both without removing them.

A receiver only has to tell a 1 from a 0, so noise picked up on the way is discarded rather than accumulated. The thousandth copy is identical to the first.

Because the world is analogue at both ends. A microphone senses continuous air pressure, the signal is digitised for processing and transmission, then converted back for a loudspeaker.

Voltage — volt (V). Current — ampere (A). Resistance — ohm (Ω). Power — watt (W). Frequency — hertz (Hz). Time — second (s).

V = I × R, so I = V ÷ R and R = V ÷ I. Power P = V × I.

p (pico), n (nano), µ (micro), m (milli), the unit, k (kilo), M (mega), G (giga), T (tera). Each step is a factor of one thousand.

Ask whether the value is plausible for that component. Resistors are usually kΩ, capacitors µF, nF or pF, LED currents mA. A 4.7 MΩ resistor in an LED circuit would light nothing.

A circuit whose single output is decided entirely by the combination of its inputs, where every signal is 1 or 0. Its truth table lists every input combination and the output for each.

AND: on only when both inputs are on. OR: on when either or both are on. NOT: the opposite of the input. XOR: on when the inputs are different.

Four. Each input has two possible states, so two inputs give 2 × 2 = 4 combinations. Three inputs give eight.

A gate responds in nanoseconds, has no program to crash or hang, cannot be broken by an update, and its behaviour is fixed by the wiring — so it works when the software does not.

Passive components limit, store or divert what is already in the circuit — resistor, capacitor, switch, relay. Active ones control the flow — diode, LED, transistor.

Stores a small charge and releases it. It blocks steady voltages and passes changing ones, which is how it smooths a supply, sets a timing delay and couples an audio signal.

It carries far larger currents and keeps the two circuits electrically separate. That isolation lets a 5 V microcontroller switch a 230 V load safely.

When the current stops, the collapsing magnetic field produces a large reverse voltage spike. The diode gives that current a path to decay instead of destroying the switching transistor.

A resistance that falls as light rises — hundreds of ohms in bright light, megohms in darkness. It is not a voltage until it is placed in a divider.

A circuit cannot act on a resistance. In series with a fixed resistor across the supply, the sensor's changing share of the supply appears as a changing voltage at the junction.

Roughly equal to the sensor's resistance at the value to be detected, so the junction sits near mid-supply where the voltage change per unit of sensing is largest.

Switches are digital — on or off. LDRs, thermistors, humidity sensors and microphones are analogue, giving continuously changing values.

Making a real sensor signal usable with analogue components: amplifying it because it is tiny, filtering it because it is noisy, and comparing it with a reference so a decision comes out.

Converting the signal to a number and letting software decide. Averaging, delays, dead bands, displays and logging all cost nothing extra in components.

When the job is fixed, the response must be immediate, the cost must be pennies, and nothing must depend on software running correctly — a smoke alarm, an amplifier, a protection circuit.

As soon as the behaviour is complicated, needs changing later, needs memory or a display, or has to communicate with anything else.

A programmable integrated circuit — a processor, program and data memory, a clock and converters on one chip — with pins connecting directly to sensors and output devices.

Timers, comparators, counters and logic gates, often a whole board of them. In a washing machine it replaced a motor-driven cam stack.

Delays and sequences; averaging and dead bands; displays, menus and logging. Each would need extra components in a discrete design.

Software to write and debug, faults that may be bugs, a response in milliseconds rather than nanoseconds, and a product whose life is limited by how long its software is supported.