The systems approach
- An electronic system has an input, a process and an output. A closed-loop system uses feedback to check the output, such as a thermostat.
- Inputs (sensors): switches (push, micro, reed and tilt), light-dependent resistors (LDRs: resistance falls as light increases), thermistors (resistance usually falls as temperature rises) and potentiometers.
- Processes: transistors (electronic switches), logic gates (AND, OR, NOT), timers and programmable microcontrollers.
- Outputs: LEDs, lamps, buzzers, motors, solenoids and relays (a relay lets a small current switch a larger one).
Electrical calculations
- Ohm's law: voltage = current × resistance (V = IR). 0.5 A through 24 Ω needs 12 V.
- Power = voltage × current (P = VI). A 12 V motor drawing 2 A uses 24 W.
- Resistors in series add up: 220 Ω + 330 Ω = 550 Ω.
- An LED needs a series resistor to limit current. Resistor = (supply voltage − LED voltage) ÷ current. With 9 V, a 2 V LED and 0.02 A: (9 − 2) ÷ 0.02 = 350 Ω.
Circuits and construction
- A potential divider uses two resistors (such as an LDR and a fixed resistor) to produce a voltage that changes with conditions.
- Circuits are built on breadboards for testing, then on printed circuit boards (PCBs). Components are soldered in place.
- Microcontrollers can be reprogrammed, reducing the number of components needed.
Key terms
- Input
- A component that senses a change, such as a switch or sensor.
- Process
- The part of a system that decides what to do.
- Output
- A component that produces a result, such as light or movement.
- LDR
- A light-dependent resistor, whose resistance falls as light increases.
- Thermistor
- A resistor whose resistance changes with temperature.
- Transistor
- A component that acts as an electronic switch.
- Microcontroller
- A programmable chip that controls a system.
- Ohm's law
- Voltage equals current multiplied by resistance.