Electrical and electronic systems and control
Electrical and electronic systems and control is examined through input-process-output circuits, component selection, simple calculations and control logic.
Core rules
- Ohm's law: V = I x R. In series, current is common and voltages add; in parallel, voltage is common and currents add.
- LEDs need a series resistor and correct polarity; the resistor value is (supply - LED voltage) / LED current.
- Inputs: switch, push-to-make/break, LDR (light), thermistor (temperature), microphone. Outputs: LED, buzzer, motor, solenoid, relay.
- A transistor or relay lets a small signal switch a larger load; a relay gives isolation.
- Open-loop control has no feedback; closed-loop compares sensed output with a target.
- Logic: AND needs all inputs high; OR needs any; NOT inverts. Flowcharts show decision loops.
Worked example
A 5 V supply drives a red LED rated 2 V at 20 mA. Resistor R = (5 - 2) / 0.02 = 150 ohm. The nearest E24 value is 150 ohm, so power = I squared x R = 0.02 squared x 150 = 0.06 W; a 0.25 W resistor is safe. Connect the anode to the resistor and the cathode to 0 V. Reversing the LED blocks current, so it will not light.
Traps
Forgetting the LED resistor; using mA directly in Ohm's law; reversing LED polarity; assuming a microcontroller pin can drive a motor directly; confusing series and parallel voltage rules.