ZIMSEC O Level · Design and Technology · Form 3, Form 4
Systems
Every system has input → process → output, often with feedback.
Components
- Input: switches (push, toggle), light-dependent resistor (LDR), thermistor (temperature), moisture sensor.
- Process: transistor (switch/amplifier), integrated circuits such as the 555 timer, microcontrollers.
- Output: LED, bulb, buzzer, motor, relay.
- Power: batteries or solar panels.
Basic electricity
V = I × R. Resistors limit current. An LED needs a series resistor to prevent damage: R = (supply voltage − LED voltage) ÷ current.
Worked example: A 9 V battery lights an LED needing 2 V at 0.02 A. Find the series resistor.
R = (9 − 2) ÷ 0.02 = 350 Ω (use the next standard value, 390 Ω)
Circuit making
Breadboards for testing; stripboard or printed circuit boards (PCBs) for permanent circuits; soldering joins components.
Applications
Automatic night lights (LDR), temperature alarms, soil-moisture alarms for gardens, solar-powered phone chargers.
Exam tip: Always draw circuits with standard symbols and label component values.
Check yourself
- Name an input sensor that detects light.
- Why does an LED need a resistor in series?
- State the three parts of a system.
Show answers
- A light-dependent resistor (LDR).
- To limit the current so the LED is not damaged.
- Input, process and output.
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