🎯 Syllabus & Goals 3 min
Cambridge 3.2 · Input and output devices Paper 1 · Computer Systems
By the end of this lesson you can:
- Name the sensors in the syllabus and choose the right one for an application.
- Explain the difference between a monitoring system and a control system, including feedback.
- Describe step by step how sensors, an ADC, a microprocessor and a DAC run a given application.
Textbook: Chapter 3, §3.2.3 (pp. 111–118) — sensors, monitoring and control applications.
Recap / Warm-Up 5 min
Lesson 9 met actuators, which turn signals into movement. Lesson 10 met the DAC, which turns digital data into a current. Today we add the input side: sensors.
Quick starter
A mercury thermometer reads "somewhere between 21 °C and 22 °C". Why can a computer not read it directly?
Reveal the answer
Temperature is analogue — it varies continuously, with infinitely many possible values. A computer only handles discrete digital values, so the reading must be converted first (by an ADC).
🧠 Key Concept 14 min
1 · What a sensor does
- Real-world data is analogue: it changes continuously.
- An ADC (analogue-to-digital converter) turns it into digital values the processor can use.
- To drive a motor or valve, the processor may need a DAC to turn its digital output back into analogue.
- Sensors send readings constantly. They do not "wait for a change". The processor analyses the data and decides.
2 · The sensor catalogue
| Sensor | What it measures / how | Example uses |
|---|---|---|
| Temperature | Signal changes as the temperature of the surroundings changes. | Central heating; chemical process; greenhouse. |
| Moisture | Measures water in e.g. soil, using the sample's electrical resistance. | Greenhouse soil; food-processing factory. |
| Humidity | Measures water vapour in air (air's conductivity changes with water). | Buildings; microchip factory; greenhouse air. |
| Light | A photoelectric cell gives a current that depends on brightness. | Street lights; automatic car headlights. |
| Infrared (active) | An invisible IR beam hits a detector; breaking the beam changes the reading. | Rain-sensing wipers; intruder alarm beam. |
| Infrared (passive) | Measures heat radiation given off by an object. | Alarm detecting body heat; freezer temperature. |
| Pressure | A transducer: its current changes with the pressure applied. | Weighing lorries; gas pressure in a reactor. |
| Acoustic / sound | Basically a microphone: sound becomes electric signals. | Footsteps in a security system; dripping pipes. |
| Gas | Output varies with O₂ or CO₂ levels (other gases too). | Airport pollution; greenhouse; car exhaust. |
| pH | Measures acidity through changes in voltage. | Greenhouse soil; chemical process acidity. |
| Magnetic field | Output depends on changes in a magnetic field. | Anti-lock brakes (ABS); phones and CD players. |
| Accelerometer | Measures acceleration / change in velocity (piezoelectric cell). | Airbags in a crash; phone portrait/landscape. |
| Proximity | Detects that an object is nearby. | Phone screen turns off when held to the ear. |
| Flow (rate) | Measures how fast a liquid or gas flows past it. | Hospital respiratory devices; gas pipes. |
| Level | Ultrasonic, capacitance, optical or mechanical sensing of level. | Car fuel tank; powder in tablet making; leaks. |
3 · Monitoring or control?
Both start the same way. They differ in what happens when a reading is out of range.
Monitoring examples
- A patient's vital signs in hospital.
- Intruders in a burglar alarm system.
- Temperature in a car engine.
- Pollution levels in a river.
Control examples
- Street lights on at dusk, off at dawn.
- Central heating / air conditioning.
- Chemical process temperature and pH.
- Anti-lock brakes; a greenhouse environment.
4 · Monitoring in action
Real world · security systemThe alarm is set with a password on a keypad. Infrared sensors detect movement, acoustic sensors pick up footsteps or breaking glass, and pressure sensors feel weight at a door. Readings pass through an ADC. The processor samples them, e.g. every 5 seconds, and compares them with stored values. If one is out of range, it signals a siren or flashing lights. They stay on until the password resets the system.
Real world · hospital patientSensors on the patient measure temperature, heart rate and breathing rate. They send data constantly; the computer samples it often. Nurses key in the acceptable range for each value. In range → values shown as graphs or digital read-outs. Out of range → an alarm sounds. It is monitoring: the computer never treats the patient.
5 · Control in action
Real world · anti-lock brakes (ABS)Magnetic field sensors measure each wheel's rotation. If one wheel turns too slowly (locking up), the microprocessor compares it with the other three. It signals the brakes to reduce pressure on that wheel until the speeds match. A wheel turning too fast gets more pressure. This happens several times a second — the driver feels a judder in the pedal.
Real world · chemical processA reaction only works above 70 °C with a pH below 3.5. Temperature and pH sensors feed an ADC and the computer. Temperature < 70 → heater on; ≥ 70 → heater off. pH > 3.5 → open the acid valve; ≤ 3.5 → close it. A DAC converts the signals for the heater and valve.
Real world · greenhouseFive sensors can run a greenhouse: humidity (high → motors open windows; low → valves spray water), moisture (low → valve adds water; high → heater dries), temperature (high → open windows; low → close windows, heater on), pH (high → add acid; low → add alkali) and light (bright → close blinds; dark → artificial light on). If a reading equals the pre-set value, nothing happens.
Worked Example 12 min
(a) Central heating — the full description
- The owner keys in the required temperature; it is stored in memory as the pre-set value.every control answer needs a stored value to compare against.
- The temperature sensor constantly sends readings.name the sensor precisely — never "heat sensor".
- An ADC converts each analogue reading to digital and sends it to the microprocessor.the processor cannot use analogue data.
- The microprocessor compares the reading with the pre-set value.the most-awarded phrase in these questions.
- If reading ≥ pre-set: no action. If < pre-set: signals go, via a DAC, to an actuator to open the gas valve and to turn on the pump.give the condition and the action for both outcomes.
- The process repeats until the heating is switched off.the loop / feedback mark.
(b) Trace the chemical-process controller
Rules: temperature < 70 → heater ON, else OFF. pH > 3.5 → acid valve OPEN, else CLOSED.
| Sample | Temp (°C) | pH | Heater | Acid valve | Reason |
|---|---|---|---|---|---|
| 1 | 68 | 3.9 | ON | OPEN | too cold and not acidic enough |
| 2 | 71 | 3.6 | OFF | OPEN | warm enough; pH still > 3.5 |
| 3 | 72 | 3.5 | OFF | CLOSED | pH = 3.5 is not > 3.5, so close |
| 4 | 69 | 3.2 | ON | CLOSED | cooled below 70; acidic enough |
- Check each sensor against its own rule separately.two sensors means two independent decisions.
- Watch the boundaries: 70 °C turns the heater OFF; pH 3.5 closes the valve.the textbook uses < and ≥, > and ≤ — equality belongs to the "OFF/CLOSED" side.
- Sample 4 shows feedback: switching the heater off let the temperature fall, so it comes back on.the output changes the next input.
(c) The street lamp as Cambridge pseudocode
A light sensor is sampled; below the stored value, the lamp goes on. Either way, the lamp then holds its state for 30 minutes. That stops it flickering when a heavy cloud passes.
Pseudocode · street-lamp control
// Switch a street lamp using a light sensor
CONSTANT StoredLevel ← 40
DECLARE LightLevel : INTEGER
DECLARE SystemOff : BOOLEAN
SystemOff ← FALSE
REPEAT
// reading already converted to digital by the ADC
INPUT LightLevel
IF LightLevel < StoredLevel
THEN
OUTPUT "Lamp ON"
ELSE
OUTPUT "Lamp OFF"
ENDIF
// hold for 30 minutes before sampling again
CALL WaitMinutes(30)
INPUT SystemOff
UNTIL SystemOff = TRUETry It Yourself 12 min
🟢 Easy
Goal: Name the best sensor for each: rain on a windscreen; a lorry's weight; a phone switching to landscape; a car's fuel tank; oxygen in a car exhaust; a phone held to your ear.
🟡 Medium
Goal: Sort into monitoring or control, with one sentence of reason each: patient heart rate; ABS brakes; river pollution; greenhouse windows; burglar alarm; air conditioning.
🔴 Stretch
Goal: A car's air conditioning is run by a microprocessor. Describe how it works, naming at least two sensors and explaining the part played by feedback.
Hint
Use temperature and humidity sensors. Follow the central-heating steps: pre-set value, ADC, compare, signal an actuator (fan motor, cooling valve) via a DAC, repeat. Then say how the new air changes the next reading.
📝 Exam Practice 10 min
Explain[2]
Explain what is meant by the term sensor.
Mark scheme
- An input device (1)…
- …that measures / reads a physical property (of its surroundings) and sends the data to a processor (1).
State[6]
A farmer has a system that monitors the conditions for growing fruit. State two sensors that could be used and describe how each is used. (Based on Cambridge O Level CS 2210, Paper 12 Q9, Oct/Nov 2017.)
Mark scheme
- Per sensor, up to 3 marks — e.g. temperature sensor (1); readings sent to the processor and compared with stored values (1); an alert / warning if it is too hot or too cold (1).
- Second sensor, e.g. moisture (soil water) / humidity / light / pH (1); readings compared with the stored range (1); warning sent to the farmer if outside it (1).
- Do not accept actuator actions as this is a monitoring system.
Describe[2]
Describe the main difference between control and monitoring of a process.
Mark scheme
- Monitoring only observes the process and warns / sounds an alarm; it has no effect on the process (1).
- Control sends signals to actuators to change the process; the output affects the next input (feedback) (1).
Describe[6]
Describe how a microprocessor and a light sensor are used to control a street lamp.
Mark scheme
- The light sensor constantly sends data (1).
- An ADC converts it to digital before it reaches the microprocessor (1).
- The microprocessor compares the reading with a value stored in memory (1).
- If the reading is below the stored value, a signal is sent to switch the lamp on (1).
- If the reading is equal to or above it, a signal is sent to switch the lamp off (1).
- The lamp holds its state for a set time (e.g. 30 minutes) before sampling again / the process repeats (1).
🗝️ Recap & Key Terms 3 min
Sensors measure analogue quantities constantly; an ADC makes them digital. The processor compares readings with stored values. Monitoring only warns. Control drives actuators, often through a DAC, and the result feeds back into the next reading.
- Sensor
- An input device that reads or measures a physical property and sends the data to a processor.
- ADC (analogue-to-digital converter)
- Converts analogue data, e.g. from sensors, into digital data a computer can process.
- DAC (digital-to-analogue converter)
- Converts digital output into analogue signals that can drive motors, valves and other actuators.
- Monitoring system
- Uses sensors to watch a process and warn when values leave a range; it does not change the process.
- Control system
- Uses sensor readings to send signals to actuators that change the process.
- Feedback
- When the system's output affects the next input received, bringing the process back to its target values.
Homework 1 min
Task (≤ 15 min): Using the greenhouse example, describe how a pH sensor and a microprocessor keep the soil acidity at the right level. [5]
Model answer
- The pH sensor constantly sends readings of the soil's acidity (1).
- An ADC converts the readings to digital for the microprocessor (1).
- The microprocessor compares each reading with the pre-set pH value (1).
- If the pH is too high, a signal (via a DAC) opens a valve to add acid; if too low, a valve adds alkali (1).
- If equal, no action; readings continue and the new pH feeds back into the next comparison (1).