Input, process, output: how a control system works
A control system watches something and reacts to it. It has three parts.
- Input: a sensor or switch. It turns light, heat, touch or distance into a voltage.
- Process: a microcontroller (a tiny computer on one chip) or a PC. It runs a program that decides what to do.
- Output: an LED, buzzer, display, motor or servo. It acts.
A robot is a control system that can move. Its sensors are the inputs, its controller is the brain, and its motors are the outputs.
Common input devices
- Push switch: gives a digital signal, 0 V (off) or 5 V (on).
- LDR (light-dependent resistor): its resistance falls when light rises. It gives an analogue signal.
- Thermistor: resistance changes with temperature.
- Ultrasonic sensor: sends a sound pulse and times the echo to find distance.
- Potentiometer: a turning knob that gives a changing voltage.
Common output devices
- LED, buzzer, LCD screen, DC motor, servo motor (turns to an exact angle), stepper motor (moves in exact steps), solenoid (pushes a rod).
Specification words
Data sheets describe parts with words like rated voltage, maximum current, power rating (watts), tolerance (how exact a resistor is, e.g. ±5%) and forward voltage (the voltage an LED or diode uses up).
Electronic components and how to choose their values
- Resistor: limits current. Unit ohm (Ω).
- Capacitor: stores charge for a short time; smooths voltage or makes timing delays. Unit farad (F), usually µF.
- Diode: lets current flow one way only. An LED is a diode that gives light.
- Transistor: an electronic switch or amplifier. A small base current controls a large collector current.
- Relay: a switch moved by an electromagnet. A small circuit safely switches a separate big circuit.
- Integrated circuit (IC): many parts on one chip, e.g. a 555 timer, a logic gate chip, a microcontroller.
Using the laws
Ohm's law: V = I × R. Power: P = V × I. Series resistors: R = R1 + R2. Voltage divider: Vout = Vin × R2 ÷ (R1 + R2) (this is how an LDR gives a changing voltage).
LED resistor: R = (Vsupply − VLED) ÷ ILED. With 5 V, a red LED (2 V) and 20 mA: R = 3 ÷ 0.02 = 150 Ω. Pick the next standard value up (e.g. 150 Ω or 220 Ω). Check power: P = 3 × 0.02 = 0.06 W, so a 0.25 W resistor is fine.
Interfacing: desktop computer or microcontroller?
Interfacing means connecting a computer to the real world so a program can read sensors and drive outputs.
| Desktop PC | Microcontroller | |
|---|---|---|
| Cost | High | Very low |
| Size, power use | Large, needs mains | Tiny, runs on batteries |
| Computing power | Very high (video, AI) | Low but enough for control |
| I/O pins | None direct; needs USB interface board | Built-in digital, analogue and PWM pins |
| Start-up | Slow (boots an OS) | Instant, runs one program |
Many robots use both: a microcontroller handles motors and sensors, and a single-board computer or PC handles cameras and planning.
Discrete vs integrated: a circuit of separate resistors and transistors is easy to learn and repair. An IC packs thousands or billions of transistors into one chip, so it is smaller, cheaper and more reliable.
Why advances in electronics matter
Transistors keep getting smaller, so chips get faster and cheaper. That gives us phones, electric cars, medical implants and affordable robots. It also brings e-waste, so recycling and repair matter.
Design, draw, simulate, build and troubleshoot
Design process
- Define the problem and the requirements.
- Draw a block diagram (input → process → output).
- Draw the schematic with standard symbols and calculate values.
- Simulate in circuit software to check currents and voltages before buying parts.
- Build on a breadboard, then solder on a PCB or stripboard.
- Test, fix, and evaluate against the requirements.
Safety
- Power off before changing wires. Use low voltage (5–12 V) for school projects.
- Solder in a ventilated area; rest the iron in its stand; wear eye protection.
- Never connect a motor directly to a chip pin. Add a flyback diode across motors and relay coils.
Troubleshooting
- Look: loose wires, wrong polarity (LED, capacitor), solder bridges, burnt parts.
- Measure supply voltage with a multimeter.
- Follow the signal stage by stage from input to output.
- Check continuity (power off) to find broken tracks.
- Swap a suspect part with a known good one.
Try it: a night-light on paper or in 3D
In the 3D scene, move the light slider. When light falls below 40%, the program turns the LED on. At home: draw the block diagram of your room's automatic device (for example a fridge light or a water-tank alarm). Label input, process and output.
Key formulas and definitions
- V = I × R (Ohm's law)
- P = V × I
- LED resistor: R = (Vsupply − VLED) ÷ ILED
- Series: R = R1 + R2
- Voltage divider: Vout = Vin × R2 ÷ (R1 + R2)
- 1 mA = 0.001 A
Worked examples
1. A 9 V battery lights a blue LED (3 V, 20 mA). Find the series resistor.
Spare voltage = 9 − 3 = 6 V. R = 6 ÷ 0.02 = 300 Ω. Use the next standard value, 330 Ω.
2. Find the power in that 300 Ω resistor and choose a power rating.
P = V × I = 6 × 0.02 = 0.12 W. A 0.25 W resistor is safe (about twice the need).
3. A voltage divider has R1 = 10 kΩ and an LDR as R2. In the dark the LDR is 90 kΩ. With 5 V in, find Vout.
Vout = 5 × 90 ÷ (10 + 90) = 5 × 0.9 = 4.5 V. In bright light (LDR 1 kΩ): Vout = 5 × 1 ÷ 11 ≈ 0.45 V. So the voltage rises in the dark.
4. A microcontroller pin gives up to 20 mA. A 12 V motor needs 0.5 A. Can the pin drive it?
No. 0.5 A = 500 mA, 25 times more than 20 mA, and the voltage is also too high. Use a transistor or relay with a separate 12 V supply.
5. An LED circuit does not light. The battery reads 5 V. What do you check next?
Check LED polarity (long leg to +), then measure voltage across the resistor and LED, then check continuity of each wire with power off.
Common mistakes
- Connecting an LED without a resistor. It draws too much current and burns out.
- Putting an LED or electrolytic capacitor in backwards. Both have polarity.
- Driving a motor straight from a chip pin. Use a transistor or relay.
- Measuring continuity or resistance with the power on. Always switch off first.