Unit 2 Option A: Electronic and Microelectronic Control Systems
Chapter: Inputs and Outputs
Welcome to your study guide for Inputs and Outputs! Whether you love electronics or find circuit diagrams a bit daunting, do not worry. This guide breaks down everything step-by-step using clear explanations, easy formulas, and practical examples to help you succeed in your CCEA GCSE Technology and Design exam.
1. The Systems Approach
Every electronic and microelectronic control system operates on a simple three-stage framework known as the Systems Approach:
Input \(\rightarrow\) Process \(\rightarrow\) Output
Here is what each stage does:
• Input Devices: These sense physical changes in the environment (such as changes in light, temperature, moisture, or movement) and convert that physical energy into an electrical signal (voltage).
• Process Devices: These act as the "brain" of the system (such as logic gates, timers, transistors, or a microcontroller / PIC chip). They take the input signal, make a decision based on set rules or code, and send a signal forward.
• Output Devices: These take the electrical signal from the process stage and convert it into a useful form of energy (such as light, sound, or mechanical movement).
Analogy: Think of your body! Your eyes sense light (Input), your brain decides what you are seeing (Process), and your hand turns on a lamp (Output).
Key Takeaway: Inputs convert physical energy to electrical energy; outputs convert electrical energy back into physical energy.
2. Input Devices
In CCEA GCSE Technology and Design, you need to understand two main groups of inputs: manual switches and environmental sensors.
A. Manual Switches
Switches allow humans to manually break or complete an electrical circuit:
• Push-to-Make (PTM): Normally open (off). When you press it, the circuit closes and electricity flows. When you release it, it springs back open. Example: A doorbell button.
• Push-to-Break (PTB): Normally closed (on). When pressed, it breaks the circuit, stopping the flow of current. Example: A fridge light switch (the door pushes the switch to break the circuit when closed).
• Toggle Switch: Stays in position (latched) once flipped until flipped back. Example: A standard room light switch.
• Slide Switch: Uses a sliding slider to bridge electrical contacts to stay on or off.
• Reed Switch: Operates via an external magnetic field. Two small metal reeds inside a glass capsule make contact when a magnet is placed nearby. Example: Burglar alarm sensors on doors and windows.
B. Environmental Sensors
Sensors change their electrical properties automatically based on the environment:
• Light Dependent Resistor (LDR): A component whose resistance changes with light levels.
• Thermistor (NTC Type): A Negative Temperature Coefficient component whose resistance changes with temperature.
• Moisture Sensor (Probes): Two metal probes placed in a material (such as soil). Moisture allows electrical current to pass between the probes; as moisture increases, resistance between the probes decreases.
• Pressure Pad: Conductive layers that touch when physical weight or pressure is applied, completing a circuit.
Memory Aid: The "Up-Down" Rule for LDRs and Thermistors
A very common exam mistake is confusing how LDRs and NTC Thermistors behave. Remember this golden rule:
• LDR: As light intensity INCREASES, resistance DECREASES.
• NTC Thermistor: As temperature INCREASES, resistance DECREASES.
Helpful Mnemonic: Think LURD (Light Up \(\rightarrow\) Resistance Down) and TURD (Temperature Up \(\rightarrow\) Resistance Down)!
C. Potential Dividers
Sensors like LDRs and thermistors only change their resistance. However, microcontrollers and logic circuits need a changing voltage to detect inputs. A potential divider circuit is used to convert this changing resistance into a measurable output voltage (\(V_{out}\)).
A potential divider consists of two resistors (\(R_1\) and \(R_2\)) connected in series across a supply voltage (\(V_{in}\)). The output voltage (\(V_{out}\)) is taken across the lower resistor (\(R_2\)).
The Potential Divider Formula:
\(V_{out} = \frac{R_2}{R_1 + R_2} \times V_{in}\)
Worked Example:
A potential divider has a supply voltage \(V_{in} = 9\text{ V}\). Resistor \(R_1 = 10\text{ k}\Omega\) and Resistor \(R_2 = 5\text{ k}\Omega\). Calculate the output voltage \(V_{out}\).
Step 1: Add the two resistances together: \(R_1 + R_2 = 10\text{ k}\Omega + 5\text{ k}\Omega = 15\text{ k}\Omega\)
Step 2: Divide \(R_2\) by the total resistance: \(\frac{5}{15} = \frac{1}{3}\)
Step 3: Multiply by the supply voltage: \(V_{out} = \frac{1}{3} \times 9\text{ V} = 3\text{ V}\)
Key Takeaway: In a potential divider, the voltage across a resistor is directly proportional to its share of the total resistance.
3. Output Devices
Output devices carry out the action in an electronic system.
A. Light Emitting Diode (LED)
An LED produces light when current passes through it in the forward direction. LEDs are polarized: current must enter through the anode (positive, longer leg) and leave through the cathode (negative, shorter leg / flat edge).
Protective Resistor for LEDs:
LEDs are sensitive and will burn out if too much current flows through them. Therefore, an LED must always be connected in series with a protective current-limiting resistor.
Formula for the LED Protective Resistor:
\(R = \frac{V_{supply} - V_{LED}}{I}\)
Where:
• \(R\) = Required resistance (in Ohms, \(\Omega\))
• \(V_{supply}\) = Power supply voltage (in Volts, \(\text{V}\))
• \(V_{LED}\) = Operating voltage drop of the LED (in Volts, \(\text{V}\))
• \(I\) = Operating current of the LED (in Amperes, \(\text{A}\))
Exam Note on Current: If current is given in milliamperes (\(\text{mA}\)), remember to convert it to amperes (\(\text{A}\)) by dividing by \(1000\) before calculating! (e.g., \(20\text{ mA} = 0.02\text{ A}\)).
B. Sounders
• Buzzer: An electromechanical or electronic device that produces a continuous tone when connected to DC voltage. Buzzers are usually polarized (must connect positive to positive).
• Piezo Sounder: Uses a piezoelectric crystal that flexes when voltage is applied. It consumes very little power and can produce tones or clicks depending on the frequency of the input signal.
C. Movement & High-Power Outputs
• Solenoid: An electromagnetic coil containing a movable iron plunger. When energized, it pulls or pushes the plunger to create linear (straight-line) motion. Example: Electronic door latches and vending machine dispensers.
• Motor: Converts electrical energy into continuous rotational movement. Motors draw significant current.
• Relay: An electrically operated switch. A small, low-voltage control signal energizes an internal electromagnet, which closes heavy-duty switch contacts. This allows a safe, low-power circuit (like a PIC microcontroller) to control a separate high-power or mains-voltage circuit.
Driver Circuits (Transistors)
Outputs like motors, solenoids, and relays require far more current than a logic gate or a microcontroller pin can supply directly (which is usually limited to a few milliamps). Therefore, these output devices require a transistor or a dedicated driver circuit (like an H-bridge for reversing motors) to act as an electronic switch to supply the required current.
Key Takeaway: LEDs need series resistors to protect them from high current; motors and solenoids need transistors to provide them with enough current.
4. Microcontrollers (PICs) & Circuit Conventions
In microelectronic control systems, Programmable Interface Controllers (PICs) are used as the central process component.
A. Microcontroller Pin Mapping
Microcontrollers (such as 8-pin or 18-pin integrated circuits) have designated pins for power and input/output connections:
• Digital Inputs/Outputs: Handle simple ON/OFF (HIGH/LOW, \(1\) or \(0\)) signals (e.g., PTM switches, LEDs, buzzers).
• Analog Inputs: Can measure a continuous range of varying voltages (e.g., the output from a potential divider using an LDR or thermistor). In microcontrollers, these pins connect to an internal Analog-to-Digital Converter (ADC).
• Power Pins: Pins designated specifically for supply voltage (\(V_{DD}\) / positive) and Ground (\(V_{SS}\) / \(0\text{ V}\)).
B. British Standard (BS 3939) Symbols
In all CCEA examination questions, you must recognise and draw components using standard BS 3939 schematic symbols. Make sure you practice drawing:
• Switches (PTM, PTB, Toggle, Reed)
• Resistors, LDRs, and Thermistors
• LEDs, Buzzers, and Motors
• Relays and Transistors
5. Common Exam Pitfalls & How to Avoid Them
Pitfall 1: Swapping \(R_1\) and \(R_2\) in the Potential Divider Formula
How to avoid: Always identify which resistor is connected to the \(0\text{ V}\) ground rail. In standard formulas, \(R_2\) is the bottom resistor across which \(V_{out}\) is measured. Double-check your formula: \(V_{out} = \frac{R_2}{R_1 + R_2} \times V_{in}\).
Pitfall 2: Reversing Sensor Logic
How to avoid: Do not write that an LDR's resistance increases in bright light. Remember LURD: Light Up \(\implies\) Resistance Down. For NTC thermistors, Temperature Up \(\implies\) Resistance Down.
Pitfall 3: Forgetting the Protective Resistor
How to avoid: Whenever you draw or design a circuit containing an LED, always place a fixed resistor in series with it to prevent it from burning out.
Pitfall 4: Connecting High-Current Outputs Directly to Logic/PIC Pins
How to avoid: Never connect a DC motor, solenoid, or relay coil directly to a logic gate or PIC output pin without an intervening transistor or driver circuit to handle the high current demand.
Pitfall 5: Unit Conversion Errors in Ohm's Law
How to avoid: Always check units before calculating. Convert \(\text{mA}\) to \(\text{A}\) (divide by \(1000\)) and \(\text{k}\Omega\) to \(\Omega\) (multiply by \(1000\)) when working with LED formulas.
Quick Review Summary
• System Structure: Input (Sensors/Switches) \(\rightarrow\) Process (Logic/PIC) \(\rightarrow\) Output (Light/Sound/Movement).
• LDR & NTC Thermistor: Both have resistances that decrease as the environmental level (light or temperature) increases.
• Potential Divider: \(V_{out} = \frac{R_2}{R_1 + R_2} \times V_{in}\).
• LED Resistor Calculation: \(R = \frac{V_{supply} - V_{LED}}{I}\).
• High-Power Outputs: Motors, solenoids, and relays require a transistor driver switch because they draw more current than microcontrollers can directly provide.