Mr Kidman's Classroom
Revise the key ideas
Selection and components
- System selection — A domestic appliance or vehicle combines sensing, control, power and mechanical output. Specify the task, operating environment, voltage/current, torque/speed, dimensions and safety before selecting components. A component's low purchase price is only part of reliability and repair cost.
- Resistors and sensors — A resistor limits current or forms part of a signal circuit; E12 is a preferred series of nominal resistance values. Tolerance gives permitted variation. Choose a sensor for range, accuracy and environment; an LDR or thermistor is not a complete controller by itself.
- ICs and microcontrollers — A dual-in-line package has two parallel rows of pins; package shape does not specify function. A PIC is one family of programmable microcontrollers. Confirm pinout, supply and program before assembly; a library symbol alone does not prove an actual component is connected correctly.
A pinout identifies what each physical pin connects to or does. Two components with similar-looking packages may have different pin functions.
- Drivers and motors — A driver allows a low-power control signal to switch a larger output current. A motor needs a matched supply and mechanical coupling; starting or stalled current may exceed running current. Appropriate protection for inductive loads and faults depends on circuit design.
A stalled motor is powered but unable to turn and can draw a large current. Inductive loads include coils, such as those in motors; their switching behaviour needs appropriate circuit protection.
- Feedback — Closed-loop control measures a relevant quantity and corrects deviation from a target. A room thermostat controls measured temperature; a motor speed controller can measure speed. Poor sensor placement, delay or excessive gain can lead to oscillation or inaccurate control.
Gain describes the strength of the controller’s response to an error. Oscillation is repeated movement above and below the target rather than settling near it.
Closed-loop control. Original schematic; apply the qualifications in the explanation. View full-size diagram - Mechanical integration — Align shafts, gears and bearings and allow appropriate clearances. Friction produces heat and wear; lubrication suits the materials and operating conditions. Enclosures should protect moving parts and electrical connections while allowing necessary ventilation and access for maintenance.
Manufacture and assembly
- Circuit prototyping — A breadboard permits temporary connections without soldering; know which holes are internally connected. Test a schematic section by section with a safe low-voltage supply. Avoid mains experimentation; teacher supervision and approved equipment are required for practical work.
- PCB manufacture — A PCB provides conducting tracks and component support. Photosensitive board can be exposed through an artwork mask, developed and etched to remove unwanted copper. Exposure/development/etching times need control; chemicals, waste and UV sources require approved supervised procedures.
PCB means printed circuit board. The light-sensitive coating and artwork create a protective pattern; etching removes copper that is not protected, leaving the required conducting tracks.
- Drilling and soldering — Secure boards and drill appropriately sized holes where required. Solder wets clean suitable surfaces to make an electrical/mechanical joint; it does not glue any dirty materials together. Control heat, inspect joints and follow extraction, eye protection and hot-tool procedures.
- Pick and place — Pick-and-place machinery positions components rapidly and consistently on boards. Flow or wave soldering passes suitable assemblies over molten solder; reflow soldering melts solder paste for suitable surface-mount assemblies. Component layout, heat profile and inspection remain necessary.
Surface-mount components connect to pads on the board’s surface, rather than using leads passed through holes in the board.
- Mechanical fabrication — Cut, drill and assemble chassis parts from suitable stock with accurate datums and tolerances. Standard fixings and modular parts support repair. Deburr edges, protect wires from abrasion and check moving parts can travel safely without trapping or contacting the enclosure.
A chassis is the supporting frame or body that holds the system’s parts in position.
Quality, finish and footprint
- Electrical testing — Inspect polarity, track separation and solder joints before powering. Continuity tests can detect an open connection or unintended short when used appropriately on an unpowered circuit. Then test expected and boundary inputs and outputs under safe conditions; continuity alone does not establish full function.
Continuity means an unbroken conducting path. An unintended short circuit is an unwanted low-resistance connection that can let current bypass the intended route.
- Finishes — Suitable PCB lacquer protects against some moisture/corrosion but must respect contacts and service needs. Anodising can improve aluminium chassis surface properties; lubrication reduces mechanical friction. A finish cannot replace insulation, grounding where required or physical guards.
- Reliability and repair — Provide replaceable modules, accessible fixings and clear component identification. A sealed product can resist water but impede repair; judge the intended environment and safety. Use rated parts and realistic testing, documenting failure modes rather than promising permanent reliability.
- Electronic footprint — Metals, polymers and electronic manufacture use energy and resources; mining and hazardous processing can affect workers and communities. Reduce standby consumption, extend life and use appropriate electronic waste collection. Separate batteries for their correct recovery route; do not put them into ordinary mixed waste.
Selection and components
- Test feedback control — A temperature controller measures temperature, compares it with a set value and switches an output through a suitable driver. Without separation between switching thresholds, small fluctuations can cause rapid switching. In a fictional model, turn heating on below 18°C and off above 20°C; between them retain the current state. Test below, within and above the interval, including an invalid sensor reading. This explains a control principle, not a ready-to-build mains circuit or a component-rating specification.
