Young children often need a gentle light in their bedroom at night. A nightlight with a light sensor would switch on automatically when the room becomes dar...

(a) Four additional functional points [4 marks]

The brief specifies a nightlight for a child's bedroom with an electronic light sensor that switches on automatically in the dark. Additional points:

1. Child safety - the nightlight must have no sharp edges, no hot surfaces (LED rather than incandescent bulb), and no small detachable parts. All edges should be rounded. If it falls off a shelf, it should not shatter. [1]
2. Low power consumption - the light runs all night (approximately 10 hours). Low-power LEDs and an efficient sensor circuit ensure batteries last weeks, or a low-voltage USB supply keeps running costs minimal. [1]
3. Gentle, non-disruptive glow - the light must be soft enough not to disturb sleep. A warm white or amber LED (2700K colour temperature or lower) diffused through a translucent shell creates a calming glow without harsh glare. [1]
4. Robustness - children may knock the light off a bedside shelf. The housing must withstand drops without breaking or exposing the circuit. Impact-resistant plastics (polypropylene, ABS) achieve this. [1]

(b) Two types of light-sensing circuit [4 marks]

1. LDR potential divider with transistor switch - a light-dependent resistor (LDR) and a fixed resistor form a voltage divider. In bright conditions, the LDR resistance is low, so the voltage at the junction is low and the transistor (e.g., BC547 NPN) stays off, keeping the LED unlit. In darkness, the LDR resistance rises, increasing the junction voltage above the transistor's base-emitter threshold (approximately 0.7 V), switching it on and lighting the LED. A variable resistor in series with the LDR allows sensitivity adjustment. [2]
2. LDR with comparator (op-amp) - an LDR potential divider feeds one input of a comparator (e.g., LM393). A reference voltage from a second potential divider (adjustable) feeds the other input. When the LDR voltage crosses the reference threshold (room going dark), the comparator output switches high, turning on a transistor or MOSFET that powers the LED. This circuit gives a crisp on/off transition with no gradual dimming. [2]

(c) Three design ideas [12 marks]

1. Idea 1: Animal-shaped nightlight - a hollow animal form (e.g., owl, 100 mm tall) moulded from translucent polypropylene. The body glows when the internal LED lights up. The circuit and 3 x AAA battery holder sit inside the base. The LDR pokes through a small hole in the back, facing the room. A magnetic base closure allows battery access. Child-friendly, attractive, robust. [4]
2. Idea 2: Wall-plug nightlight - a compact unit (60 mm x 40 mm x 30 mm) that plugs directly into a wall socket via integrated plug pins. The front face is a translucent acrylic diffuser panel. The LDR faces forward through a small aperture. Powered by mains via a step-down transformer and rectifier inside the housing. Convenient (no batteries) but mains voltage inside a child's room requires robust insulation and adds design complexity. [4]
3. Idea 3: Mushroom-shaped desk light - a mushroom cap (80 mm diameter dome) in frosted acrylic sits on a cylindrical ABS base (50 mm diameter x 40 mm tall). Three warm-white LEDs inside the cap point upward into the dome for even diffusion. The LDR is mounted on the outside of the stem. Powered by 2 x AA batteries in the base, accessed by unscrewing the base plate. Cute design, stable on a shelf. [4]

(d) Evaluation and justification [8 marks]

Idea 1 (animal-shaped) is the strongest choice. The translucent polypropylene body creates an even, gentle glow across the entire surface, which is visually calming for a child. The animal shape is appealing and serves as a bedroom ornament even when off. Polypropylene is impact-resistant and child-safe if knocked off a shelf. Battery power avoids any mains voltage concerns. The LDR transistor circuit is simple and reliable. The magnetic base closure is tool-free for battery changes. [2 marks for justified selection]

Idea 2 introduces mains voltage near a child's bed, raising safety concerns and requiring regulatory compliance. Idea 3's separate frosted dome is fragile at the stem joint and the screw-in battery access is fiddly for parents. [up to 6 marks]

(e) Detailed final drawing [12 marks]

• Body: hollow owl shape, approximately 80 mm wide x 100 mm tall x 60 mm deep, vacuum-formed or blow-moulded from 3 mm translucent polypropylene
• Base: 80 mm x 60 mm x 20 mm ABS moulded tray housing the circuit and batteries
• Circuit: stripboard (30 mm x 20 mm) carrying an LDR, 10K fixed resistor, 50K preset potentiometer (sensitivity adjust), BC547 NPN transistor, 330 ohm current-limiting resistor, and a warm-white LED (5 mm, 20 mA)
• Power: 3 x AAA batteries in a holder, giving 4.5 V supply (sufficient for LED forward voltage of approximately 3.2 V plus transistor and resistor drops)
• LDR window: 5 mm hole in the back of the body, with the LDR soldered to leads and positioned facing outward
• LED position: mounted upright inside the body at the centre, pointing upward for maximum diffusion through the translucent shell
• Base closure: two small neodymium magnets (5 mm diameter) in the body rim and two corresponding magnets in the base tray, providing secure but tool-free closure
• On/off switch: miniature slide switch on the base underside

(f) Materials and reasons [4 marks]

1. 3 mm translucent polypropylene for the body shell - polypropylene diffuses light evenly, creating a soft ambient glow across the entire surface. It is impact-resistant (will not shatter if dropped), lightweight, and safe for children. It can be vacuum-formed over a mould to create the animal shape. [1+1]
2. ABS (acrylonitrile butadiene styrene) for the base housing - ABS is tough, rigid, and can be injection-moulded or vacuum-formed with rounded edges for child safety. It is opaque (containing the circuit and batteries out of sight) and provides a stable, flat base for the nightlight to stand on. [1+1]

(g) Manufacturing the polypropylene body shell [6 marks]

1. Make a mould - carve or turn the owl shape from a solid block of MDF on a lathe (for the symmetrical body) and refine details (eyes, beak) by hand with files and sandpaper. Seal the MDF surface with sanding sealer to prevent the plastic sticking. [1]
2. Clamp the polypropylene - secure a sheet of 3 mm translucent polypropylene in the vacuum-forming machine frame. [1]
3. Heat the sheet - position the heater above the clamped sheet. Heat until the polypropylene becomes uniformly soft and begins to sag (approximately 160-170 degrees C). [1]
4. Form over the mould - raise the mould into the softened sheet and apply vacuum. The atmospheric pressure forces the polypropylene tightly around the mould, reproducing the owl shape. Allow to cool for 30 seconds before releasing vacuum. [1]
5. Trim the formed shape - remove from the machine. Cut away the excess flange material using a band saw or scroll saw, leaving a clean edge approximately 5 mm below the base rim. [1]
6. Finish - smooth the trimmed edge with fine sandpaper (240-grit). Drill a 5 mm hole in the back for the LDR window. Check the shell fits over the base tray and the magnetic closures align. [1]