Electronic systems application and selection practice questions.

A washing machine must confirm that the drum door is fully closed before the motor can start. The sensing element sits behind a sealed plastic moulding, is never accessible to the user and must survive constant vibration and hot detergent water. Which input component best meets these constraints?

• A reed switch operated by a magnet in the door
• A negative temperature coefficient thermistor set into the door seal
• A light dependent resistor facing the drum
• A membrane push switch on the door frame

Answer: A reed switch operated by a magnet in the door

A reed switch is operated magnetically through the sealed non-magnetic moulding, so no contact is exposed to water or wear. A membrane switch would have to sit in the wet zone, while a thermistor and an LDR sense the wrong physical quantity altogether.

A desktop 3D printer must hold its print bed at 60 degrees C plus or minus 2 degrees C in a workshop where draughts change the rate of heat loss from the bed. Which control arrangement meets this requirement?

• Open-loop control with a fixed heater on-time set by a timer
• Open-loop control with the heater current set by a preset resistor
• A thermal cut-out that disconnects the heater above 80 degrees C
• Closed-loop control using a thermistor as feedback

Answer: Closed-loop control using a thermistor as feedback

Only a feedback signal from the measured bed temperature lets the process block correct for a disturbance the designer cannot predict, such as a draught. Open-loop settings are fixed when the machine is built, and a cut-out is a safety device rather than a controller.

A greenhouse controller varies the brightness of a 24 V LED grow-light strip that draws 3.2 A. The dimming has to be silent and free of visible flicker, so the controller produces a pulse width modulated signal at 20 kHz. Which switching device should be specified for the strip?

• An electromagnetic relay switched by the modulated signal
• A logic-level MOSFET switched by the modulated signal
• A wirewound rheostat adjusted by hand in series with the strip
• A fixed series resistor sized to drop the surplus voltage

Answer: A logic-level MOSFET switched by the modulated signal

Only a semiconductor switch can open and close twenty thousand times a second, and because it is either fully on or fully off almost no power is turned into heat inside it. Relay contacts would wear out within minutes at that rate and would be audible, while a rheostat or a fixed resistor sets one brightness and burns the surplus power.

Two 6 V, 0.5 A filament lamps are the only lamps available for a battery inspection light with a 12 V supply. Both must run at their rated brightness and no additional components may be fitted. How should they be wired?

• In parallel across the 12 V supply
• In parallel, with a resistor in each branch
• In series across the 12 V supply
• In series with the supply reduced to 6 V

Answer: In series across the 12 V supply

A series connection shares the 12 V equally between two identical lamps, giving each its rated 6 V while the same 0.5 A passes through both. Dropping the surplus voltage in a resistor beside each lamp would also work electrically, but the specification rules out fitting anything else.

A vending machine board switches a 24 V solenoid valve several times a minute for a ten-year service life. The switching transistor is already specified. Which further component must appear on the parts list if that transistor is to reach the ten years?

• A flyback diode fitted across the valve coil
• A series resistor between the transistor and the coil
• A smoothing capacitor across the 24 V supply rail
• An optocoupler between the board and the transistor

Answer: A flyback diode fitted across the valve coil

When the transistor switches the coil off, the collapsing magnetic field produces a large reverse voltage spike that eventually destroys the transistor; a diode across the coil gives the coil current a path to circulate harmlessly. A series resistor only starves the coil, a supply capacitor smooths the wrong node, and an optocoupler isolates the control side but leaves the spike at the transistor.

An outdoor lamp is switched by a light sensor. Once it has come on at dusk it stays lit right through the following day, yet the sensor, the lamp and the supply all behave correctly when they are tested separately on the bench. Which single design decision has produced this behaviour?

• The switching threshold was set too close to full daylight.
• The lamp and the sensor were fed from the same supply rail.
• A light-dependent resistor was specified instead of a photodiode.
• The sensor was sited under the lamp's shade, away from daylight.

Answer: The sensor was sited under the lamp's shade, away from daylight.

A sensor that daylight never reaches keeps reporting darkness, so the output block stays energised whatever the real light level is; the fault lies in where the input transducer was placed, not in which one was chosen. A threshold set too high would make the lamp come on early, not stay on permanently.