Introduction to electronic systems practice questions.

A rotary potentiometer wired as a divider and a push-to-make switch both feed the input of the same controller. Which statement describes the signals these two components produce?

• The potentiometer gives an analogue signal and the switch a digital one
• The potentiometer gives a digital signal, while the switch gives an analogue one
• Both give digital signals, because a controller can only read two voltage levels
• Both give analogue signals, because both are fed from the same steady voltage rail

Answer: The potentiometer gives an analogue signal and the switch a digital one

An analogue signal can take any value between its limits, so a moving wiper gives a continuously variable voltage, while a switch is either open or closed and so gives one of two levels. Which of the two arrives decides whether the signal has to pass through an analogue-to-digital converter before the process block can work with it.

A pop-up toaster heats the bread for a period set by a dial and then ejects it, whatever colour the bread has reached. Which statement classifies this control system and gives the correct reason?

• Closed loop, because the timer measures how long the process has run
• Open loop, because the output is not measured and fed back
• Closed loop, because the user can change the dial between slices
• Open loop, because the heating element is driven by mains electricity

Answer: Open loop, because the output is not measured and fed back

A loop closes only when the controlled quantity itself is sensed and used to alter the process, and elapsed time is not the same quantity as the browning of the bread. A thermostat differs precisely because the temperature it controls is the temperature it senses.

Three identical lamps are wired in series across a battery. Which statement describes the current and the voltage in that circuit?

• The current divides between the lamps and each lamp receives the full supply voltage
• The current divides between the lamps and the supply voltage divides between them
• The current is the same in every lamp and each lamp receives the full supply voltage
• The current is the same in every lamp and the supply voltage divides between them

Answer: The current is the same in every lamp and the supply voltage divides between them

A series circuit offers only one path, so the charge cannot split and the current is common to every component, while the supply voltage is shared out across them. Swapping these two rules is the standard error and it inverts every calculation that follows.

In the input-process-output model of an electronic system, an interface stage is placed between the process block and the output device. Which statement gives the function of that stage?

• It switches the larger current the output device needs, using the low-power control signal
• It converts the physical quantity that is being monitored into a voltage the process block can read
• It compares the measured output with the required value and works out the difference between them
• It turns the electrical signal back into a physical effect such as movement or heat

Answer: It switches the larger current the output device needs, using the low-power control signal

A logic output carries information, not load power: it is rated in milliamps and can only tell the interface when to conduct. The interface is what draws the load current from a supply able to deliver it, which is why the output device never hangs on the process block itself.

Two control systems are running and their internal wiring cannot be inspected. Which single observation identifies one of them as a closed-loop system?

• It responds to a change in the surroundings without the user acting.
• Its process block is a microcontroller rather than fixed logic gates.
• Its output quantity is measured and that measurement alters the drive signal.
• Its output device changes state whenever the input device is operated.

Answer: Its output quantity is measured and that measurement alters the drive signal.

Closed loop is defined by a feedback path: the controlled output quantity is measured and returned to the process block, which then changes the drive to the output. A sensor at the input, or a programmable process block, does not by itself make a system closed loop.

A resistor is connected across a supply of fixed voltage. It is replaced by a resistor of twice the resistance, the supply being unchanged. What happens to the current in the resistor and to the power it converts?

• The current doubles and the power doubles.
• The current halves and the power halves.
• The current halves and the power falls to one quarter.
• The current is unchanged and the power halves.

Answer: The current halves and the power halves.

The voltage is fixed, so I = V/R halves when R doubles, and P = V x I therefore halves as well. The quarter answer comes from halving the current in P = I squared x R while forgetting that the resistance has doubled.