Systems and control (3)
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1.
Question 1
A student is designing a system to automatically control the brightness of a LED lamp using a light sensor. The system should turn the lamp on when the room is dark and off when the room is bright. Describe the input, process, and output devices that would be suitable for this system. Explain how these devices would interact to achieve the desired functionality.
Input Device: A light-dependent resistor (LDR) would be a suitable input device. An LDR's resistance changes depending on the amount of light falling on it. In a dark room, the LDR's resistance is high, and in a bright room, the resistance is low. The LDR provides an analogue electrical signal that represents the light level.
Process Device: A microcontroller, such as an Arduino or Raspberry Pi Pico, would be an appropriate process device. The microcontroller would read the analogue signal from the LDR using an analogue-to-digital converter (ADC). It would then compare this digital value to a pre-set threshold. If the light level is below the threshold (dark), the microcontroller will activate an output. If the light level is above the threshold (bright), the microcontroller will deactivate the output.
Output Device: A transistor or MOSFET would be used as an output device to control the LED lamp. The microcontroller would send a signal to the transistor/MOSFET, which would then switch the current flow to the LED lamp, turning it on or off. A resistor would be needed in series with the LED to limit the current and prevent damage.
Interaction: The LDR provides the input (light level). The microcontroller processes this input by converting it to a digital value and comparing it to a threshold. Based on the comparison, the microcontroller sends a signal to the transistor/MOSFET. The transistor/MOSFET then controls the power supplied to the LED lamp, resulting in the desired brightness control. A block diagram illustrating this interaction would be beneficial.
2.
Question 1
A farmer needs to lift a heavy bale of hay. He decides to use a lever system. Describe how a lever works, identifying the fulcrum, effort, and load. Explain how changing the position of the fulcrum would affect the amount of effort required to lift the bale. Consider the advantages and disadvantages of using a lever in this scenario.
How a Lever Works: A lever is a simple machine consisting of a rigid bar that pivots around a fixed point called the fulcrum. The effort is the force applied to one part of the lever, and the load is the resistance being overcome (in this case, the weight of the hay bale). The lever multiplies the effort applied to move the load.
Effect of Fulcrum Position: Moving the fulcrum closer to the load reduces the effort required but increases the distance the effort needs to be applied. Conversely, moving the fulcrum further from the load increases the effort required but reduces the distance the effort needs to be applied. The mechanical advantage (MA) is calculated as: MA = Load / Effort. A higher MA means less effort is needed.
Advantages of using a lever:
- Requires less effort than lifting the bale directly.
- Simple and relatively inexpensive to construct.
- Can lift very heavy loads.
Disadvantages of using a lever:
- The effort distance must be greater than the load distance.
- Can be unstable if not designed properly.
- Requires a stable support for the lever.
3.
Question 2
Explain the difference between a digital and an analogue signal. Give an example of a device that uses each type of signal and describe how the signal is converted to and from the other type.
Analogue Signal: An analogue signal is a continuous signal that varies smoothly over time. It can take on any value within a given range. Voltage and current are typical examples of analogue signals. The amplitude of the signal represents the value being measured.
Digital Signal: A digital signal is a discrete signal that can only take on a limited number of values, typically two (e.g., 0 and 1). These values represent binary digits (bits). Digital signals are more resistant to noise than analogue signals.
Example of Analogue Device: A thermistor, which measures temperature. The resistance of a thermistor changes with temperature, providing an analogue voltage output proportional to the temperature.
Example of Digital Device: A digital thermometer. The thermistor's analogue voltage is converted to a digital value by an Analog-to-Digital Converter (ADC) within the thermometer. The digital value is then displayed on a digital screen.
Conversion:
- Analogue to Digital (ADC): An ADC converts a continuous analogue voltage into a discrete digital value. This involves sampling the analogue voltage at regular intervals and comparing it to a reference voltage. The digital value is then determined based on the comparison.
- Digital to Analogue (DAC): A DAC converts a digital value into a continuous analogue voltage. This involves using a series of switches or resistors to create a voltage proportional to the digital value.