IAN LANG ELECTRONICS

The first question of this unit eases you in gently:

 

 

1 Microprocessor control systems are now used to provide all of the timing and control

systems for most domestic electrical and electronic goods e.g. dishwashers, washing

machines etc.

1 (a) (i) State one way in which the architecture of a microcontroller differs from a

conventional computer system.

 

 

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1 (a) (ii) Give two reasons why microcontrollers are now commonly used for

providing the control functions for domestic goods, e.g. washing machines.

 

 

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(3 marks)

A Level Electronics

A Level Home

AQA Unit4 -Programmable Control Sytems

In the microprocessor there is a seperate instruction bus.

They are highly cost effective as they can combine I/0 and

processor on one board without the need for many discrete subsystems.

 

 

As  ports  can be made to be input or output, a varied combination

may be made using just one microprocessor board.

The follow on from that is not much harder:

 

1 (b) The control functions processed by the microcontroller on a washing machine can

be classified as either open or closed loop.

Explain why the functions of filling with water and heating it to the required

temperature need to be closed loop.

 

 

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(2 marks)

A closed loop is one where the endpoint is monitored.

Since both the quantity of water and the temperature

the water is at are both monitored by sensors,

both are a closed loop.

The examiners now begin to crank up a notch:

 

 

The drum of a washing machine, which is carrying out the function of a 'slow spin', has to

rotate at a steady speed of 200 revolutions per minute. To maintain this speed, negative

feedback is provided to the microcontroller from a rotation sensor attached to the drum.

1 (c) (i) What is meant by the term negative feedback?

 

 

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1 (c) (ii) How does the negative feedback control the rotation speed of the drum?

 

 

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(3 marks)

In negative feedback a portion of the output is taken and

returned (fed back) to the input, reducing its effect.

If the speed is too low the output drops allowing the input to increase and the drum to speed up. If the speed goes too high, the output increments and reduces the input, allowing the drum to slow down.

Now comes a question with some meat on it:

 

The circuit diagram for the temperature sensor is shown below. The microcontroller

produces a temperature control voltage dependent upon the temperature for the wash

selected.

1 (d) (i) Name the function of the op-amp in the circuit.

 

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It is a comparator.

1 (d) (ii) Explain how the temperature sensing circuit functions.

 

 

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(3 marks)

Following the diagram: A reference voltage is sent from the microcontroller

to one input (+) whilst the sensor voltage derived from the thermistor and

resistor sends to the other (-). Once the sensor voltage exceeds the reference,

the op amp output goes low, indicating the temperature has been reached.

The initial reference voltage from the microcontroller is dependent on

the temperature selected by the user.

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