Street lighting is controlled automatically. A light sensor and a microprocessor are used to decide when to switch each street light on or off. Describe how the sensor, microprocessor and light interact to switch the street light on or off. Include in your answer how the microprocessor stops the street lights being frequently switched on and off due to brief changes in the light intensity.

Mark scheme

Marking points from the official mark scheme (May/June 2015). The mark or mark type is shown in brackets.

1. [5] (a) Maximum 5 marks in total for question part Description of how street light is controlled: (max 4 marks) – sensor sends signal/data to the microprocessor – signal/data converted to digital/using ADC – microprocessor compares value to a stored value – if input value < stored value – ... signal sent from microprocessor to actuator – and light is switched on/off – whole process continues in an infinite loop Avoiding frequent on/off switches: (max 2 marks) – microprocessor continues to keep light on/off for a pre-determined period – after pre-determined period, sensor output is again sampled
2. [6] (b) 1 mark for correct sensor, 1 mark for its matching application (all THREE applications must be different) sensor application infra-red/motion automatic doors burglar alarm systems temperature chemical process central heating/air con system greenhouse environment oven sound/acoustic burglar alarm systems leak detection system disco lighting moisture/humidity clothes drier environmental control (greenhouse, air con) pressure burglar alarm system traffic light control chemical process carbon dioxide/ oxygen/gas pollution monitoring in a river greenhouse environment (growth control) confined area (e.g. space craft) Fish tank/Aquarium magnetic field mobile phone anti-lock braking CD players