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| SYSTEMS PREPARATION QUESTIONS 2008 - 5 |
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| V. Ryan © 2008-2017 |
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| PDF FILE - CLICK HERE FOR PRINTABLE WORKSHEET |
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| As part of a GCSE project a student has designed a
barrier system for a roller coaster platform. The specification drawn up
by the student says - As a carriage approaches the platform it breaks a
light beam and the barrier is lowered, stopping excited and unruly riders
getting too close to the stopping carriages. When the full carriage moves
away, the barrier should lift allowing the next set of riders to access
the platform, ready for the next carriage. |
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| The student’s prototype control circuit can
be seen below. |
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| Name component ‘A’:
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| ANSWER |
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| Name component ‘A’: Light Dependent Resistor |
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| Give one reason why the circuit shown above
will not meet the specification. |
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| ANSWER |
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| As the carriage passes the sensor the barrier will
lower. However, the moment the carriage goes beyond the sensor the barrier
will begin to rise. This means that it is possible that a rider could be
hit by the carriage or get on a carriage whilst it is still moving. There
is no time delay holding the barrier in the lowered position. |
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| Write a modification that would solve the
problem with the specification |
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| ANSWER |
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| A timer circuit such as a 555 circuit will
hold the barrier in the lowered position until the train has passes the
barrier |
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| Why is a darlington pair positioned between
the sensor and the relay? |
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| ANSWER |
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| The darlington pair amplifies the current (signal) from
the sensor to the relay ensuring that the current is high enough to
energise the relay, turning on the motor that raises and lowers the
barrier. |
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| The darlington pair circuit has been tested but it
regularly fails. Draw a modification on the circuit diagram below that
will ensure that the circuit works without problems. |
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| ANSWER |
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| Explain how your modification works. |
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| ANSWER |
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| The diode protects sensitive components such as the
transistors from back E.M.F. When a current to a relay is removed the
relay can emit current stored in its coils. This can be sent the wrong way
round the circuit damaging components. The diode operates like a one way
valve preventing this from happening. |
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