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SYSTEMS PREPARATION QUESTIONS 2008 - 5

V. Ryan © 2008

 

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.

 
The student’s prototype control circuit can be seen below.
Name component ‘A’: __________________________________  
   

ANSWER

 
Name component ‘A’: Light Dependent Resistor
 
Give one reason why the circuit shown above will not meet the specification.
 
 
 
 

ANSWER

 

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.

 
Write a modification that would solve the problem with the specification
 
 
 
 

ANSWER

 
A timer circuit such as a 555 circuit will hold the barrier in the lowered position until the train has passes the barrier
 
Why is a darlington pair positioned between the sensor and the relay?
 

ANSWER

 

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.

 

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.

ANSWER

Explain how your modification works.
 
 
 
 
   
ANSWER
   

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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