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SYSTEMS PREPARATION QUESTIONS 2007 - 3   

V. Ryan 2007

 

The railway barrier shown below has been designed and manufactured for a model railway system. The motor inside the post drives the barrier up and down. The motor is the output to a 555 timer circuit.

 

 

1. There are two types of 555 timer. What is the name of the 555 circuit shown below?

   
CIRCUIT NAME: ___________________________
   
ANSWER
   
CIRCUIT NAME: 555 ASTABLE
   
2. Label the components on the circuit diagram below (A TO G).
   
ANSWER  
     
 

3. The relay is causing major problems because an important component is missing. Name the component and draw its symbol in the correct position on the circuit diagram above.

 
   
COMPONENT NAME: ___________________________
   

ANSWER

   
COMPONENT NAME: DIODE
   
4. What is the function of the component ?
 
 
 
   
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.

   

 

A second version of the railway barrier has been manufactured. This is mechanically operated by a rotating device.

5. What is the function of the pivot ?
 
 
 
   
ANSWER
   

The pivot allows the barrier to rotate at a specific point. In this case the barrier rotates upwards and downwards about the pivot.

   
6. Draw the mechanical device on the diagram above.
   
ANSWER
7. Name the device and explain how it works.
 
NAME:____________________________
 
HOW IT WORKS:
 
 
 
   
ANSWERS
   
NAME: ECCENTRIC CAM
   

HOW IT WORKS: As the eccentric cam rotates in a clockwise direction the barrier is pushed upwards at a constant rate. It then descends at a constant rate back to its original position.

   
8. What is the function of the barrier stop ?
 
 
 
   
ANSWER
   
The stop prevents the barrier from dropping below the horizontal.
     
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