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555 TIMER EXAMINATION QUESTIONS - 2

V. Ryan © 2006 - 2009

 

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1. The following question is based on the 555 monostable circuit. The circuit shown is part of a production line and controls the timing sequence. The automated production line is shown below. Parts / components move along the production line where they are sensed, clamped down, drilled and then released, moving down the production line again. The 555 monostable circuit controls the timing of the entire production line.

 
   
   

The timer circuit is found inside the control circuit box. The incomplete circuit diagram is found below. Add the following components to complete the timer circuit:

A. 0v and 9v supply
B. A variable resistor for altering the length timing sequence.
C. A buzzer that sounds during the timing sequence.

   
     

POSSIBLE ANSWER

 
     
 
     
D. Explain how your completed circuit works.  
 
 
 
 
   

POSSIBLE ANSWER

   
When SW2 is pressed current flows into pin 2 of the 555 timer starting the timing sequence. Current flows from pin 3 into the transistor, allowing current to flow from positive to negative, sounding the buzzer. The buzzer sounds for the entire timing sequence.
   

2. What is the role of the variable resistor and capacitor?

 
 
 
   
POSSIBLE ANSWER
   

Both components are key to the length of time of the timing sequence. If a resistor and capacitor of higher values are used then the timing sequence is longer.

   
3. The 555 timer is an IC. What does IC mean?
   

POSSIBLE ANSWER AND FURTHER INFORMATION

   
IC means ‘Integrated Circuit’ - click here for further information
   
4. The 555 timer has a DIL layout. What does DIL mean?
   
POSSIBLE ANSWER AND FURTHER INFORMATION
   
DIL means ‘Dual In-line. This means that there are two parallel lines of pins - click here for further information
   

5. It has been found that the sound emitted by the buzzer is too low. A circuit designer has suggested that a relay be added. The circuit also needs protecting by the addition of another component placed in parallel with the relay. (See layout below)

Complete the circuit diagram by adding the relay and a suitable additional ‘protecting’ component.

   
POSSIBLE ANSWER
   
   
6. Name the component that protects the circuit and explain how it does this.
   
NAME:_____________________________
   
How the component protects the circuit.
 
 
 
   
POSSIBLE ANSWER
   
NAME : DIODE
   
How the component protects the circuit.
   

The diode is placed in parallel with the relay, placed the opposite way round to its normal direction. When the relay is switched off it can cause back E.M.F. This is current that is sent round the circuit in the wrong direction. This ‘spike’ can damage sensitive components such as transistors. The diode stops current flowing round the circuit in the wrong direction. It acts like a one way valve.

click here for more on diodes

   
7. What is back EMF ?
 
 
 
   
POSSIBLE ANSWER
   

E.M.F is Electro Motive Force. Back E.M.F is current that is sent round a circuit in the wrong direction. This can damage sensitive components such as transistors and integrated circuits.

   

8. The incomplete timer circuit is shown below. Some components are missing. Draw the missing components in the correct positions, adding suitable labels. The missing components are shown below the circuit diagram.

   
   
POSSIBLE ANSWER
   

9. In the space below explain how the entire circuit works. Include an explanation of the key components and their role in the circuit.

 
 
 
 
 
 
 
 
 
 
 
 
   
POSSIBLE ANSWER
   

When switch 1 is closed current is available for the circuit. When switch 2 is pressed current flows into pin 2 of the 555 timer, starting the timing sequence. Current flows from pin 3 of the timer into the base of the transistor allowing the relay to become energised. The relay switches on the remaining part of the circuit by allowing current to flow through the buzzer which sounds.
The diode protects sensitive components by preventing back E.M.F. The combination of the variable resistor and the capacitor determines the length of time the 555 timer counts. When the timer has finished counting, current no longer flows from pin 3 and the buzzer stops sounding.

   
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