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741 OPERATIONAL AMPLIFIER COMPARATOR EXAMINATION QUESTION AND INFORMATION

V. Ryan © 2007 - 2009

 

PDF FILE - CLICK HERE FOR PRINTABLE VERSION OF EXERCISE SEEN BELOW

A home-made anemometer can be seen below. It is part of a system that calculates the wind speed. It is composed of four cups that rotate on a central shaft. As it rotates a light / dark sensor, housed in the ‘sensor bracket’ detects light from the light bulb found inside the sensor bracket. The sensor is connected to a circuit that counts each time the disk rotates (light from the bulb is detected).

 

NOTES

 
 

 
 
 
 
 
   
SAMPLE ANSWER
   

NOTES

As the disk spins the light from the bulb shines through the two holes in the disk as it rotates. The sensor detects the light. The circuit it is connected to counts each time light is detected.
 
 
 
 
 

2. Below is a 3D version of the sensor / counter circuit.

 
 

The circuit diagram of the 3d circuit is seen below. When the light from the bulb shines on the light / dark sensor the resistance of the LDR decreases. This allows current to flow into pin 2. The 741 compares the current of pin 2 and pin 3. When a change in current occurs in either pin 2 or 3 the 741 outputs current at pins 6. This energises the relay. The energised relay activates the counter circuit. Each time the counter circuit is activated it adds a number.

However, there is a fault with the circuit due to the position of the light bulb and LDR. Sometimes the LDR does not detect the light from the bulb. Explain why this may happen and how it could be corrected.

   
3. WHY THE LIGHT IS NOT ALWAYS DETECTED:
 
 
 
   

POSSIBLE ANSWER

   

The bulb is not directly above the LDR. This means that much of the light from the bulb shines away from it.

   
4. HOW THE CIRCUIT COULD BE CORRECTED SO THAT LIGHT IS ALWAYS DETECTED:
   

POSSIBLE ANSWER

   

The LDR could be moved so that the light shines directly into it. This will reduce the resistance of the LDR so that current flows through the circuit energising the relay.

   

5. A 741 Operational Amplifier is represented by a distinctive symbol. Draw the symbol in the space opposite

 

 

 

 

 

   
POSSIBLE ANSWER
   

The sensor circuit has been altered slightly and it is now suitable for use in the sensor bracket. The LDR is not soldered directly to the PCB as it is fixed in position with a electrical connector. This means that the LDR is directly above the bulb. As the bulb lights the LDR detects the light immediately.

 

 

6. How does the position the LDR in relation to the bulb make the circuit more efficient ?

 
 
 
 
   
POSSIBLE ANSWER
   

The circuit is more efficient because light from the bulb shines directly into the LDR. Light is either present or not present. The circuit is now an accurate light / dark sensor.

   

7. Identify the components listed below (label the components on the circuit diagram) :
 

Preset Resistor - Op Amp - Diode - LDR - Bulb - Relay

   
POSSIBLE ANSWER

8. Label the 741 Op Amp shown in the circuit above according to the Pin Table shown opposite.

     
     
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