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OP-AMPS AS COMPARATORS

V. Ryan © 2002-09

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Below are some examples of 741 I.C. based circuits. However, this time the 741 is used as a comparator and not an amplifier. The difference between the two is small but significant. Even if used as a comparator the 741 still detects weak signals so that they can be recognised more easily. It is important to understand these circuits as they very regularly appear in examinations.

   

A ‘comparator’ is an circuit that compares two input voltages. One voltage is called the reference voltage (Vref) and the other is called the input voltage (Vin).

When Vin rises above or falls below Vref the output changes polarity (+ becomes -).

Positive is sometimes called HIGH.
Negative is sometimes called LOW.

   

EXAMPLE CIRCUIT - LIGHT ACTIVATED ALERTER

   

The buzzer emits a tone when light falls on the light dependent resistor. Resistor 2 controls the sensitivity of the circuit.

The 741 is working as a comparator and the piezo buzzer sounds when the output form the 741 goes ‘low’ or in other words, changes from a positive to a negative.

   

 

   

EXAMPLE CIRCUIT - DARK ACTIVATED ALERTER

   

This is a dark activated circuit, the reverse of the circuit above. Do you notice the difference ?

If you look carefully you will notice that resistor 1 and the LDR have changed positions. Also, the inputs to the 741 are reversed.
Replace the LDR with a thermistor for a temperature circuit.

   
 
   

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