Window Comparator
The basic idea of a Window Comparator is to produce a circuit that will turn on a light,
ring an alarm etc when the input voltage lies in a ‘window”. The circuit is built using LM
311 single supply Op Amp comparators.
For the circuit shown the “Window” lies between 1/3 and 2/3 of 5 volts or 1.67 to 3.33
volts
The circuit has 3 indicators – a Below Window Voltage Indicator, an Above Window
Voltage Indicator and an In Window Voltage Indicator.
Example: The In Window voltages could represent a desired minimum and maximum
temperature range for a refrigeration unit. If the temperature is Ok then the In Window
Indicator LED is On. If the temperature rises or falls too much then the appropriate
other indicator LED turns on.
LED Voltmeter
This is a variation of a Window Comparator where a series of LEDs are turned on as the
input voltage is increased. The number of illuminated LEDs is a measure of the
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magnitude of the input voltage. Adding more LM 311 and LEDS would improve the
resolution of the voltmeter. The series of LEDs will turn on at 1 V, 2 V, 3 V and 4 V. If an
audio signal such as music was used as the input, the circuit could also be viewed as a
Peak Audio Voltage Detector - the louder the music the more LEDS would light up.
Application Note
Written by David Lloyd
Computer Engineering Program
Humber College
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