Light Meter Circuit

Posted on Jul 26, 2012

The outputs from the comparators will swing, in sequence, from high to low as the input voltage rises above the reference voltage applied to each comparator. The output LEDs will then switch on in sequence as the voltage rises. The inverting inputs of the comparators are connected in common to the collector of photo-transistor Ql. When Ql is illuminated, its collec-tor-emitter junction conducts, thereby placing all the inverting inputs within a few millivolts of ground. For most settings of Rl, each of the four reference voltages exceeds the value.

Light Meter Circuit
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Therefore, when Ql is illuminated, the output from each comparator is high and its respective indicator LED is off.

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