Light intensity detecting circuit with dark current compensation

A received light intensity detecting circuit with an optical sensor having first and second terminals and a parallel capacitor and being connected at the first terminal with a predetermined potential, first potential applying means connected between the second terminal of the optical sensor and a source of a first potential and rendered conductive at an initial setting, and a comparator having a first input terminal connected to the second terminal of the optical sensor and a second input terminal receiving a comparative reference potential of a level between the predetermined potential and the first potential such that the intensity of received light is detected as the time period between the initial setting and the instant when an output signal is generated from the comparator, comprising:

• a time constant circuit including a capacitor having a first terminal connected to the predetermined potential and a second terminal connected to the second input terminal of the comparator;
• second potential applying means for applying a second potential of a level between the predetermined potential and the first potential to the first input terminal of the comparator prior to the initial setting;
• third potential applying means for operating said time constant circuit for raising the second terminal of the capacitor of said time constant circuit to a third potential having a level between said predetermined potential and said second potential while said second potential is being applied by said second potential applying means to said first input terminal of said comparator;
• and
• interruption means for interrupting the operation of said time constant circuit in response to the generation of the output signal of the comparator.

A received light intensity detecting circuit with an optical sensor having first and second terminals and a parallel capacitor and being connected at the first terminal with a predetermined potential, first potential applying means connected between the second terminal of the optical sensor and a source of a first potential and rendered conductive at an initial setting, and a comparator having a first input terminal connected to the second terminal of the optical sensor and a second input terminal receiving a comparative reference potential of a level between the predetermined potential and the first potential such that the intensity of received light is detected as the time period between the initial setting and the instant when an output signal is generated from the comparator, comprising:

• an optically shielded mimic sensor having a first terminal connected to the predetermined potential and a second terminal connected to the second input terminal of the comparator and having the same structure as that of the optical sensor including a corresponding parallel capacitor;
• a time constant circuit including the parallel capacitor of said mimic sensor corresponding to the parallel capacitor of the optical sensor;
• second potential applying means for applying a second potential of a level between the predetermined potential and the first potential to the first input terminal of the comparator prior to the initial setting;
• third potential applying means for operating said time constant circuit for raising the second sensor input terminal of said mimic sensor to a third potential having a level between the predetermined potential and said second potential while said second potential is being applied by said second potential applying means to said first input terminal of said comparator;
• and
• interruption means for interrupting the operation of said time constant circuit in response to the generation of the output signal of the comparator.