Photodetecting circuit

• US 4,851,689 A
• Filed: 12/23/1987
• Issued: 07/25/1989
• Est. Priority Date: 04/15/1987
• Status: Expired due to Term

• Current Assignee: ALPS Electric Company Limited (Alps Alpine Co., Ltd.)
• Original Assignee: ALPS Electric Company Limited (Alps Alpine Co., Ltd.)
• Inventors: Hasegawa, Kazuo
• Primary Examiner(s): Nelms, David C.
• Assistant Examiner(s): Messinger, Michael
• Application Number:
  US07/137,414
• Time in Patent Office: 580 Days
• Field of Search: 250/214 B, 250/214 A, 250/221, 250/222.1, 307/311, 340/555-557
• US Class Current: 250/214.00B
• CPC Class Codes: G01J 1/42 using electric radiation detectors optical or mechanical part G01J1/04; by comparison with a reference light or electric value G01J1/10
• CPC Class Codes: G01J 2001/182 with SH sample and hold circuits
• CPC Class Codes: G01J 2001/4238 Pulsed light
• CPC Class Codes: G01J 2001/444 Compensating; Calibrating, e.g. dark current, temperature drift, noise reduction or baseline correction; Adjusting
• CPC Class Codes: G01N 2201/0621 Supply
• CPC Class Codes: H03K 17/14 Modifications for compensating variations of physical values, e.g. of temperature
• CPC Class Codes: H03K 17/941 using an optical detector H03K17/968 takes precedence

Abstract

A photodetecting circuit for detecting radiation emitted from a light emitting element in the presence of ambient radiation is disclosed. The circuit includes a first and a second synchronizing on/off input signal having a predetermined period, each signal being complementary to the other. When the first synchronizing signal is "on", the second synchronizing signal is "off" and the circuit can detect the amount of ambient radiation. When the first synchronizing signal is "off", the second synchronizing signal is turned "on" and the light emitting element is energized and the circuit detects the amount of radiation emitted therefrom. The circuit prevents erroneous readings of radiation emitted from the light emitting element when the amount of detected ambient radiation is high.

Claims

1. A photoconducting circuit comprising:

a light emitting element driven by a first synchronizing on/off input signal having a predetermined period;

a light receiving element correspondingly arranged for detecting light emitted by said light emitting element and for outputting a current signal varied in response to an amount of said light received from said light emitting element;

a current-to-voltage converting circuit for converting said current signal from said light receiving element into a corresponding voltage output signal;

a first negative feedback circuit for negative-feeding, at a specified first constant, said voltage output signal of said current-to-voltage converting circuit back to an input circuit of said current-to-voltage converting circuit;

a sample-and-hold circuit for detecting and holding said voltage output signal of said current-to-voltage converting circuit, said sample-and-hold circuit being driven by a second synchronizing on/off input signal which is the complement of said first synchronizing on/off input signal;

a second negative feedback circuit for negative-feeding an output signal from said sample-and-hold circuit back to said input circuit at a second constant which is greater than said first constant of said first negative feedback circuit.

2. A photoconducting circuit as claimed in claim 1, wherein said light emitting element is a light emitting diode.

3. A photoconducting circuit as claimed in claim 1, wherein said light receiving element is a phototransistor.

4. A photoconducting circuit as claimed in claim 1, wherein said current-to-voltage converting circuit is constructed by a first operational amplifier, and said first feedback circuit is arranged by a first resistor connected to the first operational amplifier so as to feed an output signal of said first operational amplifier back to an input terminal thereof.

5. A photoconducting circuit as claimed in claim 1, wherein said sample-and-hold circuit is arranged by a second comparator, a switching element, and a capacitor.

6. A photoconducting circuit as claimed in claim 1, wherein said second feedback circuit is constructed by at least a second operational amplifier and a second resistor.

7. A photoconducting circuit as claimed in claim 1, wherein said first feedback circuit is connected parallel to said second feedback circuit.

8. A photoconducting circuit as claimed in claim 7 wherein said second operational amplifier has an output terminal and an input terminal directly connected to said output terminal, thereby functioning as a buffer amplifier.