Photo-detecting method and photo-detecting device with varying modulation frequency
A photo-detecting method of the present invention includes the steps of projecting from a light emitting element a pulse-modulated light as pulse-modulated at a light-emitting element drive circuit toward an object; and counting a light-receiving signal, which is converted by the light-emitting element based on either reflected light reflected from the object or transmitted light transmitted through the object, plurality of times in synchronism with a modulation frequency of the pulse-modulated light projected toward the object, so as to detect the object, wherein the modulation frequency of the pulse-modulated light projected toward the object is changed while carrying out the step of counting the light-receiving signal plurality of times. With this method, it is possible to prevent faulty operation when disturbance light or noise enters the light-receiving element without the light-receiving signal.
1. A photo-detecting device, comprising:
an oscillation circuit provided with a plurality of constant current sources for outputting a plurality of signals having different oscillation frequencies;
light-emitting means for projecting toward an object a light that is pulse-modulated in response to a signal from said oscillation circuit;
light-receiving means for receiving either light reflected from the object or light transmitted through the object, and for converting the received light into a light-receiving signal;
counting means for counting the light-receiving signal a plurality of times in synchronism with a modulation frequency of the projected light;
detecting means for detecting the object in response to said counting means; and
frequency changing means for, in accordance with a count of the counting means, controlling the plurality of constant current sources to vary an oscillation frequency of said oscillation circuit in a determinable manner, while said counting means counts the light-receiving signal a plurality of times.
2. The photo-detecting device as set forth in claim 1 , wherein:
said counting means is a shift register in which a plurality of flip-flops are connected in a cascade manner; and
said frequency changing means changes the oscillation frequency of said oscillation circuit in response to said output signal from at least one of the flip-flops except the flip-flop at a last stage.
3. The photo-detecting device as set forth in claim 2 , wherein:
said frequency changing means changes the oscillation frequency of said oscillation circuit when said output signal from one of the flip-flops changes.
4. The photo-detecting device as set forth in claim 2 , wherein:
said frequency changing means changes the oscillation frequencies of said oscillation circuit in response to said output signal from at least one of the flip-flops except the flip-flops at a first and last stage.
5. The photo-detecting device as set forth in claim 2 , wherein:
said oscillation circuit outputs three or more signals having different oscillation frequencies;
said counting means is a shift register in which four or more flip-flops are connected in a cascade manner; and
said frequency changing means changes the oscillation frequencies of said three or more signals outputted from said oscillation circuit, in response to output signals from two or more flip-flops except the flip-flop at a last stage.
6. The photo-detecting device as set forth in claim 1 , wherein:
said plurality of constant current sources of said oscillation circuit are provided in a same number as a number of variable oscillation frequencies.
7. The photo-detecting device as set forth in claim 1 , wherein:
said oscillation circuit changes the oscillation frequency so that any one of oscillation frequencies is not an integer multiple of another oscillation frequency.
8. A photo-detecting device, comprising:
an oscillation circuit provided with a plurality of constant current sources for outputting a plurality of signals having different oscillation frequencies;
light-emitting means for projecting toward an object a light that is pulse-modulated in response to a signal from said oscillation circuit;
light-receiving means for receiving either light reflected from the object or light transmitted through the object, and for converting the received light into a light-receiving signal;
counting means for counting the light-receiving signal a plurality of times in synchronism with a modulation frequency of the projected light;
detecting means for detecting the object in response to said counting means; and
frequency changing means for, in accordance with a count of the counting means, controlling the plurality of constant current sources to vary an oscillation frequency of said oscillation circuit dependent on a previously received light-receiving signal while said counting means counts the light-receiving signal a plurality of times.
9. The photo-detecting device as set forth in claim 1 , wherein the oscillation circuit further comprises a condenser; and
said frequency changing means controls which of the plurality of constant current sources charges and discharges the condenser.