JPH09249339A - Paper detection device and portable printer using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a paper detection device which can prevent erroneous detection caused by the effect of external light. SOLUTION: A paper detection device is provided with an optical sensor consisting of a light emitting part and a light receiving part. It detects a light receiving level of the light receiving part (St.2) in a condition in which the light emitting part is not lit (St.1) and adds a predetermined value which is determined in advance to the light receiving level to set a threshold value (St.3). Then, it detects a light receiving level of the light receiving part (St.5) in a condition in which the light emitting part is lit (St.4) to recognize the presence of paper (St.7, St.8) based on the comparison between the light receiving level and the threshold value (St.6). Consequently, it is possible to remove the effect of external light from the results of detection.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sheet detection device for detecting the presence / absence of a sheet and a portable printer using the same.

[0002]

2. Description of the Related Art Some printers include a paper detection device for detecting the presence or absence of paper in a paper passage to control a printing operation for printing an image on paper and a paper feeding operation for conveying paper. is there. Such a printer detects whether or not there is a sheet in a sheet passing path by a sheet detection device,
Control the printing operation and the paper feeding operation based on this result,
Print on the paper by aligning the print start position with the leading edge of the paper.

Some sheet detection devices have an optical sensor including a light emitting portion and a light receiving portion. This paper detection device detects the light receiving level of the light receiving portion in the light emitting portion lighting state in which the light emitting portion is made to emit light, and detects the presence or absence of paper by comparing the light receiving level with a predetermined threshold value.

[0004]

By the way, as the printer, there is a portable printer which can be carried. In this portable printer, a plastic mold is often used for easy carrying, and the thickness of the mold is also thin. In addition, a mold having a color that easily transmits outside light such as sunlight may be used. Therefore, in such a portable printer, external light passes through the mold and enters the light receiving portion of the optical sensor.

In addition, since the portable printer is portable, it may be used not only indoors but also outdoors.
When the portable printer is used in a sunny place such as outdoors, the light-receiving level of the light-receiving portion of the optical sensor changes greatly due to the effect of sunlight.

Therefore, there is a problem that it is detected that there is paper even though there is no paper.

[0007]

According to the first aspect of the present invention,
An optical sensor including a light emitting unit and a light receiving unit, a natural light detecting unit that detects a light receiving level of the light receiving unit in a state where the light emitting unit is not lit and the light receiving unit detects the light receiving unit. Threshold value setting means for setting a threshold value by adding a predetermined value to the light receiving level of the light receiving section, and the light receiving level of the light receiving section in the light emitting section lighting state where the light emitting section is turned on A paper presence / absence recognition unit for recognizing the presence / absence of paper by comparing the level with the threshold value is provided. Therefore, the light receiving level of the light receiving section is detected by the natural light detecting means in the light emitting section non-lighting state, and the threshold value is set by adding the predetermined value preset to the light receiving level by the threshold value setting means, and then the light emitting section is turned on. By detecting the light receiving level of the light receiving section in this state and comparing the light receiving level with the threshold value by the paper presence / absence recognizing means to recognize the presence / absence of the paper, the influence of outside light can be removed from the detection result. False detection caused by the influence of external light is prevented.

According to a second aspect of the present invention, there is provided an optical sensor comprising a light emitting portion and a light receiving portion, and natural light detecting means for detecting a light receiving level of the light receiving portion when the light emitting portion is not turned on and the light emitting portion is not turned on. A lighting light detection means for detecting a light receiving level of the light receiving portion in a light emitting portion lighting state in which the light emitting portion is turned on; and a natural light detecting means for detecting the light receiving level of the light receiving portion detected by the lighting light detecting means. The paper presence / absence recognition means for recognizing the presence / absence of a paper by subtracting the received light level of the light receiving unit and comparing the obtained value with a predetermined threshold value. Therefore,
The light receiving level of the light receiving section is detected by the natural light detecting means when the light emitting section is not lit, the light receiving level of the light receiving section is detected by the lighting light detecting means when the light emitting section is lit, and the natural light is detected from the light receiving level detected by the lighting light detecting means. By subtracting the received light level detected by the detection means and comparing the obtained value with a predetermined threshold value by the paper presence / absence recognition means to recognize the presence / absence of paper, the influence of external light from the detection result Since it can be removed, false detection caused by external light is prevented.

According to a third aspect of the present invention, an optical sensor is provided in the sheet passage path, and the sheet detection device according to the first or second aspect is provided. Detection of the sheet by the sheet detection device is performed for printout of the sheet. Run every time. Therefore, even if the location where the portable printer is used changes every time the printout is performed, the false detection of the sheet detection device caused by the external light is prevented. The malfunction of the portable printer caused by the false detection of the detection device is prevented.

According to a fourth aspect of the present invention, in the portable printer according to the third aspect, the portable printer is provided with a light blocking member that is provided in the paper passage and reduces an incident amount of external light to the light receiving portion of the optical sensor. Therefore, it is possible to reduce the change in the light receiving level of the light receiving unit caused by the influence of the external light, and thus the erroneous detection of the sheet detection device caused by the external light is prevented.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A sheet detecting device according to a first embodiment of the present invention and a portable printer using the same will be described with reference to FIGS. The portable printer according to the present embodiment is a portable label printer that is used in a state where it is in close contact with the user's waist. As shown in FIG. 2, the portable label printer body 1 accommodates a label roll 3 obtained by winding a mount 2 in a roll shape. Labels 4 are attached to the mount 2 at predetermined intervals. A transport path 5 for transporting the mount 2 is formed in the portable label printer body 1. A print unit 6 for printing an image on the label 4 and a label issuing unit 7 for issuing the label 4 with the image printed thereon are sequentially arranged in the transport path 5 along the transport direction of the mount 2. There is.

The portable label printer body 1 is formed with an opening 8 for opening the front surface (upper surface in FIG. 3),
A cover 9 for opening and closing the opening 8 is rotatably arranged by a support shaft 9a. A hopper 10 in which the label roll 3 is rotatably accommodated is formed in the opening 8. The cover 9 is provided with a paper pressing portion 11 for pressing the floating of the label roll 3 in the hopper 10.

The transport path 5 is formed from the hopper 10 toward the front surface of the portable label printer. A transmissive optical sensor 12 is arranged on the transport path 5 upstream of the printing unit 6 in the transport direction of the mount 2.

A thermal head 13 for printing an image on the label 4 is provided in the printing section 6, and the thermal head 1
A platen roller 14 that presses the label 4 against the sheet 3 is disposed across the transport path 5. The thermal head 13 is arranged on the cover 9. The platen roller 14 has a paper feed motor 15 (see FIG. 6).
Are connected. The platen roller 14 is black and does not transmit light.

The label issuing unit 7 sends out a label on which an image is printed as shown in FIG.
6 are provided. The label issuing port 16 is formed in the cover 9. At a position facing the label issuing port 16, the mount 2 is bent at an acute angle and the mount 3 is attached.
A label peeling body 17 for peeling the label 4 is provided. The label peeling body 17 is formed such that when a part of the label 4 is peeled from the mount 2, the label 4 projects from the label issuing port 16.

Further, the label issuing section 7 is formed with a paper discharge path 18 which is a paper path through which the mount 2 from which the label 4 is peeled is conveyed. The paper discharge path 18 has a pinch roller 1 that contacts the platen roller 14.
9 are arranged. A reflection type optical sensor 20 is provided on the downstream side of the pinch roller 19 in the conveyance direction of the mount 2.
A light blocking member 21 that reduces the amount of external light incident on the reflective optical sensor 20 is arranged. On the downstream side of the reflective optical sensor 20 and the light shielding member 21 in the conveyance direction of the mount 2, the mount discharge port 2 for sending the mount 2 is provided.
2 are arranged. The mount discharge port 22 has one end of the cover 9 and the opening 8 when the cover 9 is closed.
And (see FIG. 5).

The pinch roller 19 is biased toward the platen roller by a spring (not shown), and the mount 2 is bent by the label peeling body 17 in the conveying direction at an acute angle.
With the platen roller 14. The pinch roller 19 is black and does not transmit light.

The reflection type optical sensor 20 is arranged on the back side of the portable label printer body 1 with respect to the paper discharge path 18. The reflection type optical sensor 20 is provided with a light emitting diode (hereinafter referred to as LED) 23 which is a light emitting portion and a photo sensor 24 which is a light receiving portion (FIG. 6).
reference).

The light shielding member 21 is arranged on the cover 9. The light blocking member 21 is the pinch roller 1
It is of a size that covers a space from near 9 to the mount outlet 22. A fine protrusion called a grain is formed on the surface of the light shielding member 21 facing the paper discharge path 18 in order to suppress the reflection of light.

Further, the portable label printer main body 1
The back surface of the is formed with a gentle curved surface, and a belt hooking portion (not shown) for hooking this portable label printer on the bell of the user's waist is provided. A battery 25 is built in the portable label printer body 1 (see FIG. 2).

Next, the hardware configuration of the portable label printer of this embodiment will be described with reference to FIG. This portable label printer is provided with a CPU 26 that controls the operations of the printing unit 6 and the label issuing unit. The CPU 26 includes a thermal head driver 27 that drives the thermal head 13, a motor driver 28 that drives the paper feed motor 15, and the LED 2
The LED driver 29 for driving the LED 3 is connected via the bus 30. An A / D converter 31 is connected to the CPU 26 via the bus 30. Said A
The photo sensor 24 is connected to the / D converter 31. An I / O port 32 is connected to the CPU 26 via the bus 30. The transmissive optical sensor 12 and the operation unit 33 are connected to the I / O port 32. The operation unit 33 includes a reissue key (not shown) for reissuing the image printed on the previous label to reissue the label and a switch (not shown) for switching ON / OFF of the main power source. It is provided. The CPU 26 has a UART 34 (Universal Asynchronous).
nous Receiver and Transmitter; Asynchronous receiver /
A transmitter) and a RAM 35 are connected via the bus 30. The UART 34 receives image information and the number of printed sheets transmitted from, for example, a handy terminal (not shown) by infrared light or wireless. The R
The AM 35 temporarily stores, for example, the image information and the number of printed sheets received by the UART 34. The RAM 35 is provided with an image buffer 36 for expanding image information. A ROM 37 is connected to the CPU 26 via the bus 30. The ROM 37 stores, for example, a program relating to control of operations of the printing unit 6 and the label issuing unit 7. ROM 37
Is provided with a character ROM 38. The character ROM 38 stores specific print patterns such as numbers and bar codes.

In such a structure, the operation of the portable label printer will be described first. This portable label printer has a continuous issuance mode in which the label 4 on which the image is printed is issued from the label issuing port 16 while the label 4 is still attached to the mount 2, and the label 4 on which the image is printed is peeled from the mount 2. And the peeling issuing mode of issuing from the label issuing port 16 are selectively executed.

Whether the continuous issuance mode or the peeling issuance mode is executed is selected depending on whether or not the mount 2 is present in the paper discharge path 18. The continuous issuance mode is executed when the mount 2 is not present on the discharge path 18, and the peeling issue mode is executed when the mount 2 is present on the discharge path 18. Paper ejection path 18
Whether or not there is a backing sheet 2 is detected by a sheet detection device described later.

The operation when the portable label printer executes the continuous printing mode will be described. In the portable label printer, the mount 2 pulled out from the label roll 3
When the image information transmitted from the handy terminal is received or the reissue key is pressed after the tip of the sheet is inserted into the label issuing port 16 and the cover 9 is closed, the sheet detecting device causes the sheet to be ejected to the sheet ejection path 18. It is determined that there is no backing sheet 2 and the continuous issuance mode is selected. Then, the paper feed motor 15 is driven by the motor driver 28, the platen roller 14 is rotated to convey the backing sheet 2, and the thermal head driver 27 adjusts the thermal head according to the position of the label 4 conveyed on the conveyance path 5. 13 is driven,
The label 4 is heated by the thermal head 13 to develop a color, and an image is printed on the label 4. After that, when an image is printed on the predetermined number of labels 4, the printing operation ends.

Next, the operation when the portable label printer executes the peeling issue mode will be described. In the portable label printer, after the cover 9 is closed so as to cover the platen roller 14, the label peeling body 17 and the reflection type optical sensor 20 with the mount 2 pulled out from the label roll 3, the image transmitted from the handy terminal. When the information is received or the reissue key is pressed, the paper detection device determines that there is the mount 2 in the paper discharge path 18 and selects the peeling issue mode. Then, the motor driver 28
The paper feed motor 15 is driven by the platen roller 1
4 rotates and the mount 2 is conveyed, and the thermal head 13 drives the thermal head 13 in accordance with the position of the label 4 conveyed on the conveyance path 5, and the thermal head 13 heats the label 4 to heat the label 4. An image is printed on 4. Then, based on the number of rotations of the paper feed motor 15, the label 4 is peeled from the mount 2 by the label peeling body 17 and protrudes from the label issuing port 19 and a part of the label 4 is still attached to the mount 2. Then, the paper feed motor 15 is temporarily stopped. And label issuing slot 1
When the user completely peels off the label 4 protruding from the mount 9 and presses the reissue key, the printing operation is started again. In this way, the operation of printing images on the labels one by one is performed, and when the images are printed on a predetermined number of labels, the printing operation ends.

Next, the operation of the sheet detection device will be described with reference to FIG. When the image information transmitted from the handy terminal is received, the LED 23 is turned off (step St. 1). This process is forcibly performed regardless of whether the LED 23 is lit.
Then, the light reception level X of the photo sensor 24 is detected while the LED 23 is not lit (step S
t. 2). The function of the natural light detecting means is to detect the light receiving level X of the photo sensor 24 while the LED 23 is not lit.

Next, St. 2 is added with the predetermined value A set in advance to the light reception level X of the photo sensor 24 detected in 2 to set the threshold value (step S
t. 3). The function of the threshold value setting means is to add a predetermined value set in advance to the light reception level X of the photo sensor 24 detected when the LED 23 is not turned on to set the threshold value.

St. When the threshold value is set in step 3, the LED driver 29 drives the LED 23 to turn on the LED 23 (step St. 4). Then, with the LED 23 lit, the photo sensor 2
The light reception level Y of No. 4 is detected (step St. 5).

After that, St. 5, the light reception level Y detected in St. 3 is compared with the threshold value (X + A) set in 3, and it is determined whether or not the light receiving level Y is greater than or equal to the threshold value (X + A) (step St. 6). This St. When it is determined in 6 that the light receiving level Y is greater than the threshold value (X + A), it is determined that the light from the LED 23 is reflected by the mount 2 and is detected by the photosensor 24, and the mount 2 is present in the discharge path 18. It is recognized (step St. 7).

St. If it is determined that the light receiving level Y is smaller than the threshold value (X + A) in No. 6, it is recognized that there is no backing sheet 2 in the paper discharge path 18 (step St. 8).

These St. 4 to St. In 8, L
It is possible to detect the light receiving level Y of the photo sensor 24 in a state where the ED 23 is lit, and to recognize whether or not there is the backing sheet 2 in the paper discharge path 18 by comparing the light receiving level Y with the threshold value (X + A). This is the function of the paper presence / absence recognition means.

Then, when it is recognized whether or not the backing sheet 2 is present in the sheet discharge path 18, the operation of the sheet detecting device is ended.

As described above, the light receiving level X of the photosensor 24 is detected in a state where the LED 23 is not turned on, and a predetermined value A set in advance is added to the light receiving level X to set the threshold value (X + A). After that, the LED 23 is turned on to detect the light receiving level Y of the photo sensor 24,
By comparing the received light level Y with the threshold value (X + A) and recognizing the presence or absence of the paper, the influence of outside light can be removed from the detection result. Therefore, erroneous detection caused by the influence of external light is prevented. Further, the reflection type optical sensor 20 is arranged in the paper discharge path 18, and whether the mount 2 is present in the paper discharge path 18 every time the image information transmitted from the handy terminal is received or the reissue key is pressed. Whether or not the continuous issuance mode and the peeling issuance mode are selected is determined. As a result, even if the place or direction in which the portable label printer is used (light receiving direction of the photosensor 24) changes every time the printing operation is performed, the mount 2 is placed in the paper discharge path 18.
It is possible to prevent that the mount 2 is determined to be present despite the absence. Therefore, for example, when the portable label printer is used in a sunny place, only the peeling issue mode is selected and the continuous issue mode is prevented from being deselected. Further, the platen roller 14 and the pinch roller 19 are made black so as not to transmit light, and the light shielding member 21 is arranged from near the pinch roller 19 to the mount paper discharge port 22 to pass through the mold of this portable label printer. Since the change in the light receiving level of the photo sensor 24 caused by the influence of external light can be reduced, the false detection of the paper detection device caused by the external light can be prevented.

A sheet detecting device according to a second embodiment of the present invention will be described with reference to FIGS. 7 and 8. The same parts as those described in the first embodiment described above are denoted by the same reference numerals, and the description thereof will be omitted. In the paper detection device of the present embodiment, the first and second threshold values A and B are set in advance. The first threshold value A is for determining whether or not the mount 2 is present in the discharge path 18. Second threshold value B
Is for preventing erroneous detection caused by saturation of the light receiving level of the photo sensor 24. The erroneous detection occurs when the actual light receiving level becomes 5 V or more because the photo sensor 24 cannot measure the light receiving level of 5 V or more, for example.

The operation of the sheet detecting device having such a structure will be described with reference to FIG. In this paper detection device, when the image information transmitted from the handy terminal is received, the LED 23 is turned off (step St. 9). Then, the light reception level X of the photo sensor 24 is detected with the LED 23 not lit (step St. 10). The function of the natural light detecting means is to detect the light receiving level X of the photo sensor 24 while the LED 23 is not lit.

After that, the LED driver 29 causes the LED
23 is driven to turn on the LED 23 (step S
t. 11). Then, the light reception level Y of the photo sensor 24 is detected with the LED 23 turned on (step St. 12). The function of the lighting light detection means is to detect the light reception level Y of the photo sensor 24 while the LED 23 is lit.

Next, St. 12 from the received light level Y detected in St. The light reception level X detected in 10 is subtracted to obtain the light reception level Z (step St. 1).
3). This St. Received light level Z obtained in 13
Is compared with a preset first threshold value A, and it is determined whether the received light level Z is greater than or equal to the first threshold value A (step St. 14).

St. 14, when it is determined that the received light level Z is greater than the first threshold value A or more, it is recognized that there is the mount 2 in the discharge path 18 (step St. 15).

St. 15, when it is determined that the light reception level Z is smaller than the first threshold value A, St. The light-reception level Y detected in 12 and the preset second threshold value B are compared to determine whether the light-reception level Y is higher than the second threshold value B (step St. 16). . This St.
16, when it is determined that the light reception level Y is larger than the second threshold value B, St. At 15, it is recognized that the paper 2 is on the paper discharge path 18.

St. In 16, when it is determined that the light receiving level Y is less than or equal to the second threshold value B, it is recognized that the mount sheet 2 is not present in the discharge path 18.

FIG. 8 shows a range in which it is determined that the mount 2 is present in the discharge path 18 at this time. In FIG. 8, the range in which the oblique line descending to the lower right is drawn is the range in which it is determined that the mount 2 is in the discharge path 18.

These St. 13, St. 14, St.
15 and St. In FIG. 17, the light reception level Z detected by subtracting the light reception level X detected when the LED 23 is not turned on from the light reception level Y detected when the LED 23 is turned on is calculated. The function of the paper presence / absence recognizing means is to compare the first threshold position A with the first threshold position A to recognize whether or not there is the mount 2 in the paper discharge path 18.

Then, when it is recognized whether or not there is the backing sheet 2 in the sheet discharge path 18, the operation of the sheet detecting device is ended.

As described above, the light receiving level X of the photosensor 24 is detected while the LED 23 is not lit, and L
The light receiving level Y of the photo sensor 24 is detected with the ED 23 turned on, the light receiving level X is subtracted from the light receiving level Y to obtain the light receiving level Z, and the obtained value and the first threshold value A that is predetermined. Compared with
By recognizing whether or not the backing sheet 2 is present, it is possible to remove the influence of external light from the detection result, so that erroneous detection caused by external light is prevented. Further, when the light receiving level of the photo sensor 24 is high without the LED 23 being turned on, it is possible to prevent the continuous issuance mode from being selected instead of the peeling issuance mode even though the mount 2 is present in the paper discharge path 18. It is possible to prevent the label 4 from being attached to the label issuing unit 7.

In the embodiment of the present invention, the portable label printer which is used in a state of being in close contact with the user's waist is described, but the portable printer is not limited to the portable label printer. . For example, it can be applied to a printer that can be carried and used both in a dark warehouse and a bright office.

[0046]

According to the invention described in claim 1, the light receiving level of the light receiving portion is detected by the natural light detecting means in the light emitting portion non-lighting state in which the light emitting portion is not turned on, and the predetermined light receiving level is set in advance. After setting the threshold value by adding the values by the threshold value setting means, the light receiving level of the light receiving part is detected in the light emitting part lighting state with the light emitting part turned on, and the light receiving level and the threshold value are detected by the paper presence / absence recognition means. By recognizing the presence or absence of the paper sheet by comparison, it is possible to remove the influence of external light from the detection result, so that it is possible to prevent erroneous detection caused by the influence of external light.

According to the second aspect of the invention, the light receiving level of the light receiving portion is detected by the natural light detecting means in the light emitting portion non-lighting state where the light emitting portion is not turned on, and the light emitting portion is turned on in the light emitting portion lighting state. The light receiving level of the light receiving portion is detected by the lighting light detecting means, the light receiving level detected by the natural light detecting means is subtracted from the light receiving level detected by the lighting light detecting means, and the obtained value and the predetermined threshold value are obtained. By recognizing the presence / absence of the paper by comparing the paper presence / absence recognizing means with the paper presence / absence recognizing means, it is possible to remove the influence of the external light from the detection result, and thus it is possible to prevent the erroneous detection caused by the external light.

According to the third aspect of the invention, the optical sensor is provided in the paper passage path, and the sheet detection device according to the first or second aspect is provided, and the detection of the sheet by the sheet detection device is performed to print the sheet. Even if the location where the portable printer is used changes every time it is printed out, it is possible to prevent erroneous detection of the paper detection device caused by outside light, so that the portable printer can be used in a sunny place. It is possible to prevent the malfunction of the portable printer caused by the erroneous detection of the paper detection device when using the.

According to the fourth aspect of the present invention, by providing the light-shielding member which is provided in the paper passage and reduces the incident amount of the external light on the light-receiving portion of the optical sensor, the light-receiving portion caused by the influence of the external light is provided. Since it is possible to reduce the change in the light receiving level of the sheet, it is possible to prevent erroneous detection of the sheet detection device caused by external light.

[Brief description of drawings]

FIG. 1 is a flowchart showing an operation of a sheet detection device according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional plan view of a portable printer.

FIG. 3 is a perspective view showing a state in which a paper passing path of the portable printer is released.

FIG. 4 is a cross-sectional plan view showing a light shielding member provided in a paper passage.

FIG. 5 is a perspective view showing a portable printer using a paper detection device.

FIG. 6 is a block diagram showing an electrical hardware configuration of a portable printer.

FIG. 7 is a flowchart showing an operation of the sheet detection device according to the second embodiment of the present invention.

FIG. 8 is a graph showing a range in which it is determined that there is a sheet by a sheet detection device.

[Explanation of symbols]

 2 Paper 18 Paper Passage 20 Optical Sensor 21 Light-shielding Member 23 Light-Emitting Section 24 Light-Receiving Section

Claims (4)

[Claims] 1. A light sensor comprising a light emitting portion and a light receiving portion, a natural light detecting means for detecting a light receiving level of the light receiving portion in a non-light emitting portion non-lighting state of the light emitting portion, and the natural light detecting means. A threshold value setting means for setting a threshold value by adding a predetermined value to the detected light receiving level of the light receiving section, and a light receiving level of the light receiving section in a light emitting section lighting state in which the light emitting section is turned on. A sheet detecting device, comprising: a sheet presence / absence recognizing unit that detects the presence or absence of a sheet by comparing the received light level with the threshold value. 2. An optical sensor comprising a light emitting section and a light receiving section, natural light detecting means for detecting a light receiving level of the light receiving section in a light emitting section non-lighting state in which the light emitting section is not turned on, and the light emitting section is turned on. Lighting light detecting means for detecting the light receiving level of the light receiving portion in the light emitting portion lighting state, and the light receiving portion of the light receiving portion detected by the natural light detecting means from the light receiving level of the light receiving portion detected by the lighting light detecting means. A sheet detecting device, comprising: a sheet presence / absence recognizing unit for recognizing the presence / absence of a sheet by subtracting the received light level and comparing the obtained value with a predetermined threshold value. 3. A paper detection device according to claim 1, wherein an optical sensor is located in a paper passage, and the paper detection device detects the paper every time the paper is printed out. Characteristic portable printer. 4. The portable printer according to claim 3, further comprising a light shielding member which is provided in the paper passage and reduces an incident amount of external light on the light receiving portion of the optical sensor.