LED photosensitive voltage and current signal

▌01 LED reverse photosensitive current

1. Experimental background

In , can we use tandem LEDs (or photodetector LEDs?) [1] discusses the use of ordinary LEDs as photoelectric sensors to reduce the national college student smart car competition [2] 's electromagnetic induction coil used in the timing system_ span12span [3] . In the message after the TSINGHUAZHUOQING public account corresponding to the tweet [4] , Tong Chao also gave the results of their previous experiments: LEDs are used as photosensitive devices, and the frequency The response is low (only a few hundred Hz) , which may not be able to meet the requirements of monitoring accuracy when the model passes.

In order to facilitate the production, I ordered the LED strip light strip [5] ,It can reduce the cost of production. But what about its performance as a sensor light?

▲ Ordinary LED lamp current

For the above problems, some tests will be used to determine later.

2. Test LED

There are a variety of surface mount and in-line LEDs at hand, the colors also include red, green, blue, white and so on.

LED ▲ for test

test indicators include:

• LED photovoltaic voltage;
• LED reverse Photosensitive current (this is the main test difference);

test the amplitude and frequency response of the corresponding signal.

▌02 LED photovoltaic signal

measurement process,They are all done in paper boxes painted black on the inside.

▲ The black box used for the measurement

1. The measured response time

▲ LED measured PV signal

◎ measurement conditions:
source: high-intensity white LED used as the light source;
measuring voltage: Use FLUKE45 DC voltage measurement
    measurement time interval: 1 second

manually turn on and turn off the light source, repeat twice, read the corresponding voltage and record.

The following is the measured voltage change curve.

▲ Change of LED photovoltaic signal

You can see the change of photovoltaic voltage in about 3 seconds.The voltage rises to about 63%.

2. The relationship between voltage and light intensity

According to the relationship between LED current and light intensity [6] [6] The light intensity emitted by the white LED is approximately linear with the operating current. Therefore, measuring the current of the luminous white LED can reflect the intensity of the light.

◎ measurement:
illumination light : using high-power white LED generating, collecting
PV voltage measurement: direct measurement using FLUKE45;

▲ The relationship between the current flowing through the lighting LED and the light discharge voltage of the red LED

From the above measurement results, we can see that the photovoltaic voltage and light intensity show a roughly linear relationship.

3. The photovoltaic voltage corresponding to different color LEDs

In the same environment,Measure the photovoltaic voltage of several LEDs of different colors.

different color LED [Table 2-3] PV voltage

color LED Diameter (mm) photovoltaic voltage (V) red 5 0.0683 red (transparent encapsulation) 5 0.347 green (transparent encapsulation) 5 0.065 yellow 5 1.43 _span12sp an blue 3 0.0993 infrared LED (transparent package) 5 0.236

the color of the LED,The diameter of the package varies. They include red, green, red, blue, and infrared lights. From the above test results, the relationship between the corresponding photovoltaic voltage and color is not obvious. For example, the red output voltage actually has 1.43V. The red color corresponds to different photovoltaic voltages for different packages (transparent, red).

▲ Several different colors of LEDs

▌03 The reverse current of the LED span tube is very small because the reverse current of the LED span

There are two Darlington transistors, KSP13, and BC517.

▲ Two types of Darlington transistors: BC517, KSP13

strongspan strong13span 13 strong_span 13 strong_span 13 span13 strong_span 13 strong_span 13 strong span13 span13 KSP13 [7] Basic parameters of : Type: NPN VCEO: 30V VCBO: 30V Icmax: 500mA fT: 125MHz

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▲ KSP13 Darlington Transistor Parameters

(2) BC517

BC517 Darlington [8]

▲ BC517 Darlington Transistor

span10 span Let's compare the DC current gain between BC517 and KSP13,At that time, it was roughly around 40,000.

▲ The relationship between DC current gain and Ic of BC517

2. For Darlington span1

10h15, span10h15, span12p

2. Use to test resistors, capacitors, diodes, transistors, and transistors to test the display module [9] to measure their basic parameters.

(1) Measure BC517 parameters

▲ Measure BC517's pin distribution and basic parameters strongspan

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▲ Measuring the basic parameters of KSP13 Darlington tube

From the results of the above measurement, KSP13 is at Ie=5.4mA.The DC current gain is larger than BC517.

3. Use KSP13 to amplify the LED reverse current

According to the figure below, after the LED reverse current is amplified by Darlington, it is measured by the voltage drop on its collector resistor R1.

▲ Darlington amplified using reverse LED current measurement program

◎ measurement parameters:
VCC: + 5V
R1: 995Ω

(1) Measure under the desktop light

[Table 3-3-1 LED reverse current measurement under the desktop]

LED model (span12span) strong13span (span12span) Ic red (red package 0.156V 0.156mA yellow (transparent encapsulation) 4.22V 4.22mA span1 0span red (transparent encapsulation) 1.72V 1.72mA green (transparent encapsulation) 0.366V 0.366mA infrared (transparent encapsulation) 0.616V 0.616mA blue (transparent encapsulation) 0.909V 0.909mA

can be seen,The reverse current of the yellow LED is the largest, and the reverse current of the original ordinary red packaged LED is the smallest.

(2) Measuring the relationship between light intensity and directional current

Using the same method as in [2-2: The relationship between voltage and light intensity], high-power white LEDs are used as light sources , Measure the relationship between the light intensity of the LED light source (expressed by its working current) and the reverse current of the LED in the dark box.

▲ The cassette used to measure the reverse current and light intensity

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▲ The relationship between the reverse current and the light intensity current of the yellow LED

【Ⅱ. The reverse current and light intensity of the red LED] p _p18span _span12

0 ▲ The relationship between the reverse current of the red LED and the light intensity (represented by the operating current of the supplementary LED)

From the results of the above measurement,There is a certain linear relationship between reverse current and light intensity.

4. LED reverse current dynamic characteristics

LED reverse current reflects the dynamic relationship between the reverse current and the external input light intensity.

Test method: Use DG1062 to generate a square wave signal, and use an NPN (8050) transistor to drive the white LED to flash. Use an oscilloscope to measure the Darlington collector voltage waveform.

▲ measure the dynamic characteristics of the LED reverse current

strong13MHz strong13span_strong13MHz according to SP_strong13span_strong_strong13MHz In the following measurement, if the measured dynamic spectrum is within 1MHz, it should reflect the electrical dynamic characteristics of the LED itself.

(1) Red LED reverse current waveform

▲ Measure the Darlington collector voltage waveform under 10 Hz flicker _p

(2) Yellow LED reverse current waveform

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From the measurement results above,The dynamic characteristics of the LED should be around 1kHz.

▌Experiment summary

Through the previous experiment, we can see that the photosensitivity of the LED is reflected in its photovoltaic characteristics and strong13 _strong16 reverse light detection_spanstrong_span_strong16 Relationship between.

The photovoltaic characteristics, reverse current and detection light intensity all show linear characteristics within a certain range.

The photovoltaic characteristics are basically between tens of millivolts and 1 kilovolt. The photovoltaic characteristic of the yellow LED is the greatest.

The reverse current is usually only a few microamperes. Using Darlington transistors can amplify the reverse current to several milliamps. Similarly, the yellow LED corresponds to the largest.

In terms of dynamic characteristics, the photovoltaic characteristics under no-load conditions are very slow. This is mainly because the parasitic capacitance of the PN junction of the LED is relatively large without reverse bias, which causes the change to be slow.

But the reverse current, when the reverse bias is 5V, the dynamic frequency will be around 1kHz.

▲ The response of the yellow LED reverse current to 2kHz light pulse

The research of the reverse current detection of the LED in this paper is given by the Darlington tube means.This method is also suitable for the reverse current amplification process of the purchased LED strip in the future.

References

[1]

Can a series LED (or photosensitive LED) be used to make a photoelectric detection board? : https://zhuoqing.blog.csdn.net/article/details/115817993

strong 10 _span12 _span12 strong span10 _span12 _span12 _strong_strong_span12 https://zhuoqing.blog.csdn.net/article/details/110253008

[3] strong span_strong_span10 867 _strongspan10 _strongspan_strong_p10span10 ://zhuoqing.blog.csdn.net/article/details/104120744

[4]

AZHUspan_ _ _span_strong13span_strong13span_strong13span_strong13span_span_strong //mp.weixin.qq.com/s?_ _biz = MzA5NjQyNjc2NQ & mid = 2452239485 & idx = 1 & sn = 139d3c5e9bb534042c807e7173af754e & chksm = 876eda1fb0195309d1d01df98d5d14a2602db8cb4dc4307965e39d6885baba05fb83a011b70c & token = 24919686 & lang = zh_CN # rd *

_strongspan12 span10 _strongspan12span12 : https://www.mouser.cn/Semiconductors/Discrete-Semiconductors/Transistors/Darlington-Transistors/KSP13-Series/Datasheets/_/N-ax1sb?P_emxk8span 872 p_emsk8span =1yxk8span [8]

A good helper for testing resistors, capacitors, diodes, transistors, and transistor test display modules : 867__strong. /details/109223139