▌01 Can LEDs Be Used as Light Sensors?

We know that a moving coil electromagnetic speaker can play sound and also act as a microphone to detect sound. Similarly, a DC motor can convert electrical energy into kinetic energy and can also convert kinetic energy back into electrical energy. So can an LED also convert light into electrical signals and thus act as a photodetector?

Why is this question raised?

The reason for mentioning this question is to hope to replace the electromagnetic induction detection car model^[1] scheme in the referee system of the National College Student Intelligent Car Competition^[2] with optical detection.

1. Electromagnetic Detection Scheme

The earliest timing scheme used for intelligent car racing models was the infrared (laser) cross-beam photoelectric switch. Since this method requires installing transmitting and receiving photoelectric tubes on both sides of the track, it involves troublesome alignment and is easily affected by collisions with the car model, leading to timing errors.

Later, the scheme was changed to the electromagnetic induction principle detection scheme^[3], which only requires laying coils under the detection location on the track, using permanent magnets installed on the car model to generate trigger signals. The detection coils are relatively concealed and not easily damaged by collisions, but there are certain limitations: for example, when the car model runs slowly or when the permanent magnet installed on the car model is small, it may cause timing failures. It may also be triggered incorrectly due to the installation of wireless charging devices nearby.

2. Photodetection Scheme

Using semiconductor photodiodes to detect the pressure exerted by the car model has overcome some limitations of the electromagnetic scheme. In October 2018, a detection scheme based on phototransistors was proposed by Tong Chao from the Intelligent Car Team of Beijing University of Science and Technology.

The scheme was tested, and it could automatically identify the performance of the car model detection both in indoor environments with low light and outdoors under sunlight.

To avoid the impact of the color of the track surface on the measurement light band, white tape was used to fix the detection board on the track. Although there is white tape, it has no impact on the detection performance.

The image below shows the pulse situation that the detection light board can detect when the tape is pressed.

In the intelligent car competition in Sichuan Province, a photodetection board was also used^[5] as a method for car model detection, which improved the stability and accuracy of the competition timing measurement. In particular, the surface of the photodetection board was set to a black-and-white alternating pattern, seamlessly integrating with the zebra crossing at the finish line of the track, reducing the impact on the height and visibility of the track.

3. How to Reduce Costs

The photodetection boards used previously mainly employed phototransistors or similar semiconductor devices, converting the changes in current and voltage caused by the light intensity incident on the photodetector when the car model tires are pressed.

To accommodate different widths of car model wheels, the spacing of the photodiodes needs to be controlled between 1 cm and 3 cm. Therefore, within the limited detection range of 45 cm, a single photodetection board may require 20 to 50 photodiodes. In this year’s competition, due to the addition of intersections, the AI intelligent vision group required measurements of the stay time at intervals, which may increase the number of required detection boards to 5 to 10, making it meaningful to explore ways to reduce the cost of detection boards.

Compared to the cost of photodiodes, regular LEDs are much cheaper. For example, for 0603 packaged red LEDs, the price for 100 pieces is only a few yuan, far lower than the price of ordinary phototransistors, thus raising the question of whether LEDs can be used to make photodetection boards.

▌02 LEDs for Light Detection

A review of light sensors is provided in Light Sensors^[6]. The following detected articles are a portion of the literature obtained by searching for “LED as light sensor” on Bing.

1. Photovoltaic Voltage Generated by LEDs

In LEDs As Light Sensors^[7], the author Zorink demonstrated that the voltage generated across a red LED varies under different light intensities. The stronger the light, the higher the voltage produced by the LED.

According to the referenced Wikipedia content, the wavelength of light that LEDs respond to is equal to or less than the wavelength of the light emitted by the LEDs. For example, a green LED can generate a corresponding voltage for green or blue light, but it cannot respond to red light.

However, Zorink also experimentally showed that while LEDs can generate a corresponding DC voltage, they cannot be tested for the current output using a regular multimeter’s current setting. Therefore, the efficiency of LEDs converting light into electrical energy is very low.

In LED AS a LIGHT SENSOR^[8], an experiment was demonstrated where LEDs were used as light sensors, and an Arduino UNO was used to read the voltage on the LED via ADC to control the on/off state of another LED.

Similar experiments were also presented in the article USING LED AS A LIGHT SENSOR? SEE HOW IS IT POSSIBLE^[9].

2. Using LED Matrix to Create Touch Screens

In Multi-Touch Sensing through LED Matrix Displays^[10], the author uploaded a touch screen made from an LED array, showing how measuring the signals from LEDs for light detection can determine the position and pressure of a finger on the touch screen.

Unfortunately, this webpage did not reveal more technical details.

Additionally, this blog post mentioned that as early as 1977, Forrest M. Mins mentioned in his “Engineer’s Notebooks” that LEDs could be used as photodiodes.

A white paper from Altera titled Using LEDs as Light-Level Sensors and Emitters^[11] described how to use LEDs during off periods to sense ambient light levels, using this feedback to control the PWM duty cycle of the emitted light signal to maintain consistent contrast.

3. Using Transistors to Amplify LED Reverse Light Sensitive Current

Activity: LED as light sensor^[12] is an article from the AD Company website Wiki that introduces an experiment using transistors and Darlington pairs to amplify the reverse light-sensitive current of a red LED to detect changes in ambient light.

 ◎ Experimental Components:   NPN Transistor: 2N3904, or SSM2212   Resistors: 100kΩ, 2.2kΩ   LEDs: Red, Yellow, Green   Infrared LED: QED-123

In LED As Light Sensor^[13], operational amplifiers are used to amplify the reverse light-sensitive current of the LED (across a 1MΩ resistor) to produce a light intensity signal.

4. Dynamic Characteristics of LEDs as Photodiodes

Jon Clift‘s blog post Experimenting With Using an LED as a Light Sensor^[14] demonstrated a scheme using OP177 to amplify the reverse light-sensitive current of a red LED and measured the dynamic response characteristics of the LED as a photodiode.

The image below shows the amplification circuit diagram.

Using another LED to emit red light can detect the dynamic response characteristics of the LED as a photodiode.

The image below shows the dynamic characteristics of the LED detection tube. The rise and fall time of the circuit signal is approximately 40 microseconds.

Interestingly, Jon Clift also found that light emitted by green LEDs could still trigger a response when detected by red LEDs, although the sensitivity decreased significantly.

▌03 Conclusion of the Review

From the above review, it can be seen that using LEDs as light sensors is indeed possible. However, compared to photoresistors, photodiodes, phototransistors, and solar cells specifically designed for light detection, the electrical signals generated by LEDs are relatively weak and require effective current and voltage amplification to be usable.

If the photovoltaic characteristics of LEDs, that is, the voltage signal generated by LEDs under light, are directly amplified, the entire circuit is in a high-resistance state, so the voltage signal response speed is relatively slow.

Using the reverse light-sensitive current signal of LEDs through an I-V conversion circuit, like those used in sections [2-5], can provide a fast response time for the LED photodetection signal, which can meet the needs of detecting the operation of intelligent cars.

▲The atmosphere of the school anniversary is becoming more intense

The schemes mentioned earlier are all based on single LEDs to detect ambient light, while using multiple LEDs to create the photodetection board used in intelligent car competitions will require reasonable arrangements for the circuit connections of these LEDs. For example, should they be connected in series? In parallel? Or in a combination of both? This part of the design still needs further verification.

For scientific questions, perhaps the truth exists only in one form. But for engineering problems, it is necessary to weigh various factors related to practical applications, such as functionality, performance, cost, size, and power consumption, to find the best balance point. The fun in this may be left for those who enjoy technology and have a perfectionist mindset.

References