LEDs are a popular technology known for being energy-efficient, cost-effective, and readily available. While commonly used for lighting, LEDs have an interesting hidden ability – they can also be used as sensors!
Multi-Purpose Devices
The concept of utilizing an LED as a sensor is akin to repurposing a speaker as a microphone. Rather than emitting light, an LED can detect light. This unique characteristic can be observed by connecting an LED to a multimeter set to measure current and directing it towards a light source like the sun. Unlike traditional photodiodes, LEDs are sensitive to light within specific wavelength ranges.
Taking innovation a step further, in 2003 Mitsubishi Electric Research Laboratories (MERL) introduced a method to use LEDs as sensors by connecting them to a microcontroller in a “reverse biased” mode. In this setup, the LED can function as a sensor when connected to an IO pin instead of emitting light. By grounding the LED’s anode and connecting the cathode to an IO pin set in a high state, the LED’s capacitance is charged, which then discharges into the microcontroller pin. The time taken for the voltage to drop below the digital logic level of the IO pin can effectively detect light levels.
The MERL team expanded on this idea by exploring the use of LEDs for short-distance data transmission, configuring the LED between two tristate IO pins on a microcontroller to act as a light transmitter and receiver. This innovative approach opens up possibilities for applications like adjusting screen brightness based on ambient light detected by a TV’s power LED or developing proximity sensors using LED arrays for emitting and sensing light interchangeably. Additionally, the team proposed using status LEDs for bidirectional communication on small devices.
While LEDs can function as sensors, they might not be as accurate as dedicated phototransistors or photodiodes. Despite some limitations, LEDs offer real-world applications where they have been successfully used as sensors.
This method showcases the versatility of LEDs beyond just lighting and presents fascinating opportunities for innovative sensor applications.
Utilizing LEDs as sensors has advantages, such as selective wavelength sensitivity and long-term stability. Forrest Mims has effectively incorporated LED sensors into scientific instruments, including an ozone measurement device that exposed a drift error in NASA’s Nimbus-7 satellite.
Perovskite technology, particularly Perovskite LEDs (PerLEDs), shows promise as efficient light sources and capable photodetectors. The aim is to create touchable displays that integrate the PerLED array as both a display and a sensor, though the current instability of perovskites hinders practical application.
It’s worth noting that this sensor technique may not be compatible with modern LEDs like addressable LEDs or self-flashing LEDs that do not allow direct connection to the LED die. Many contemporary smart LEDs lack exposed pins for connection to a microcontroller, restricting the application of this method to LEDs like NeoPixels or WS2812Bs.
In conclusion, utilizing LEDs as sensors is a fascinating and practical approach for specific light wavelength applications. It can be a valuable tool to have on hand for various scenarios when needed.
There are multiple ways to leverage this capability. For those preferring analog methods, connecting the LED to an op-amp can amplify the output. Nearly any standard op-amp can be used to enhance the output voltage proportional to the light falling on the LED.
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