LEDs are a favored technology recognized for their energy efficiency, cost-effectiveness, and wide availability. While frequently utilized for illumination, LEDs possess an intriguing hidden capability – they can also serve as detectors!
Versatile Devices
The notion of using an LED as a detector is similar to repurposing a speaker as a microphone. Rather than producing light, an LED can sense light. This distinctive trait can be witnessed by attaching an LED to a multimeter configured to gauge current and aiming it towards a light source such as sunlight. Unlike traditional photodiodes, LEDs are responsive to light within specific wavelength bands.
Pushing the boundaries of innovation, in 2003 Mitsubishi Electric Research Laboratories (MERL) introduced a technique to employ LEDs as detectors by linking them to a microcontroller in a “reverse biased” stance. In this configuration, the LED can act as a detector when connected to an IO pin instead of emitting light. Grounding the LED’s positive terminal and connecting the negative terminal to an IO pin set in a high state, the LED’s capacitance is charged, which then discharges into the microcontroller pin. The duration for the voltage to drop below the digital logic level of the IO pin can effectively gauge light intensities.
The MERL team further developed this concept by investigating the utilization of LEDs for short-range data transmission, configuring the LED amidst two tristate IO pins on a microcontroller to serve as a light emitter and receiver. This inventive method unveils possibilities for applications like modifying screen brightness based on ambient light sensed by a TV’s power LED or devising proximity detectors using LED arrays for emitting and sensing light interchangeably. Also, the team suggested the use of status LEDs for bidirectional communication on compact devices.
While LEDs can operate as detectors, they may not be as precise as dedicated phototransistors or photodiodes. Despite some constraints, LEDs present practical applications where they have been effectively utilized as detectors.
This approach demonstrates the adaptability of LEDs beyond just illumination and illuminates captivating opportunities for innovative detector applications.
Employing LEDs as detectors offers benefits, such as selective wavelength sensitivity and long-lasting stability. Forrest Mims has seamlessly incorporated LED detectors into scientific instruments, including an ozone measurement apparatus that exposed a drift error in NASA’s Nimbus-7 satellite.
Perovskite technology, particularly Perovskite LEDs (PerLEDs), displays potential as efficient light sources and efficient photodetectors. The objective is to develop touch-sensitive displays that integrate the PerLED array as both a display and a detector, although the current instability of perovskites hampers practical implementation.
It’s essential to note that this detection technique may not be compatible with contemporary LEDs like programmable LEDs or self-flashing LEDs that do not enable direct linkage to the LED die. Numerous modern intelligent LEDs lack exposed pins for connection to a microcontroller, limiting the application of this technique to LEDs such as NeoPixels or WS2812Bs.
In summary, employing LEDs as detectors is a captivating and pragmatic approach for specific light wavelength tasks. It can be a valuable resource for various scenarios as needed.
There exist numerous techniques to capitalize on this ability. For individuals preferring analog approaches, coupling the LED with an op-amp can magnify the output. Virtually any standard op-amp can be employed to escalate the output voltage proportionate to the light falling on the LED.
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