LED with sensing function and display of ambient light intensity

作宇:Dhananjay V Gadre，Sheetal Vashist，ECE Division, Netaji Subhas Institute of Technology, New Delhi

In addition to their traditional roles as indicators and illumination, modern LEDs can also act as photovoltaic detectors (References 1 and 2). Simply connecting a red LED to a multimeter and shining a bright light source, such as a similar red LED, on it will produce a reading greater than 1.4V (Figure 1). A model of a reverse-biased LED is equivalent to a charged capacitor in parallel with a light-dependent current source (Reference 1). If the incident light is increased, the current source is enhanced and can quickly discharge the equivalent capacitor to the supply voltage.

　　Figure 2 shows one way to use an LED as a photovoltaic detector. If the output pin 2 of the microcontroller is connected to the cathode of the LED, it will be reverse biased, charging the internal capacitance of the LED to the supply voltage. If the cathode of the LED is connected to the input pin 3, a high impedance load is connected to the LED. When light shines on the LED, a photocurrent is generated. The internal capacitance of the LED is initially charged to the supply voltage, discharged through the photocurrent source, and when the voltage on the capacitor drops below the lower logic threshold voltage of the microcontroller, pin 3 detects a logic 0. Increasing the incident light intensity discharges the capacitor more quickly, and lower light levels cause the discharge rate to decrease. The microcontroller, an Atmel AVR ATtiny15, measures the time it takes for the voltage at pin 3 to reach a logic 0 and calculates the intensity of ambient light incident on the LED. In addition, the microcontroller flashes the LED at a rate proportional to the intensity of the incident light.

　　 Figure 3 shows an Everlight Electronics Ltd. 3mm ultra-bright red LED, D1 _, encapsulated in a clear, colorless encapsulant, acting as an ambient light sensor. The circuit has only four components and operates from any DC power supply between 3V and 5.5V. The circuit uses only three of the six I/O pins of the AVR ATtiny15, leaving the remaining pins available to control or communicate with external devices. The sensor LED is connected to port pins PB0 and PB3 of the AVR microcontroller _. Another _port pin, PB3, generates a square wave with a frequency proportional to the intensity of the incident light. The circuit works by first applying a forward bias to the LED for a fixed time, then changing the bit sequence applied to PB0 _and PB1 _, thereby applying a reverse bias to the LED. Next, the microcontroller reconfigures PB0 _as an input pin. An internal timing loop measures the time interval T for the voltage applied to PB0 _to drop from logic 1 to logic 0.

　　Reconfiguring pins PB ₀ and PB ₁ to apply a forward bias to the LED completes the cycle. The time interval T is inversely proportional to the intensity of the ambient light incident on the LED. For weaker light, the LED flashes at a lower frequency, and as the incident light intensity increases, the LED flashes more frequently to provide a visual indication of the incident light intensity. The

　　LED light output intensity exhibits good linearity for lower forward current values ​​(Reference 2). To test the circuit, the light output of a second identical LED can be coupled to the sensor LED, D ₁ in Figure 3. These LEDs should be mounted in a sealed tube covered with opaque black tape to ensure that external light does not illuminate the sensor LED. Changing the forward current of the illumination LED from 0.33 mA to 2.8 mA produces a more linear sensor flicker frequency plot (Figure 4).

　　The efficiency of an LED as a sensor depends on its reverse bias internal current source and capacitance. To estimate the reverse photocurrent, place a 1MΩ resistor in parallel with the sensor LED and measure the voltage across the resistor while applying a constant illumination level from an external light source. Repeat the measurement with 500kΩ and 100kΩ resistors in place of the 1MΩ resistor.

　　For a representative LED under constant illumination and shielded from stray ambient light, we measured a photocurrent of approximately 25nA for all three resistor values. For the same illumination level applied to the sensor LED, measure the frequency generated at pin PB _3.

　　To calculate the reverse bias capacitance of the LED, substitute the delay loop time, the LED’s photocurrent, and the microcontroller’s logic 1 and logic 0 threshold voltages into the equation and solve for C, the LED’s effective reverse bias junction capacitance: (dV/dt)=(I/C), where dV is the measured logic 1 voltage minus the logic 0 voltage, dt is the measured discharge time of the LED’s internal capacitance, and I is the calculated value of the LED’s photocurrent source. The calculated values ​​for the selected LEDs range from 25pF to 60pF. This range is comparable to the data in References 3 and 4, although Reference 3 only reports the current source values. The assembly language firmware for the AVR microcontroller can be downloaded from this design example, Table 1.

参考文献
1.Dietz, Paul, William Yerazunis, and Darren Leigh, "Very Low-Cost Sensing and Communication Using Bi-directional LEDs," Mitsubishi Research Laboratories, July 2003.
2.Petrie, Garry, "The Perfect LED Light," Resurgent Software, 2001.
3.Miyazaki, Eiichi, Shin Itami, and Tsutomu Araki, "Using a Light-Emitting Diode as a High Speed, Wavelength Selective Photodetector," Review of Scientific Instruments, Volume 69, No11, November 1998, pg 3751, http://rsi.aip.org.
4."Optocoupler Input Drive Circuits," Application Note AN-3001, Fairchild Semiconductor, 2002