Using LEDs as Light-Level Sensors and Emitters

Using LEDs as Light-Level Sensors and Emitters
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Using LEDs as Light-Level Sensors and Emitters
Modulating LED power based on ambient light level increases battery life, a particularly helpful feature in a device
where battery life is measured in days. Using a very simple circuit, Altera’s MAX II and MAX IIZ CPLDs can measure
the analog light level of their environments and then drive an LED at a proportional analog intensity level.
Controlling the LED intensity based on ambient light as demonstrated reduces LED energy usage by more than 47
percent without affecting appearance.
Introduction
In portable electronic products, a common use for LEDs is a “heartbeat” indicator that shows power status, battery
condition, or Bluetooth connection activity. The LED can be a major factor in determining battery life, as its intensity
is directly proportional to power drain. LEDs are designed to be easily seen in bright ambient light, yet it can be
assumed that much of the time a portable device is in a dark purse or pocket. A low-intensity LED indicator extends
battery life but is useless in a bright environment. Modulating LED power based on ambient light level would
increase battery life, a particularly helpful feature in a device where battery life is measured in days.
Regulating the LED’s Intensity
A pulse-width modulator (PWM) is very effective at regulating the LED’s intensity with very little wasted energy.
The only feature necessary to complete a light-sensitive flash intensity system is an ambient light-level sensor, which
can be added to a CPLD or FPGA circuit with no additional components. The light sensor uses the same blinking
LED to measure ambient light level. The LED is forward biased to emit light and reverse biased to act as a light
detector. Figure 1 shows how the LED is biased for emitting light and for sensing light with a relaxation oscillator.
The frequency of oscillation is proportional to light intensity, allowing the use of a PWM to regulate LED light
intensity output.
Figure 1. Bias Schematics for Using LED as Sensor and Emitter
Sensor bias
VCCIO
Emitter bias
LPM_Register
LPM_Register
Existing
capacitance
on the line
altufm_osc
altufm_osc
~
~
4.4MHz
4.4MHz
±25%
±25%
D QQ
PINOSC
PINOSC PINOSC
PINOSC
Frequency
frequency
Increases
increases
with light
with
light
intensity
intensity
MAX IIII
MAX
MAX IIII
MAX
Current
limiting
resistor
PWM
PWM
Control
Control
altufm_osc
altufm_osc
~
~
Intensity
Intensity
Enable
Enable
Output
Output
4.4MHz
4.4
MHz
±25%
±
25%
A very simple feedback system can be created for a flashing light. The LED flash intensity is determined by a value
presented to the PWM, which is calculated when the LED is off. It is biased as a sensor connected to a relaxation
oscillator. The oscillator output feeds into a frequency counter, where the frequency is proportional to the ambient
light level. The frequency counter output is the value that controls the intensity of the LED PWM. It is possible to
have only one sensor controlling the intensity of multiple LEDs.
Figure 2 shows the block diagram for a flashing LED with a feedback loop that controls the intensity based on the
ambient light level. It would be very easy to add an additional control to enable or disable the flash function or the
flash rate (not shown).
WP-01076-2.1
October 2009, ver. 2.1
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Using LEDs as Light-Level Sensors and Emitters
Altera Corporation
Figure 2. Simplified Block Diagram of LED Flasher with Intensity Control
Current
limiting
resistor
Sensor
Sensor
Emitter
Emitter
Bias
Bias
External
oscillator
~
32 KHz
Freq.
Frequency
Counter
counter
Bias
Bias
Control
control
IN
Count
Count
Enable/Res.
Enable/Res.
PWM
~
altufm_osc
4.4 MHz ±25%
Control OUT
Enable/Res.
State Machine
machine
Flash LED
Flash
LED
using PWM
Using
PWM
Enable
enable osc.
Osc
125mS
125 mS
Reset
Reset
PWM
PWMand
&
Counter
counter
125mS
125 mS
Count LED
Count
LED
Relax.
relax. Osc.
osc.
cycles
Cycles
125
125mS
mS
Wait
Wait
625mS
625 mS
Figure 3 shows an Altera® MAX® II CPLD design implemented on a MAX II Starter Kit that generates a 1-Hz flash
rate with 125 ms on period LED flash with a PWM duty cycle from 6.25 percent to 100 percent (15 levels). Various
tests with discrete LEDs plugged into the #4 and #8 terminals of the right-side 2x5 header have determined that the
four MSBs of an 8-bit counter that samples the LED sensor PINOSC frequency for 125 ms are the optimum values to
control LED intensity.
Figure 3. LED Intensity Control Demonstration Circuit
The circuit is divided into three sections. The state machine at the bottom controls circuit operation with eight states,
each of which are active for 125 ms as determined by the Counter12 timer, which is clocked by the MAX IIZ Demo
Board’s 32-kHz oscillator input. In the case of MAX II Starter Kit, the Counter12 is clocked by the 3.3-MHz internal
oscillator. An additional block, freq_div is added to allow user to control the input frequency. The reset switch puts
the state machine into the LED PWM-controlled flash state, State0, which can be used to stop the sampling and
sustain the PWM value reached when reset was pushed. This function allows the LED intensity to be observed after it
is removed from a light source.
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Altera Corporation
Using LEDs as Light-Level Sensors and Emitters
State Machine States
The state machine states include:
■
■
■
■
■
State0 – the LED PWM-controlled flash state (reset state)
State1 – the PWM-counter reset state
State2 – the intensity-counter reset state
State3 – the light intensity measurement state
State4, State5, State6, and State7 – unused states
As shown in Figure 4, an LED is connected to pins LED_CATHODE and LED_ ANODE on the MAX IIZ Demo
Board, which in turn are connected to terminals #4 and #8 of the right-side 2x5 header. For the MAX II device on the
MAX II Starter Kit, shown in Figure 5, pins LED_CATHODE and LED_ ANODE are connected to terminal #3 and
terminal #4 of the 2x17 header. The bias of the LED_CATHODE and LED_ ANODE terminals are determined by the
state of the state machine: in State3 they are biased as a sensor, in all other states they are biased as an emitter. State0
is the only state in which LED_CATHODE can be a “0” to light the LED.
Figure 4. Testing LEDs with the MAX IIZ Demo Board
Figure 5. Testing LEDs with the MAX II Starter Kit
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Using LEDs as Light-Level Sensors and Emitters
Altera Corporation
The middle light intensity measurement block is simply an LED biased as a detector, then connected to a relaxation
oscillator connected to an 8-bit counter. The counter measures the number of LED oscillations in 125 ms, the duration
of State3 that enables the Counter8 counter. The Counter8 block’s carry-out signal, COUT, is used to saturate the
count to FF and prevent it from wrapping around to 00, while the four MSBs of the counter control the PWM
intensity.
The top block is a simple PWM circuit that uses the internal 4.4-MHz oscillator as the modulation carrier frequency.
The Counter4 block drives a simple 4-bit adder with the carry-in signal tied to the VCC. The other adder input is
connected to the light intensity value (the four MSBs of Counter8). Counter4 cycles from 0 to 15 and then repeats.
When the sum of Counter4 plus the intensity value plus one is greater than 15, then the COUT of the adder is “1.”
Thus, the carry-out signal of the 4-bit adder is the PWM output, which drives the LED. For example:
■
■
■
A “0” from the intensity measurement results in a “0” at carry out when Counter4 is 0–14 and a “1” when Counter4
is 15. This is a 6.25 percent duty cycle or a very low intensity level.
A “7” from the intensity measurement results in a “0” at carry out when Counter4 is 0–7 and a “1” when Counter4
is 8–15. This is a 50 percent duty cycle or medium intensity level.
A “15” from the intensity measurement results in no “0” at carry for any Counter4 value and a “1” when Counter4
is 0–15. This is a 100 percent duty cycle or full intensity level.
Figure 6 shows a similar design with additional features to aid in the development of a design using a specific LED or
environment. The first feature added is an 8-bit PWM, which can be easily increased in resolution if necessary.
Figure 6. LED Sensor and PWM Development Platform
The next addition is connection of the outputs of the intensity counter Intensity[7..0] to the eight green LEDs on
the MAX IIZ Demo Board. This makes it easy to monitor the light intensity measurement. When the push button is
held, the sampling stops, the unit displays the last reading, and the LED stops flashing and stays lit at that intensity
until the button is released. For the MAX II Starter Kit, the push button, SW2, performs the same function.The red
LED D1 on the demo board is connected to LED_PWM and is a second output of the PWM value. This makes
pointing the sensor LED at a light and seeing the resulting change in flash brightness easier to observe. In addition to
this feature, the design is modified to contain only one clock source.
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Altera Corporation
Using LEDs as Light-Level Sensors and Emitters
Setting the I2C Interface
The LED sensor and emitter reference design support an I2C interface on both the MAX IIZ Demo Board and the
MAX II Starter Kit. For the I2C interface (Figure 7) in this design, the CPLD has a built-in 7-bit address that can be
easily modified. The general-purpose I/O ports in this design, GPIO_input [7..0] and GPIO_output [0], are connected
to the Intensity_reg8 and one LED respectively, as in the I2C design example.
Figure 7. I2C Interface in LED Sensors and Emitter Design
f
Refer to the LED sensor and emitter reference design with an I2C interface using the MAX IIZ Demo Board
for detailed information on the connection for I2C interface.
f
For more information for the GPIO pin expansion using I2C Bus interface, refer to AN 494: GPIO Pin
Expansion Using I2C Bus Interface in MAX II CPLDs.
Conclusion
Using a very simple circuit, the MAX II CPLD can measure the analog light level of its environment and then drive
an LED at a proportional analog intensity level. The sensing and emitting is performed with the same LED and an
optional bias resistor. The programmability of the CPLD makes adjusting the parameters of the circuit to the
characteristics of any LED fast and easy. The power consumption of a flashing LED can be reduced by increasing the
flash period, decreasing the flash pulse width, or decreasing intensity, but it is assumed that designers have already
adjusted these values to optimum levels. Controlling the LED intensity based on ambient light as demonstrated
reduces LED energy usage by more than 47 percent without affecting appearance.
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Using LEDs as Light-Level Sensors and Emitters
Altera Corporation
Further Information
The following reference designs must be opened with Quartus® II design software:
●
“Light-Sensing LED and PWM Flashing LED Development Platform” reference design (Figure 6):
– For the MAX IIZ Demo Board: www.altera.com/literature/wp/MAXIIZ_LED_Sensor_Display.qar
– For the MAX II Starter Kit: www.altera.com/literature/wp/MaxIIStarterKit_led.qar
●
“LED Sensor and Emitter” reference design (Figure 7):
www.altera.com/literature/wp/LED_Sensor_PWM_I2C.qar
■
■
■
■
■
■
■
■
■
■
Implementing a Flexible CPLD-Only Digital Dashboard for Automobiles:
www.altera.com/literature/wp/wp-01072-implementing-flexible-cpld-only-digital-dashboard-automobiles.pdf
A Flexible Architecture for Fisheye Correction in Automotive Rear-View Cameras:
www.altera.com/literature/wp/wp-01073-flexible-architecture-fisheye-correction-automotive-rear-view-cameras.pdf
Creating Low-Cost Intelligent Display Modules With an FPGA and Embedded Processor:
www.altera.com/literature/wp/wp-01074-creating-low-cost-intelligent-display-modules-with-fpga.pdf
Applying Graphics to FPGA-Based Solutions:
www.altera.com/literature/wp/wp-01075-applying-graphics-to-fpga-based-solutions.pdf
AN 494: GPIO Pin Expansion Using I2C Bus Interface in MAX II CPLDs:
www.altera.com/literature/an/an494.pdf
Rafael Camarota “Use an LED to sense and emit light,” EDN, May 14, 2009:
www.edn.com/article/CA6656305.html
Geoff Nicholls, “Red LEDs function as light sensors,” EDN, March 20, 2008:
www.edn.com/article/CA6541376
Howard Myers, “Stealth-mode LED controls itself,” EDN, May 25, 2006:
www.edn.com/article/CA6335303
Dhananjay V Gadre and Sheetal Vashist, “LED senses and displays ambient-light intensity,” EDN, Nov. 9, 2006:
www.edn.com/article/CA6387024
Paul Dietz, William Yerazunis, and Darren Leigh, “Very Low-Cost Sensing and Communication Using
Bidirectional LEDs,” Mitsubishi Research Laboratories, July 2003:
www.merl.com/reports/docs/TR2003-35.pdf
Acknowledgements
■
Rafael Camarota, Non-Volatile Product Line Manager, Low-Cost Products Group, Altera Corporation
101 Innovation Drive
San Jose, CA 95134
www.altera.com
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