Intelligent Desk Lamp

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Intelligent Desk Lamp

• Sinan Farmaka
• Rade Kuljic
• Spring 2009

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System
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Introduction

• Minimizing power consumption is an essential part
  of illumination systems.
• Power loss is inconvenient.
• Solution
• Build an Intelligent Desk Lamp that will
  monitor the level of brightness on the desk and
  adjust itself in such a way to maintain a desired
  level of light.
• Benefits
• Can be implemented in remote areas
• Reduced power dissipation
• Environmental

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Performance

• Features
• Constant level of brightness at a working spot
• 3 levels of brightness
• Intelligent and Standard modes
• Adjusts brightness within 5 of the desired
  level in less than 1 second even in cases of
  sudden changes of light
• LCD indicator of levels of brightness mode
• No need to turn on/turn off in certain situations
• Low cost device

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System Overview
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Dimmer Circuit

• Based on chopped AC waveform
• The chopping is accomplished by a triac which is
  controlled by a microcontroller
• When triggered by a pulse at its gate (3), triac
  starts to conduct current when the current
  reaches zero, triac turns off by itself
• Wider chops gt more brightness and vice versa

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Dimmer Circuit

• Triacs characteristics
• L2004L3 - Littelfuse
• Voltage capability 200 V
• Current capability 4A
• Average Power Dissipation 0.3W
• Sensitive gate Igt3 mA
• Symmetric for all four quadrants, hence for
  unipolar pulses

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Microcontroller PIC16F877A
Tasks
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1. I/O Interface

• Determines Mode Level
• Mode standard/intelligent (0/1) - simple switch
• Level 3 discrete levels L1, L2, L3 push
  button
• Debouncing implemented by software
• Displays Mode Level
• Mode simple LED
• Level 7-segment LED display

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2. Monitoring Level of Brightness

• Light sensor Panasonic AMS302
• built in optical filter for
• spectral response similar
• to that of the human eye
• linear output
• prop delay 8.5 ms
• optical angle 50
• power dissipation 40mW
• A/D conversion done by PIC internally

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3. Determination of Triggering Delay

• 8000 µs range of modulated width
• Binomial search method
• Precision within 2 µs in 13 steps (213 8192)
• Search step of 65 ms
• Calculation cycle 0.85 sec

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4. Detection of Zero Crossings

• AC line sampler Scales down and rectifies AC
  line signal in order to be compatible with the
  PIC
• PICs internal voltage reference module set 200
  mV
• PICs internal analog comparator module
• An interrupt is generated when a low voltage is
  detected (C1OUT1)

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5. Triggering the Triac

• When a zero crossing is detected, a timer is set
  to a certain value which corresponds to the
  previously calculated triggering delay
• On the overflow of the timer, an interrupt is
  initiated and PIC sends out a short pulse to its
  digital output which controls the triac.

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Microcontroller Programming

• C code, CCS compiler, PIC Start Plus Programmer
• Interrupts in priority order
• COMP generated by the comparator module when a
  zero-crossing is detected
• TIMER0 generated when a triggering pulse has to
  be sent
• TIMER2 generated every 65ms for purpose of
  calculation the triggering delay
• EXT generated when desired level is changed by
  pressing the pushbutton
• RB generated when mode is changed by the switch
• main() the only purpose is initialization after
  powering up

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Power Supply

• Powers PIC, the light sensor and display
• Dc open circuit voltage of 5.1V
• Output voltage stays within 5 of the desired
  level when load current varies between 0 and 30mA
  (in our case 30mA will be enough current to
  supply the PIC)
• Open circuit voltage stays within 2 of the
  desired voltage as the dc line varies from 105
  120 Vrms.
• Ripple voltage at the output is less than 2 of
  open circuit voltage

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Power Supply

• Transformer 115V/6.3VCT (center tapped)
• Rectifier two diodes.
• Filter capacitor of 3.3 mF.
• Regulator zener diode with breakdown voltage
  5.1V

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Power Supply

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Testing Dimmer Circuit

• Send short pulses to triac from function
  generator to determine minimum pulse width
  required to trigger the triac
• Used a 250 ? , 100 W resistor to mimic a bulb
• Observed triacs pin 2
• Voltage across the bulb is the missing part of
  the sinusoid.

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Testing Dimmer Circuit
Triac voltage for level L2

• Desired output observed
• Voltage chopping works

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Testing Sensor

• Observed voltage with different RL
• Desired
• 0V for very dark environments
• 5V for very bright environments
• RL 12k ? is the best in terms of sensitivity
• 0.67 V ? dark
• 4.22 V ? bright

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Testing Power Supply

• Tested the power supply for
• Vac 105Vrms, 110Vrms, 115Vrms
• IL 0mA, 15mA, 30mA.
• The output voltage is monitored on the
  oscilloscope and all mentioned specifications are
  verified.
• Varied the capacitance to minimize ripples.

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Testing Power Supply

• Output voltage 5.2 V DC
• Ripple of about 175 mV 3
• Output current 30mA

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Entire System Testing Results

• Hex Display
• Push button, toggle switch and display interface
  checked
• Sensor Value
• LED array to display 8 bit value after ADC

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Entire System Testing Results

• Bulb Light levels
• Both modes
• Triac output on oscilloscope for necessary time
  delays
• Intelligent Mode
• Change of light with dark and bright environments
• Re-adjustment of the voltage level (response
  delay) is about 1 second
• Power loss
• PIC, oscillator, sensor, display 86mW
• Power supply lt 400mW
• Triac 300mW
• Total lt 790mW
• Very high efficiency

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Issues

• Dimmer circuit --Asymmetric triac
• Sensor calibration
• Interrupts priority
• Delay Adjustments
• Intelligent mode - brightness levels
  re-adjustment

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Potential Improvements

• Better sensor sensitivity
• Use more sensors and take an average
• Improve noise rejection using a DISO buffers in
  front of sensors

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Questions ???