Kitchen Lighting Project

By Martin Bernier, Engineer, MBA, Future Electronics Canada

READ THIS TO FIND OUT ABOUT Lumileds Luxeon® K2 STMicroelectronics STM32 (ARM Cortex-M3) - 32-bit microcontroller STCS1 1.5A max. constant current LED driver ST7 8-bit microcontroller - ST7Lite QST108 capacitive touch sensor

This technical view is different from previous FTM releases and offers an opportunity to share my personal project experience while I walk you through my design process.

 

The Project

The idea came from my kitchen renovations last fall; we decided to include solid-state lighting under the top cabinet, for ambiance and working illumination on the counter area. So I decided to build my own LED lighting system.

Before starting the project, I defined some guidelines:

• Wanted to be able to drive eight (8) individual lighting zones
• Each zone should have five (5) light fixtures, with each fixture being controlled individually
• Would like to use new Capacitive Touch technology for control switch
• Would like to use 9V AC to power the fixture (approx 12.7V DC)
• Use a supplier that can bring me a solution for each section of my design
• Use a supplier with new parts so I can learn more about them and help my customers

 

 

 

The block diagram in Figure 1 shows the total capability of the system; for the actual project I only used 4 zones and 1 touch switch.

 

Networking

A serial RS-485 network was chosen for communication between the main controller, the light fixtures and the touch sensor. The RS-485 communication network is robust and allows a daisy chain configuration. Before the installation of the new kitchen cabinets, installed a control cable with two twisted pair wires (one pair for 9V AC and the second pair for the RS-485 network).

 

Touch Switch

The touch switch is composed of 2 components from STMicroelectronics: the QST108 as capacitive touch sensor and the ST7Lite39 MCU. The MCU is used to configure and interface the QST108 with the main controller board. If a contact pad is touched and released within 2 seconds, the microcontroller sends an ON or OFF (according to the previous state) to the main controller which activates or deactivates the appropriate group of fixtures. Dimming of a particular zone can be achieved by holding the touch pad for more than 2 seconds. The microcontroller will send continuous dimming commands until the touch is released (when the lowest dimming level is reached, the dimming value resets to the highest level).

 

 

QST108

The QST108 from STMicroelectronics is a solution for capacitive sensing based on QProx™ patented technology developed by Quantum Research Group (QRG).

The QST108 offers as a standard:

• Up to 8 independent QTouch™ keys supported
• Individual key state outputs or I²C interface
• Fully “debounced” results
• Patented AKS™ Adjacent Key Suppression
• Self-calibration and auto drift compensation
• Spread-spectrum bursts to reduce EMI
• Up to 5 general purpose outputs

 

In my design, I used the QST in an I²C configuration. This configuration gives five (5) general purpose outputs (GPOs), access to low power mode and internal capacitive sensing parameters. I used the GPOs for driving the LED in the touch pad behind the face plate.

As you can see in the Touch Switch Block diagram (Figure 2), I used only four keys (of the eight available); this is related to the space I had on my PCB. For aesthetic reasons, I used a standard Decora-Style wall plate; this plate is about 1.25” x 2.50”. Figure 3 shows a picture of the actual PCB; the top portion represents the four 0.5” diameter touch pads on the top layer with the LED behind the bottom layer. The other components are on the bottom layer below the touch pad.

 

 

To design this PCB, I used Quantum Research Application Note AN-KD02 “Secret of a successful QTouch™ design”.

 

ST7Lite39 MCU

STMicroelectronics’ ST7Lite series consists of low memory size and general purpose 8-bit Flash microcontroller.

 

KEY FEATURES

• 8 Kbytes Flash, 384 bytes RAM and 256 bytes data EEPROM
• Internal RC 1% oscillator
• Five power saving modes
• 15 multifunctional bidirectional I/O lines, 7 high sink outputs
• A/D Converter (7 input channels with 10-bit resolution)
• Master/slave LINSCI™ asynchronous serial interface and SPI synchronous serial interface
• 10 interrupt vectors plus TRAP and RESET and 12 external interrupt lines (on 4 vectors)
• 5 Timers
• Two 8-bit Lite Timers with prescaler
• 1 real-time base and 1 input capture
• Two 12-bit auto-reload timers with 4 PWM outputs, input capture and output compare functions
• Configurable Watchdog Timer
• 40 to 125°C extended temperature range

 

The only weak point of the ST7Lite39 in this application is that only an SPI interface is available while the QST108 has an I²C interface. I could use the individual key state outputs, but I wanted the full control on the QST108. To remedy this situation, I programmed a software I²C interface using the STMicroelectronics Application Note AN1045 “ST7 S/W implementation of I²C bus master”.

For the development of the ST7Lite firmware, I used Raisonance Integrated Development Environment (RIDE). The RKitLST7 Lite Kit is available free of charge from Raisonance (with registration) and includes:

• The RIDE interface for Windows 2000, XP and NT
• The RC-ST7 C Compiler limited to 16 Kbytes
• The MA-ST7 Macro-Assembler (full version)
• The RBuilder-ST7 Application Builder
• The RL-ST7 Linker
• The SIMICE-ST7 Simulator/Debugger (full version)

 

For programming and debugging, I used RLink- Pro In-Circuit Debugger and Programmer from Raisonance. RLink supports both JTAG and ICC protocols and connects to your application board via one of three adapters:

• 20-pin JTAG ARM adapter for STR7, STR9 and STM32 microcontrollers
• 14-pin JTAG µPSD adapter for µPSD microcontrollers
• 10-pin ICC-ST7 adapter for ST7 microcontrollers

 

You may also use RLink’s standard version for ST7 debugging and programming, but RLink standard will only do full programming for STR7, STR9 and STM32, and the debugger will only work for applications less than 32K.

 

Main Controller

The heart of the main controller is the new STM32; a 32-bit Flash microcontroller based on the breakthrough ARM Cortex™-M3 core - a core specifically developed for embedded applications. The STM32 family benefits from the Cortex-M3 architectural enhancements, including the Thumb-2 instruction set to deliver improved performance with better code density, significantly faster response to interrupts, all combined with industry leading power consumption.

In this application, I used the STM32 as a communication bridge between the touch switch and the light fixtures. With the three UARTs, the STM32 was a perfect fit for this application. The STM32 receives a command from the RS-485 network of the touch switch and redirects the command to the appropriate lighting zone. The main controller will also poll each light fixture in the network to get status (overheat and LED disconnect).

 

 

Even if I didn’t use all the performance features of the STM32, working with the MCU allowed me to better understand the new Cortex™-M3 core.

 

 

On my board, I used the STM32F101 access line, which has the same features as the performance line (STM32F103) except for the following items:

• Max. speed 36MHz
• No USB/CAN/PWM Timer
• Only 1xADC
• SRAM up to 16K

 

Like with the ST7Lite, I used the same RLink-Pro In-Circuit Debugger and Programmer from Raisonance, but with the new Integrated Development Environment Ride7 (available free of charge from Raisonance).

STM32 (Cortex-M3) GNU tools are fully integrated into Ride7, so that there is no need for makefiles or complex command lines. Moreover, some specific libraries are provided for a better adaptation to the STM32 (Cortex-M3) microcontrollers:

• The STM32 libraries from ST allow a quick configuration and simple use of the embedded peripherals
• The original ANSI libraries from GNU (scanf, printf, etc.) have been simplified and optimized to match with the embedded world requirements

 

Light Calculation

Based on the Future Lighting Solutions brochure “The 6 Steps to LED Lighting Success”, to illuminate a working plan for reading or writing, we need around 500 LUX. Based on Table 1, for a distance of 1 meter, I would need 112.8 lumens.

To facilitate my mechanical design, I decided to use a standard optic (#151) from Polymer Optic (see Figure 6 on the next page). This optic is normally used with a K2 LED in color mixing applications, but works perfectly with three neutral white K2 LEDs when we add a 25° diffuser (# 161) on top of it.

 

 

 

With three K2 LEDs driving at 350mA, I should get around 150 lumens based on an S bin (LXK2-PWN2-S00).

 

Light Fixture

 

All the light fixtures are based on a microcontroller ST7Lite39 previously used in the touch switch design and a STCS1 1.5A max. constant current LED driver from STMicroelectronics.

According to the command received on the RS-485 network, the microcontroller will do the following:

• Enable the LED driver
• Disable the LED driver
• Set the appropriate PWM value
• Send a status

When the LEDs are ON, the microcontroller verifies the board temperature and the load disconnect signal, to report any anomaly to the main controller.

 

STCS1

The STCS1 is a BiCMOS constant current source designed to provide a precise constant and the following features:

• Up to 40V input voltage
• Less than 0.5V voltage overhead
• Up to 1.5A output current
• PWM dimming pin
• Shutdown pin
• LED disconnection diagnostics

 

 

Power Dissipation

In the actual application the 3 white LEDs (VF=3.42) are driving at a current of 350mA and a voltage of 12.7V DC (9V AC x √2). The power dissipation in the device can be calculated as follows:

PD = (VDRAIN - VFB) x ILED + (VCC x ICC)

Based on this, the thermal resistance and the ambient temperature; the junction temperature can be calculated as:

TJ = RthJA x PD + TA

• Package: DFN8 3x3mm
• Ambient Temperature: TA = 50°C

In this case VDRAIN = 12.7 - 3 x 3.42 = 2.44V

PD = (2.44 - 0.1) x 0.35 + 12.7 x 0.0005 = 825mW at 50°C, a DFN8 could dissipate around 1.5W.

The junction temperature will be:

TJ = 37.6 x 0.825 + 50 = 81.03°C.
Maximum for a DFN8 = 150°C

 

Board Layout Considerations

The LUXEON K2 must be electrically isolated from the anode, cathode and other slugs. This requirement stems from the basic internal construction of InGaN LUXEON K2 devices.

An Aluminium Metal Core has been used for the light fixture layout. The Metal Core PCB (MCPCB) achieves very efficient heat conduction from the PCB and offers an electric isolation between the slugs.

 

Installation

The working prototype is still on my lab bench; this is why the lighting source in my kitchen is still an incandescent fixture. As soon as I complete the assembly of the LED based fixtures, I will be able to install them. I will include some pictures of my new LED illuminated kitchen counter in the next article.

 

 

Martin Bernier, an EE graduate from École Polytechnique de Montréal in Electrical Engineering, has a Masters in Business Administration from Université de Sherbrooke joined Future Electronics’ Advanced Engineering Team in 2005. Martin has more than 14 years of research and development experience.