# Lino Project: USB tests

The Lino board was correctly made and roughly tested. But now, how can I see how correctly performs? By simply integrating the load (power LEDs) and the PC environment to a complete working system. In this article will be provided a summary of these various steps, just to keep rtace of the progresses.

The load is made by 4 Glighter-S that are powering as many power LEDs. I grouped them in a proto PCB like in picture below:

These 4 boards are simply connected to the same power line, with 4 control wires that are connected th Lino. Each of their output is connectet to a single LED, with the result of 4 LEDs in total driven singularly by Lino.

### The finger test

Now, moving to the board that actually carry the LEDs, I felt the need to change some things. When doing my math, I realized that a small heatsink could work keeping the LEDs cool enough to keep them alive, as you can see here:

This “cool enough” is not enough to pass the so called finger test: this test is considered to be failed if you can’t stand a finger on the heatsink or any surface. If this happens, we are likely more than 65°C, depending on what is it your job, if you are a farmer maybe you have thicker skin and maybe you are sensible to higher temperatures.With this one, we are passing 80°C using my IR thermometer. My finger is definetely hopeless.

Since the heatsink actually shall be touched during the usage and my finger must not be burned, I needed to ovesize the heatsink itself. With a new one, the maximum temperature rise to no more than 60°C, and I extend by 20°C more the ambient operating temperature range of the LEDs:

As you can see, I need to think about the shape of the heatsink and the PCB. But for this test, what’s in the picture is acceptable.

## Turn on the board

The board can be driven using the USB/serial or with a single encoder. Herein I will show the serial test, and to generate the correct data pattern to be sent on the board, I wrote a simple Python script to simplify the testing. It uses Python 3+, exploits the pySerial and Tkinter module. Basically the Tkinter lib generates an HTML compatible color code and will be arrangend to an array sent throug the serial line, here emulated on USB. At every complete frame received, Lino will apply instantly the requested color. In other words, when receives the “stop character” described in the Lino protocol.

Here the complete assembled configuration, where the power supply is given from a 9V – 1.5A DC power supply that will be connected to the board.

Let’s connect the DC power jack and the USB to the Lino and to a PC running the Python script demo available here.

Run the script with Python 3.5+ and the following will be opened:

Just type the available COM that appears when you connect the board, a space mark and write the communication speed used by Lino. Press enter and the following appears:

It is in italian, but it is the intuitive and dummy default color table provided by Windows: select the right combination and press OK. Lino automatically displays the colors on the LEDs.

Note that it is implemented a white adjustment computed directly by the Lino’s microcontroller, the white component that controls the white LED it is not generated by the script: when all of the RGBs are greater than zero, there is a bit of white that will be reproduced using the fourth pure white LED, making it more realistic with respect the expectations. In this way I can generate any kind of white light shade. Awesome.

### Setting-up the prototype

To set up everything and test it in a more compact way, I thought to use a PVC panel. But at the moment they were missing. So to finish in the same day, I remembered that I have kept a light guide plate from an old LCD panel. I knew that wasn’t trash!

A light guide plate is a panel (in acrylic like material) with some dots that are intercepting the light from the side backlight and redirect, guiding it, it to the LCD, actually rotating the light path by 90° towards the LCD panel:

Then I have realized too late that I could make a photo before cutting the plane, so here is what remains of my panel, already cutted, bended and somehow abused:

Exactly, the old panel was used for all but lighting purposes. In the end, I can put everything in a uglyish case, sticked on the wall like a painting:

The video below shows the working system paired with the PC:

When only powered from USB, the firmware derates the total provided power to stay inside the power budget of 2.5W. This video shows the automatic power handler when disconnecting/connecting the main power supply:

# Lino Project

I am playing with LEDs for few month and during that time some little projects were born, but now I need a system to handle the logical part and the power distribution. I simply just can’t use an Arduino board, or any other development board: I need a proper power handling embedded in a small space, all in one PCB, with some particularly high power requirements. And being Arduino compatible. This is Lino.

It will be used to power up the Glighter-S modules in a compact fashion, finally reaching something similar to a complete lighting working system. What is important here, is that we have 255 combination per each color, plus an algorithm to emulate in RGB and pure white combination to generate pastel colors or shades of white. With such assumption, I do not use any LUT but instead I generate the color according to the HTML encoding executed on the Lino microcontroller.

### Project purpose and board review

This board is designed to power up any power LED, motor and/or anything which requires up to 10W and can be controlled in PWM. Lino supports in hardware the control through physical, glitch filtered, inputs like buttons and encoders.

In the picture below is shown the board with its main interfaces:

More protocols are supported onboard, like I2C and UART (real or through USB emulation). With the proper firmware running on Lino, terminal commands are supported using the serial protocol, connected with the Atmega (the main MCU) mainly in two ways: with USB through an emulated terminal, or directly with the microcontroller’s UART, supporting then any communication with a serial device, like the ESP8266 and with no additional circuitry, since everything is working at 3.3V.

Since this board can communicate using UART or I2C, this setup is scalable, making it capable to drive more Lino boards connected with each other.

## Board characteristics: the union of Atmel and Microchip in practice

Here is shown the main blocks of the board. It is a kind of celebration of the acquisition of Atmel from Microchip. It is used an Atmega328P microcontroller and the Microchip MCP2200 USB to Serial converter instead of the FTDI, which instead would costs truly a lot.

Below, the block diagram of the project and a picture which shows where physically things were arranged on the board:

Everything is almost self-explaining. Briefly, the HMI (Human-Machine Interface) can support a single Encoder with an integrated button to minimize the commands and user interface complexity, but with firmware mods can be anything else.

The controller core is the Atmega328P, which is used to keep an Arduino compatibility. With the low cost and well supported Microchip MCP2200 I can obtain a neat working system interfaced with any PC or serial device: in fact I have also a pure serial connector, with 3.3V voltage swings. For example it can talk with the famous ESP8266 with no additional circuitry.

A bit of more explanation is instead necessary for clarify the power handling.

### Dynamic power source selection

The backbone of this system is the dynamic power handling. When powered from an external supply, up to 10W can be provided to the load, while can be still controlled from the USB port. If this supply is not present, is still possible to drive the board using the USB power, which allows up to 2.5W.

The selection is performed by the MCU (green dashed line in pictuire below), while a default safe state is kept when the MCU is not driving the switch. All the voltages are monitored, in order to allow the firmware to make the right selection. The priority of the source selection is given to the external power source.

The USB power is limited, since this board can be used directly connected to a PC, able to provide, normally, no more than 2.5W per USB port. Therefore, to use the entire power capability, an external power supply shall be used.

Due to the extremely high input voltage range, the power source selection is achieved in the firmware while the hardware is fully protected against misbehaviours and short circuits. After all, it is a personal project and burning a board is tedious and I can’t afford this, prevention is better. As also in professional environments, anyway.

Below just a brief of the supply characteristics:

 Electrical characteristics Min Max Input voltage 3.3V 20V Maximum supported power (USB only) 2.5W Maximum supported power (Power Jack) 10W Maximum supported power (Power Jack + USB) 10W

#### Source selection logics

The main power supply shall be capable to provide the total power required from the load. If this supply voltage is above the minimum required of around 3V, Lino is powered up and will source from the main supply. If the USB is present, the MCU can chose from where source the power. If the main supply is lower than 3V and the USB is present and the USB is present, the power is absorbed from the USB cable, otherwise the voltage would be insufficient.

Since the USB it is used to exchange the USB Data, it is usually connected to a device that cannot provide more than 500mA per USB port, therefore the power delivered to the load shall be reduced down to 2.5W using the firmware (in this particular case, by reducing the duty cycle of the PWM which drives the load).

## The “Lino protocol” – Board demo interface

To have the board working, I setup a simple protocol used over the serial, but other physical line can be used, like I2C. I need to master even complex variations of the PWM sequence, also in a scalable way. For this reason I needed a proper protocol, with expansion potential, to open the possibilities to develop a proper higher level application in a simple way on a PC or any master controller side.

Moreover, as it will be explained, the data handled by this protocol is conceived to be developer friendly, where the developer is someone who will generate a proper software to achieve the desired data exchanged. It is not perfect, but it works.

### Physical Layer

It consist in an array of 18 bytes sent from a master PC or device (over serial/USB) made by the following sequence, from the first to the last byte sent, MSB to LSB:

[StartChar] [CTRL#00] [CTRL#01] [R#0] [R#1] [G#0] [G#1] [B#0] [B#1] [W#0] [W#1] [INT#0] [INT#1] [RES#00] [RES#01] [RES#10] [RES#11] [EndChar]

Where each byte is represented from LSB to MSB, the data is sent using UART (8 bits, 1 Start + 1 Stop bits, no parity), but can be transferred using any other simple protocol (I2C, SPI ecc).

Serial voltage levels swings are on 0V-3.3V.

### Data handling

The values inserted by the application or the user through the terminal are written in ASCII, but representing an hex format coded to save bytes. In other words, if the sender wants to send the value 254(decimal) to Lino, it will send the chars “F”(character) and “E”(character) or “f”(character) and “e”(character) on text terminal, representing the intention to send the 0xFE hex value. The first char sent is the MSB of the hex number; how each bit is transmitted is no more concerned here, but hidden in the physical layer described before, handled in UART. The chars sent are corresponding to ASCII codes, so on the serial lines are physically sent, in this example, the ASCII “70”(decimal) (for letter f)+”69″(decimal) for letter e.

On the board is made all the conversion reversed to actually compose the number 254. To ease the software front-end, the serial is completely case insensitive and look only for characters that can represent the hex numvers, so any character sent outside the range “0” to “9” and from “a” (or “A”) to “f” (or “F”) can be used in the firmware for synchronization, or can be discarded at run time but used by the user when sending data manually over the terminal for testing purposes.

Here below is listed the meaning of each byte:

1. StartChar: the char used to discriminate a begin of transmission after startup, or a StopChar, after a timeout or any subsequent StartChar in case of line issues
2. CTRL#00/#01: The combination of characters reserved for any auxiliary command. #00 is the MSB, #01 the LSB.
3. R#0/#1: channel Red data (from 0 to 0xFF). #0 is the MSB, #1 the LSB.
4. G#0/#1: channel Green data (from 0 to 0xFF). #0 is the MSB, #1 the LSB.
5. B#0/#1: channel Blue data (from 0 to 0xFF). #0 is the MSB, #1 the LSB.
6. W#0/#1: channel White data (from 0 to 0xFF). #0 is the MSB, #1 the LSB.
7. INT#0/#1: intensity which is applied to the RGBW channels (from 0 to 0x64 [100% in decimal]). #0 is the MSB, #1 the LSB.
8. RES#00/#01: reserved byte for any further necessity. #00 is the MSB, #01 the LSB.
9. RES#10/#11: reserved byte for any further necessity. #10 is the MSB, #11 the LSB.
10. EndChar: the char used to end the transmission and call the command interpreter on the receiver (Lino).

Each group of byte (numbered as #0#1, #00#01, #10#11) can be divided using a mark char (here the “.”, a dot) to make it more human readable the text on a terminal emulator, when debugging, testing and so on. These chars will enlarge the data transmission because it is a data overhead, but on the receiver side are completely invisible since are discarded, at run time.

These 3 examples can be actually copied in a terminal and sent as a text on Lino, moreover, these three commands are equivalent (colors are used to identified the fields):

$00.Aa.20.FF.bC.64.00.00#$00Aa20FFbc640000#
$00AA20FFBC640000# In which: – start char “$”
– the CTRL field is CTRL#00 = 0, CTRL#01 = 0
– the R field is R#0 = 0xA, R#1 = 0xA
– the G field is G#0 = 0x2, G#1 = 0x0
– the B field is B#0 = 0xF, B#1 = 0xF
– the W field is W#0 = 0xB, W#1 = 0xC
– the INT field is INT#0 = 0x6, INT#1 = 0x4
– the RES field is RES#00 = 0x0, RES#01 = 0x0, RES#10 = 0x0, RES#11 = 0x0
– stop char “#”

The colors are therefore represented adopting the HTML color compatibility 24bit coded. As example, the above frame will represent the following color:

at maximum intensity, and the additional pure white set to 87% to make the white component more realistic.

### Next steps

The next steps will be the usage of such board to control Glighter-S modules, but for now the hardware test is sufficient to proove that the system works, the MCU, the protocol and the serial too. More details will be provided as soon as a final release will be tested, and the protocol can be tested also more deeply in all its potential.

To be more precise, will be setup a prototyping system with Lino and 4 Glighter-S. These will be controlled by the already present initial firmware release running on Lino, and a Python script which generates the correct sequences according to a color selection made on a PC connected to Lino. All the test loads are designed in Glighter-S project page.

# Tiny, robust, low cost, fail-safe LED driver: the Glighter-S project

It has been a while since the last LED related article. Was experimented the linear current source, its pros and cons and the field of application. Now arises the need of a small version, handling the same high power, things that are contrapposed in the linear regulator. I need something that I can bring with me, connecting a bit of everywhere, potentially. Heatsinks were expensive, heavy and it is impossible to cover the entire voltage range without huge radiating elements and is also very low efficient. So what I need to do?

### A new board design

With the new version, the entire design is completely changed. I moved to a swiched solution, by exploiting an integrated buck converter controller.

The core is the Texas Instruments LM3405 buck controller, with a fixed sense resistor directly mouted on PCB. With some tricks, like using an external companion LDO boost voltage regulator, I can provide all the required drive voltage for the LM3405’s internal mosfet over an enormous input voltage range without damage or efficiency loss. More over, the output voltage required can be of any value from almost 0V to the input applied, therefore there is no concerns related to what minimum or maximum LED voltage I can use, matters only the current which the LED can withstand without generating the magic smoke.

The high switching frequency of 1.6MHz of the controller allow also to use small components: in the picture there is a reduced size inductor, mounted on pads designed to carry on the higher ones. This left me some flexibility on performance analysis and experimentation.

### Project summary: Glighter-S Power LED driver

• Technology: switching buck regulator
• Input voltage: 3V to 20V (note 1)
• Output voltage (LED Vf): 0.3V to 20V
• Sourced current: up to 1A (note 2)
• Size: 32mm x 21mm
• Efficiency: ~90% (note 3)
• Operating Temperature: -40°C to 60°C
• Safety features:
• Short circuit immune
• Open circuit immune
• Safe state (off) when no control is applied
• Control features:
• PWM control: voltage swing from <0.4V to >1.8V up to input voltage, max PWM frequency 5kHz
• Analog control: voltage swing from 0.4V to 1.8V. Outside these limits device is either shut off of fully on.
• Overtemperature protected
• It does not make the coffee, but the coffee made this. (note 4)

Note 1: active regulation starts from the forward LED voltage plus 0.6V. Full power is delivered from 4V on (device’s control voltages fully on)
Note 2: can be set by changing a resistor
Note 3:  with one led with a forward voltage of 3V is 83%. 90% can be achived my using more than 3 LEDs in series.
Note 4: with caffeine.

On the back of the board there is a brief indication of what is summarized above:

Regarding safety, if the output is shorted, the regulator instantly lower the output voltage avoiding failures. With open circuit, as any buck converter with a sense resistor removed, the output raise as the input voltage due to the broken feedback loop, but with no damage on components: when connecting the LEDs back again, the regulation starts “immediately” avoiding LED damage. Smooth delayed start of few microseconds is implemented when powering the board with the enable already active, to avoid high inrush currents, stressing also less the LED avoiding high current glitches.

It can be dimmed with an analog signal or through a PWM wave up to 5kHz. A greater frequency is just filtered away. Another safety feature is that when PWM pin is left floating or the wire is broken, is equipped with a pulldown in order to guarantee a device shutoff with no misbehaviors. If no control is needed, just connect the PWM pin safely to the input voltage to turn it always on.

### Wiring the LEDs

I wanted a very high power LEDs, but with a relatively low voltage, in order to try it also with low voltage batteries, lower than 6V. So I choose an LED with single diode chip, because when power LEDs have an high voltage (usually greater than 5V) is composed with multiple LEDs in series. My choice is a Luxeon Rebel LEDs, with a single diode, no series:

Each of these LEDs can take up to 700mA (1A peak), therefore I chosen the sense resistor on the board in order to achieve 700mA. I designed a board used as a satellite, mouting 4 LEDs, with the peculiarity of handling the thermal management of a global 9W LED lamp.

The LED board (a.k.a. satellite board) is straighforward: 8 pins, 4 anodes and 4 cathodes to drive each LED individually using 2 wires per LED connected to the driver. The A means anode (connect the +), K the cathode (connect the -). Below the RGBW LEDs soldered on the red, green, blue and white position.

On the back is mounted an heatsink capable of dissipating through natural convection up to 9W up to 40°C of ambient temperature. Bigger heatsink will extend the temperature range. But here the purpose is to have a small heatsink, and wasting time in my playground.

### Wiring the driver

Here is shown the board connections used to test the LEDs and the driver.

The input voltage was 20V from an old laptop power supply, the enable pin shorted to the 20V. I have connected 4 LEDs in series (one cathode on the next anode and so on), with a total forward voltage of around 12V, which means that in this configuration, the minimum supply voltage required was a bit higher than 12V, here 20V are provided. Since the leds in seried are dropping more than 3V (12V in total), the efficiency of the driver is near the 90%, because the ratio between internal losses and delivered power is lower.

The power absorbed is roughly 9W, I just can’t prove its light due to the exposure of the camera, but it is comparable of a white LED bulb of 10W commercially available.

Fun fact: these LEDs are also used in some Philips RGB lamps, with a similar thermal design of the PCB. That is somehow encouraging.

### Conclusion and next steps

There is no much else to say, but it works and it is a small accomplishment that allows me to play more with power LEDs, more easily than before, mainly because of the possibility to use, NOW, almost any power supply (from tablet’s charger, to laptop ones and any battery lying around).

Every microcontroller can drive these boards with no additional components. The absolute value on the pin have analog effect if inbetween 0.4V and 1.8V, while if lower than 0.4V shut off the driver, and if higher than 1.8v will fully turn on the device. Any board, 5V, 3.3V or 1.8V can drive this module even with a low voltage logic I/O pin or even using an DAC. I plan to continue the improvement, and maybe design a control board in order to control these drivers with a small power low-profile set-up.

# Current sources for LEDs – Glighter: a multichannel LED Driver

This is kind of a report of the second attempt in making a power constant current source, high speed, for power LEDs. Taken all the errors made previously, I tried a new linear based current source which should be kept “simple” and “fast”.

I have experimented with power LEDs, how they work and various solutions possible to drive them. With a pseudo evaluation board I tried the very inefficient linear based approach, with the intent of making an high speed driver. As you can read in the previous article there are some troubles that make this kind of circuit everything but not so straight for a rapid DIY approach. On the other side, this journey was very educational to me. The current absorbed per LED (700mA), thermal dimensioning and the LED descriptions are still present in the previous board description, while here I am focused on describing the improvements in this version.

Troubles with the maximum input voltage, offset, mechanical handling and power efficiency have been encoutered and herein somehow fixed with a “new” version. Moreover,  I perfectly know that there is still no input and short circuit protection, as long as various improvements that can be made. But was only the beginning of PCB design, with PTH (plated-through hole), which occupy space and size, so costs. So I said: do not put any capacitor, what could happen? Yeeah, herein I will describe how this choice was something comparable to only an evil genius would do.

#### Troubleshooting the offset

The main issue on the first version, was related to the offset introduced by the op-amps. Therefore, I need to countermeasure this by injecting some current in the sense resistor. This will lead to some dummy current absorbed by the device, thus the capability to deactivate this functionality. This can be accomplished by using SMD transistors and the proper resistors to inject up to: $I = \frac{V_{offset}}{R_{sense}}$. Where $V_{offset}$ and $R_{sense}$ are the maximum op-amp offset and the sense resistor. Let’s say 7mV of offset and 0.47Ω resistance, I need to inject at least 15mA. Since the current sink will work between 3.5V and 7V of supply, the minimum resistance to keep 15mA injected to the sense resistor will be aroud 270Ω, as you can see in the schematic. More input voltage, more losses. The enabling capability is achived efficiently by using an N-MOS digitally driven (0-5V), as a low-side switch, since the source voltage will be up to 0.33V, i.e. the maximum op-amp’s reference voltage.

This compensation will lit up the LED a bit lower, since part of the current used forthe compensation will not flow though the LED, but it is not significant on the final test. I have actually used some 330Ω and the result is achieved equally. But at the highest defined voltage here will source up to 20mA, meaning the the regulator will attemp to flow 20mA less in the LED. In picture below, R11 and T6 are the current limiting resistance and the signal MOSFET used to let deactivate this offset compensation, while Q3 is the main MOSFET which source current for the LED. The X6-4 and X5-4 are the connectors of the main board and the “satellite” which mounts the LEDs.

#### Troubleshooting input voltage range and voltage dropout

Another issue was related to the high value of the sense resistor. This high value was chosen in order to make the system more stable, since the output resistance of the op-amp combine with the MOSFET capacitance, will add a pole. The circuit will be stable if the frequency of the added pole is more than a decade above the circuit’s bandwidth. But I DON’T know the output resistance of the op-amp. Empirically, increasing the efficiency on the sense resistor by reducing the resistor’s value itself and the input voltage, lead to a greater ringing, because reduces the pole frequency. Thus, the system must be slowered down with respect the expectations.

As can be seen in the picture above, the capacitive load (here the main MOSFET) introduces a $R_O \cdot C_L$ pole at $f_p$ frequency in the picture, where $C_L$ and $R_O$ are respectively MOSFET’s input capacitance and op-amp output resistor. At the crossover frequency the closed loop phase w/out compensation is very close to -180° due to the pole contribution. In my circuit, we are at unity gain, therefore in the area labelled as “potentially unstable gains”, that reduces even more the phase margin. Now, with the compensation used in the last design it only rings a bit, improving the situation. In other words, we are practically somewhere between the dashed line (no capacitive load) and the dashed one (capacitive load and unstable control feedback), with a compensation empirically adopted, since the $R_O$ is unknown.

Reducing the input voltage, will bring to the need of reducing also the input control voltage. This reduction has been made by 10 times, so in the board also the voltage divider resistors have been added. Then a buffer placed before this voltage reducer allowing me to not interfere with the resistance divider factor. Reducing the voltage drop on the sense resistor, allow me to power the system with a lot power savings which is not wasted anymore in sensing the current.

The main pain in the amp was to choose a different MOSFET that has a lower theshold voltage, allowing me also to use a lower voltage on the opamp. The choice was on the IRLI630GPbF. Almost 2V of threshold, but big input capacitance. Below the entire schematic:

Handling and manufacturing

I preferred to desing also a separated satellite PCB which is used to store the LEDs. I wanted to try to make it more compact. In these pictures are shown the main board and the satellite, sent togheter to OSH Park as a single design. In terms of cost, that was not a good idea. I like to call it experience.

Here the final assembly:

#### Frequency troubleshooting and considerations

The oscillation was even worse, despite the compensation. That’s the evil genius’ choice. I didn’t put any filtering/bulk/tank capacitors. Putting a 100uF one on the main voltage rails, eliminate the oscillation. This let me understand that in my setup I had a lot of wires that are going from the board to the power supply, keyword: RLC oscillator.

The capacitance will likely dump an LC resonance composed by the MOSFET output capacitance and the lead/parasitic/wiring inductance on the supply rails. The frequency that is dumped thanks to the capacitor seems to be around the crossover designed freqeuncy, ~160kHz, not so strange since at crossover we had such stability issues.

But lowering the voltage will trigger again ringing. The answer is in this picture:Reducing the supply voltage to 3.5V will increase the (already fat in its own) input capacitance, Ciss and also the output capacitances of the MOSFET. This reduces the pole frequency combined with the output resistance of the op-amp, tunes the LC filter, and with no proper handling leads to to much instability (see bode plots before).

And here the LEDs powered with a stable conditions. As always, if you think there are errors or suggestions, feel free to comment or contact me.

# Current sources for LEDs: firsts DIY impressions

Recently I found that today almost lighbulbs are made with power LEDs. And surprisingly most of them are made so cheap that the controller fails far away before its lifetime. At least for what I’ve experienced with few models. Most of them seems to work well, and I think that LEDs are the future for many more things, more than we imagine nowadays.

This triggered in me the need to start experimenting how difficult is driving with consciousness a power LED system, containing mainly the driver and the LED itself, of course.

### Lighting up an LED: what does it means?

Lighting up an LED is simple. Put a resistor in series to achieve a correct current with respect to the supply and the forward voltage of the LED, and voilà, it is light. You can realize that if dimensioning badly these parameters, the LED become hot, leading to change also its colour. Becaming hot, will lower its forward voltage and then more current will flow. As you may realize, this is not ideal when color precision, high power and energy savings are the keyword of a design. An indicator LED (few mW of light) can be designed with a constant voltage, and due to the low energy absorbed, there is an absolute low wasted energy. But relatively speaking, the energy loss is very high.

Let’s consider the circuit below:

The wasted power is always the difference of the voltages between the LED and the supply. So we have that $P_{wasted} = I \cdot (6V-V_{led})$ and $I = \frac {(6V-V_{led})}{R1}$. Using a constant voltage generator will not compensate the variations of the forward voltage of the LED, here called $V_{led}$, which as said before, it is not very constant with temperature, which can vary a lot due to the dissipated power. To avoid variations in luminosity or to avoid overtemperatures, a different kind of circuit is used everywhere, which is a constant current source circuit. The more efficient way is, nowadays, to use a switching regulator, of any type. These circuits can bring average precision, if needed, despite they are quite noisy.

For now a switching regulator is part of the next steps, if making a linear regulator will be successful.

### Constant current linear regulator

The most simple linear regulator is made by two transistors. Through the 2.2K resistor is powered the T2, which turn on the LED. The drop on R is senses by T1 which choke the base of T2 on order to keep the current on T2 around to $I = \frac{V_{beT2}}{R}$.

This is fine if one does NOT need high stability over voltage range, considering also that the Vbe are almost the same, therefore given a current required, the resistor cannot be reduced to reduce wasted power. Since T1 is driven in its active region, the stability corresponds to its plot of the collector’s current VS the collector-emitter voltage:

A possible and more controllable solution is using the LM78xx in constant current source configuration. The issue is the limited linearity in current, and moreover we cannot change the reference voltage without making strange things, leading to waste a lot more power in sensing the current (dropout cannot be lower that 1.25V in these regulators).

Therefore I have ended up with an initial trial in these drivers by making something more “controlled”.

### Linear constant current: the challenge

I need to make something very precise, simple and instructive for myself. I found the classic configuration of a linear current regulator, but using an Operational Amplifier, a MOSFET and shunt resistor. Here is the principle of what I am talking about:

Assuming a sufficient supply for the OpAmp, V1 must be high enough to allow current flowing in Rs so that the current can follow the rule $I = \frac{CLK1}{R_S}$, where CLK1 is the voltage at + terminal. Now, TWO conditions must be followed:

1. V1 must be equal or greater than the forward voltage of D1 plus the voltage drop on Rs and the  Vds in linearity of the MOSFET, in worst condition (MOSFET works in saturation region if higher voltage is applied, enabling the analog control of the light, but also higher wasted power).
2. The voltage drop on Rs must be the voltage on CLK1 pin

The #2 condition is the key of this circuit, and it is the OpAmp purpose. The values and components in the figure are just for reference. The sense resistor substantially determine the minimum voltage applied to the LED: if the resistor need to drop 3.3V, these must be the starting point for the minimum voltage. We are designing a linear current regulator with a “dropout” of 3.3V! It is quite a lot of wasted power: $P = 3.3V \cdot I_{LED}$. But this is just a trial. Here, we have 3.3V of reference voltage, but reducing the resistance, the voltage can be lowered even more than the canonical 1.25V of an LM78XX regulator.

But here the advantage: I can change the reference in order to reduce the wasted power! And its linearity is order of magnitude higher that the configurations shown above (yes, with the expense to use an op-amp). In the reality I need to slow down my circuit response, because low sense resistor will lead to instability.

## The real world: demo boards

I then build a demo board to test the thermal calculus applied on power components (LEDs, MOSFET, Sense Resistors), without any concerns about efficiency: one need first to understand if the main concept works, with the simplest conditions… the roots of the tree! 🙂 To add a bit of fun, I will try to design a setup of 4 LEDs, Red, Green, Blue and White.

Which is the accuracy of the design? The precision is dependant from the op-amp’s offset: a sensed voltage of 3.3V with 9mV offset, bring uncertainty of few hundredths of percent. Then the other main source of error is the sense resistor. Here I used 5% power resistors, so the main error in current stand point can be said to be 5%.

### Thermal experiments

I choose the Luxeon Rebel series LEDs, which provides also a public well designed thermal pad for normal 2-layer PCBs. Following the basic rules of thermal design (I should write something about that in the future), I have interfaced an heatsink to a pad, which brings a thermal resistance of 7 °C/W, junction to heatsink, which is from the LED to the top PCB anding to the bottom layer:

The design takes in consideration the maximum forward voltage of the LED, and its maximum current. In this design, 4 LEDs are used (Red, Green, Blue, White) and only white have the higher voltage and so the higher power, so the dimensioning is made on the white to achieve the worst case.

#### A bit of math

The maximum junction temperature allowed is 150°C, reduced to 130°C for reliability. The white LED dissipates up to 2.2W at 700mA. Thermal resistance of the junction to case of LED is 10°C/W for these LEDs. So:

$[\Theta_{junction-case} + \Theta_{case-heatsink} + \Theta_{heatsink-ambient}]\cdot P_{LED} = T_j - T_{ambient}$     (1)

Thanks to the thermal pads, the heat is trasferred to the bottom and from that towards to ambient, using an heatsink.  Where Tambient is 40°C, in order to consider the maximum allowed one. Calculus lead to use a 24 °C/W, the used one is 25°C/W, can be unsafe for the LED to work at 40°C, but it is well cooled at 25°C. To stay even more safe in a reasonable way, the power of the LED is assumed to be entirely transformed in heat, while actually only around 60% is wasted, due to efficacy the LED to be around 40% and therefore that quantity of light is transformed in light, and this percentage will vary from one color to another. With this assumption and these LEDs, I can use them at 40°C too with a smaller heatsink.

The remaining power due to overvoltages is absorbed by the MOSFET.Again, Tj will be 130°C. Here, if I decide to power my board with maximum 9V, the drop on the MOSFET will be:

$P_{mos} = (9V - V_{led-min} - V_{sense})\cdot 0.7A = 2.5W$

And using the (1) only with the junction-to-case resistance, with parameters extracted from the datasheet of the BUZ70 used:

$\Theta_{case-ambient} = \frac{T_j - T_a}{P_{mos}} - \Theta_{junction-to-case}$

The system will work up to 40°C of ambient temperature, still air. The same happens for the power MOSFET, which is dimensioned to dissipate up to a supply voltage of 9V (my maximum supply allowed) minus the minimum forward voltage between the 4 LEDs, leading to a worst case condition and so higher power dissipated on MOSFET corresponding to lower power dissipated on LED, here is the Red one, which have the lower voltage drop. The MOSFET uses then a simple TO-220 heatsink of 21°C/W. Calculus lead to Θca = 32°C/W, but I had only these larger one at the moment of the test.

Since it is just a proof of concept, the sense resistor drops a very high reference voltage, here 3.3V. Which at 700mA of maximum current, dissipates up to 2.5W. A small vitreous enamelled wirewound power resistor is used.

### The PCB design

You can see the schematic of the final system, with different supply rails for op-amp and LEDs, which can be also merged togheter. To feel safe, I’ve added some pulldown resistors, keeping the LEDs off if no signal is applied.

The placement takes into account the dimensions of the heatsink, shown here:

The Luxeon Rebel LED pad used brings the heat down to bottom PCB with 7°C/W of thermal resistance. But how to place the heatsink? I choose a PLCC heatsink and glued to the bottom with a thermal tape. Of course, the bottom must no contain the soldermask, freeing the condutors.

Here the demo board ordered exclusively from OSH Park:

Nice work, even the tiny 0.25mm vias of thermal pads are made correctly! And here you can see the bottom section with the exposed conductor in which will be mounted the heatsink on it. Note that this solution is adopted since it is a regular FR4 and not an aluminum PCB:

Since I am an evil genius which tries to drive 4 LEDs, I bought 3 heatsinks both for LEDs and MOSFETs. For that reason I will show pictures of only white and separately the RGB test.

### First tests

To see the regulation, I probed the signal on the sense resistor. In the real world, this is how my regulator regulates, when applying a 50% duty cycle:

This beacuse we drive a capacitive load (i.e. the MOSFET gate!), introducing a pole which depends also on the output resistance of the op-amp, reducing the phase margin too much. I have found that my circuit is one of the most unstable systems. By improving the reliability inserting a resistance between opamp and MOSFET’s gate, the oscillation will increases. Such a pain in the amp.

The answer to this problem is to increase the phase margin of the system, and I chose to use a Lead Compensation circuitry, this will reduce the bandwidth of the system to a known value, in my case to around 160 kHz, choosing a values of R and C of 100Ω and 10nF, after a bit of math and trials, not depending anymore from the operational amplifier’s bandwidth. This compensation seems to work, also on various supply ranges, because reducing the Vds will increase the capacitance Ciss, so testing over various suply voltages worth something. Here the smoothed output:

Still, I did not made any rising time tests, but I will need to. This linear regulator can modulate signals up to 160kHz! Any switching LED regulator do not handle modulating frequencies up to few tens of kHz. Is this useful? It depends: we will see in the next episodes. “Is it more efficient than a linear COTS component?” “No.” “Can be improved to be more efficient?” “Yes.” “When?” “Later.” “How w…” “shut up”.

The thermal pad of LED becomes quite hot, demonstrating that some heat is tranferred. I run the LED for a while, with no damage, I need only to test it in an environment of around 40°C, since this is the temperature at which needs to survive by design. Just for curiosity, I probed the temperature, to make a quantification of the “finger test”, by putting an NTC (did you know how to use it?) as close as possible to the heatsink, fixing the probe with a thermal grease:

What is a finger test? When you put your finger on a surface and you can’t stant the finger, it is usually more than 60°C. And here the system set-up to use 3 LEDs:

### Color power

The white works, I did not made a serious comparison with other light sources, but is visible like almost as a 5W or 6W neon bulb at first approximation. The picture is taken with a partially hidden LED behind the MOSFET heatsink otherwise the camera  will close completely the shutter.

I then discovered that the offset is sufficient to light up dimly the LEDs when they are supposed to be fully off (here you can spot the blue one behind the resistor). Injecting current in sense reistors, to compensate the offset voltage will solve the problem. Must be applied a resistor in order to flow a current at least equal to the current generated by the offset, here $I = \frac{V_{off}}{Rsense}$. Next versions will take into account this. Here the temporary solution for testing the RGB setup:

Let the light begins! The 5V-3.3V voltage shift is provided through a blob of air soldered resistors placed in the inputs:

The red trial:

The LEDs were soldered using an hot air gun, heating from the bottom: the pad is used then as a heating tranferrer for the soldering phase. The Luxeon Rebel LEDs are demonstrated to be strenght enough.

The blue will shows an interesting thing: exposes the fluorescent materials!

With an another set of blue LED (here more blue, to be precise I’ve used a Royal Blue when I taken the picture below) I discovered that my small toolbox is somehow fluorescent (the light green one). In the center of the picture, you can see another board disconnected in which the white led seems to emits on its own, while it is just converting back the royal blue to a more sensible and wide wavelenght range, i.e. white, thanks to its covering resin made of fluorescent material.

Another LED from a smartphone flash, that re-emits under the Royal Blue:

### Little demo

In the video posted here, can be appreciated the variety of shades that can be represented. With a firmware handling 8bits per colour, here can be represented 255 times 255 times 255 colours, i.e. 16,581,375 color shades. Implementing the white, an even more precise white light can be generated (did you mentioned Philips Hue?). For the future improvements, I will release also the code and stuff.

### Future improvements

The next version will have the offset compensation, better LED positioning, higher efficiency. The concept works, but now a proper firmware will be developed, strarting from implementing a better script in Python which communicates with the Arduino and selects the colour.

Maybe for efficiency reasons and since I don’t need speed, I will “switch to switching” for future versions. For now, I am satisfied of having touched with hands the “inefficiency” of the linear current sources for LEDs, which is the price to pay if you want precision and, eventually, high bandwidth.

# An UART interpreter – sneak peek

In a hot summer of a couple of years ago, trying to not thinking about the stifling heat of the afternoon, I put some efforts in a system which should drive a set of motors and peripherals by means of a single serial line. The idea consists in creating a sort of instruction set, sent over the serial, to transmit and receive informations, but the main effort was to develop this under a robust software, to make the system rensonsive and interrupt driven, fully aknowledged to keep trace of errors also.

This project is born following these ideas, on order of development:

• Driving a low level peripheral using a console, with an affordable and error tolerant firmware
• HW separated by SW, being able to design them finely and independently
• Being able to drive the HW on any type of PCs
• Expliting any software that can handle strings, the only specification is to know the language of the console
• As a natural consequence, a PC connected on the Internet can drive the HW remotely

### The main idea

In order to do this, I thought to a concept made of a two stages of control: the microcontroller receive commands from the user and put them in action. But the user of the microcontroller could be another board or PC, not necessarily a human generated commands. This, as previoulsy said, can be made through a console.

In this way the microcontroller carries an “interpreter” which translate commands to the hardware. By this mechanism I split a single action in a confined memory and execution time, affordable and in a responsive way. Due to this software “modularity” it is possible to extend indefinitely the instructions that the micro can interpret and use all of its calculus power to control precisely the hardware desired or manage the instructions in a robust way. The MCU is then only a front-end of the system which receive and sent signals from/to external world. By using a coherent language, the device can communicate with other peripherals (like a PC) by using the serial, and being driven by them directly.

### The abstraction layer

The user only see a console over the serial, which work so vaguely similar to the Python line interpreter. But commands are simple, being able to be generated by a software running on another machine which send them to the console. And are simple because this project is at a very early stage. 🙂

Here the user of the console can implement all the algorithms, writing them down to the MCU generated command prompt. With this machanism we already have an abstraction to blink an LED over the Internet, or driving a robotic arm, nothing change inside the MCU. The abstraction give the possibility to personalize the behavior of the device and save in a text file on the PC without the need of reprogramming the chip. Also, but this is not structured yet, can be possible to save this content on an EEPROM or whatever memory support can be reached by the microcontroller, when the console is not directly accessible.

So, anyone can control the device directly or through a PC algorithm. The power of the idea is that if I want to drive a robotic arm, I have sufficient abstraction to drive it directly from another location, making this MCU implementation very suitable for remote control and environment analisys.

At this stage of the project, the software running on an internet connected device is written in the multi-platform Python language, due to its versatility. It reads the console and write commands to it. But here the console is just an echo taken from the microcontroller, elaborated by the Python based SW and printed on the screen. What is important, the user can not know anything of the hardware of the MCU, being free to think to the optimum driving algorithm.

Are also implemented 2 buttons where each control a device bypassing the physical serial console. But due to the modularity of the firmware, when a button is pressed, it is not activated a signal inside the MCU, but there is an auto-generated internal command sent back to the console, by parameter passing without using the USART module. By means of this mechanism, all controls are the same, the code remains modular and there are no performance losses.

A pretty stupid image is shown here, just to make the idea. The light green is what is done inside the MCU, while the light orange shows the basic algorithm of the PC. Between these two colors, there is an USART wrapper, but can be implemented on other technologies, like SPI, I2C, whatever isola supported on the MCU side.

I almost forget an analogy: we can think about a client-server structure. Here, the server is the MCU while the client the Phython based PC software. I like thinking to the old fashioned gerarchy of the computer terminals, a dumb terminal that send commands on a mainframe. But here the terminal is not so dumb compared to those in 1960.

### Some specifications

I almost forget to talk about the core. Its based on a 8-bit PIC18F47J53 microcontroller, which can handle  very efficiently two levels of interrupts priority. Here I can give more priority to the USART commands instead of those on board, or viceversa, depending on what I desire. For example, when I press a button and I want to be interrupted by the serial, I can put the serial receiving interrupt up in priority. The project was previously born on an Atmega328P by using Avrdude and AVR Studio 6, but then ported on this PIC using XC8 and MPLAB-X, due to better hardware specifications.

Moreover, and I will talk about that on next article, on the MCU board there are two buttons and here are configured to work efficiently like a joystick. The ADC samples a value of voltage on a potentiometer and can set the intensity of the command.

The software on PC is Python based and uses the Pyserial library.

### And now?

Now as soon as I can find some time to put something on more than a breadboard and when the Python based terminal is presentable (keep in mind what “presentable” means for an electronic student…), I will put something more that a bare useless presentation. I will also put the sources and I will explain better what is going on, trying to be helpful to who want to play with this stuff.

On the next article, I’ll try to explain how to control a motor by using an opposite phase PWM modulation, allowing the analog control of a motor. This can be done also without an MCU, by using only one 555. I’ll see what implementation I will adopt.

Nevertheless, this opposite phase PWM method can be integrated very well with this UART intepreter project: one can drive the motor manually and then an algorithm can decide if that speed must be kept, and so take the control over the device. Yes, I will decisely talk about that in the first release of this project. I will play with the 555 another time…
But I’m afraid that I’ll not find so much time to setup a proper sensor and data acquisition. So, I need to organize myself on order to find time, and this is not a good news.