I’ve started by reading SLVA374 app report from TI which is a Step-Down LED Driver Design Guide based on the TPS54160 dc-dc converter, normally used as a buck voltage regulator. The schematic from the app report can be easily adapted to our device with a few changes like adding a sense resistor and an output capacitor:

If you remember from my last post I needed 1.12 A to power the two led packs that I’ve  build connected in parallel. We have a simple equation that we use to find out the feedback resistor value: R=Vref/Io In our case Vref is 0.8V taken from TPS54232 datasheet, Io is our desired current of 1.12 A. Doing the math we get the R value = 0.71 ohms. This is not a standard value, but we can use two 1.5 ohms resistors in parallel(R1 and R2) to get 0.75 ohms which is pretty close. With 0.75 ohms feedback resistor I have 1.06A at the output which divided by 56(the number of white LED’s) means 18.9 mA for each white led.

We must consider the power dissipation for this resistor and we can calculate it : Pdis = Vref^2/R In our case we have Pdis = 0.8^2/0.75 = 0.85W. The only 1.5 ohms resistors that I could find locally were 5 W so I had to work with a bigger package. The EN pin is used to enable and adjust the Undervoltage Lockout but I’m not going to use that feature since my input voltage is always gonna be high enough not to cause any problems. The resistors are placed on the pcb anyways so you could use those pads and soldere the necessary resistors.

We also must consider the power dissipation in the low side diode. During the converter on time, the output current is provided by the internal switching FET. During the off time, the output current flows through the catch diode. The average power in the diode is given by: Pdiode = (1-Voled/Vin) * Vfd * Io , where: Vfd is the led forward voltage, Voled is the supplied output voltage and is approximated by: Voled = Nled * VLed + Vref where : Nled = number of LEDs, Vled = forward voltage drop of each LED. In my case Pdiode = (1-(4.2/12))*0.75*1.06=0.51W so I used a 2A 40V schottky diode part number CDBA240-G.

You would also have to consider the inductor and the input and output capacitors, but you can read all about that in the datasheet of the TPS54232 or in the app report mentioned above. After I had all of my circuit figured out, I routed the board and etched it using my photo etching technique. For such a small board its not worth taking out the solder paste so I soldered it using the soldering iron, as you can see its a mix of through hole and surface mount parts but they fit together quite nice.

The solder drops that you can see on the back of the board are a sort of DIY thermal vias. I’m not sure how efficient they are but I gave them a try with this board, probably because I had too much time available . First when I designed the board I included a area of copper in the top layer right beside the resistors. Next I drilled some 0.7 mm holes and I placed 0.7 copper wire in the wholes securing it by soldering on both sides. The trick is to make the solder joint as small as possible so it doesn’t spaces your component from the board. As I’ve said I don’t know their efficiency but I think they work, I can feel the heat transferring from one side to the other faster. If you like them you can try them.

The testing went smooth, the LED’s light up perfectly, no problems at all. During the testing I noticed one bug though: if the input wires are not firmly attached or secured and there is an imperfect contact the converter will tend to output less current than the programmed 1.02A. I’m not sure why this is happening, it might have something to do with the Undervoltage Lockout feature that I skipped on but I’m not sure. Anyway its not that much of a problem since once in place the power source will have the cables firmly attached so no worries. That’s why I used the cable connectors on the PCB in the first place, I knew it would save me some trouble later.

Now I had one last thing to do. Since this circuit is going to be operated in the outdoors the corrosion would set in pretty quickly so I improvised once again. I had this idea for quite a long time but never actually tried it. So I used my Bison universal hobby glue, which is transparent, and covered the copper traces on the PCB in glue. After it hardened, it looks like you could dip these board into water and nothing would happen to them. Well, except from the terminals which stick out of the PCB . Nonetheless I think these boards will run no problems even after a few years. If you’re wondering where the bubbles come from, they form when the glue dries out .

I’m pretty happy how this project ended up and I feel like I know more about LED’s and ways to power them. I’ll definitely need to experiment more with some high power LED’s, maybe use them to light my workbench. As for the efficiency of this circuit I don’t know if I’m calculating the right way because I’m using the equations from page 16 of TPS54232 datasheet, and those are clearly stated to be used only under continuous conduction mode. Since the circuit has been modified to act as a constant current source I’m not sure the same equations apply. But I did the math anyway and I got an efficiency of 75.36% and according to the same equation if I would connect the two led packs in series I would get an efficiency of 79.01%. Once again I’m not sure these calculations are correct and I ask the readers to comment on these.

There is more that you can do to improve the efficiency of this circuit. It turns out you can reduce the power losses in the current sense resistor by lowering the voltage across the resistor. The solution is to inject a bias voltage, but I’m not going to try this solution since I’m already happy with my design. You can read more about it in SLEA004 app report from TI.

Downloads:

• TPS54232 constant current source Eagle files

Anyway, I had everything needed on hand so I proceeded and designed a PCB in Eagle. I used two LM317 voltage regulators because I want to power two sets of LED’s. Each set of LED’s is composed of 28 LED’s tied in parallel. I’m gonna set the current on each LED to 20 mA so that is 560mA for each set of LED’s. I wanted to spread all that load on two LM317 so I used two.

The PCB were easy to route and ended up like this:

Kind of small and nice looking. The LED pack could of been smaller but since LED’s don’t spread light on a wide angle I decided to spread the LED’s a bit to spread the light. The LM317 board was designed so that it would fit perfectly under a heatsink I had around. Actually the heatsink is from an old video card I had in my junk box; its nice to reuse these things, its like recycling.

I proceeded and used my DIY photo etching method but something went wrong and I messed up all of the PCB’s, they ended up looking like this:

The thing is I stopped using ferric chloride a while ago. Instead I turned to ammonium persulfate which comes as a white powder that you need to dissolve in watter. The watter stays clear, it doesn’t smell and it doesn’t stain (not checked yet). It only catches a blueish color after you’ve used it and copper accumulates in it. The only disadvantage for using ammonium persulfate is that it etches slower than ferric chloride, other than that is great. The problem with mine is that it was mixed with water more than 6 months ago and it turns out it doesn’t last that long when mixed with water, so keep it as powder and only mix it when you need it. Unfortunately I couldn’t get more of it locally so I had to return to the good old ferric chloride. I did all the process again and the boards came out great this time.

The assembled LED packs look like this:

and the assembled LM317 board look like this:

Notice how I designed the board to fit just over the raised part of the heatsink? its the little things that count (and I’m not referring to the heatsink as you will read next). The LM317 pair was mounted on the back so that it sits firmly attached to the heatsink transferring all that heat.

here’s a photo between the heatsink and the pcb:

I was pretty happy with the overall result until I actually run it. The heatsink got very hot pretty fast and it was clear it was not dissipating enough heat. With 12 V in, LED Vf of 3.4V , 560mA and an ambient temperature of 28 degrees C, the heatsink reached almost 80 degrees C. It turns out The LM317 is not very efficient in this setup and together with the sense resistor has to dissipate allot of heat. And a simple calculation would of shown this from the beginning if I paid attention to it.  (Vin-Vf.Led-Vadj)xIo => (12V-3.4V-1.25V)x0.56A= 4.116 W for each regulator, double that and the heatsink is just too small to dissipate that kind of heat. Not to mention the poor efficiency of the circuit. Its the little things that count , I would of done that calculation earlier I wouldn’t of made that circuit just to realize that is not good for my setup.

At this point I knew this option is not good and I had to find something else to power the LED’s. I could use a specialized circuit like the LM3406 which would give me the best efficiency but I would have to wait for delivery and get some more parts, or I could use a dc-dc converter modified as constant current source which would give me decent efficiency and I could build it with parts that I already have. I decided to go with the dc-dc converter modified as constant current source. I already have the TPS54232 from another project of mine; the TPS54232 is a 2A, 28V, 1MHz, step down SWIFT™ dc-dc converter. Now its time to design a new PCB for the new power source but more about this in the next article.

Downloads:

• LM317 constant current source design files
• 28 LED pack design files

I would like to wish my readers a Happy New Year and may all your wishes come true. As you may have experienced, during the holidays,  hobbyists get some free time and they like spending it building stuff. In this case the author, Infernoz, build this 8 LED POV display, which seems like a fun way of wishing a happy new year. The device is based on the Attiny26 from ATMEL and from the video we can see it works quite nice, but unfortunately I was unable to locate any source code posted on the project page, but this is not that bad as you can find POV source code from others projects(POV1 POV2 POV3) if you’re interested in building one.

Happy new year 2010 POV: [via]

You want to learn more about DC to DC boost converters? Here is a good and practical application of such converters, an LED tail light for your bicycle. Why do you need a boost converter for that you might ask, when you can simply have a battery power up some LEDs. Well, this way you greatly increase efficiency, you increase the number of LEDs you can drive and thus increase the perceived power of light. And yes, you will learn about them.

This project makes use of the National Semiconductor LM3410 chip. This chip provides a current regulation rather than a voltage regulation and makes it suitable when working with LEDs. This current regulation is done by switching a NMOS gate at constant frequency of 525Khz ( you can also chose 1.6Mhz) and varying the duty cycle, the so called pulse width modulation. Given a constant frequency varying the pulse width results in varying the mean value in time. The regulation is done by providing a feedback current that causes a voltage drop on a resistor which in turn is fed into a comparator.

The output of this comparator goes to the PWM comparator where it faces the output of the amplifier that reads the switched current through an inductor. The result will be a PWM signal attacking the NMOS gate and thus providing a steady mean output current value. The current sensing amplifier also provides an output for switched current limiting. Some nice features of LM3410 makes things easier, it’s internally compensated,  has a very low stand-by current, has dimming possibilities and thermal shutdown.

As the author states in his article you have to be careful when choosing components. Since the operation frequency is pretty high the diodes through which the inductor discharges must be high speed or you risk blowing the chip. Taking things a step forward he added a PIC12F683 microcontroller to provide some light effects selected by a pressing a button.

If we look closer at this project we can see it can be further developed, using the dimming possibility of LM3410 you can also turn it into a stop light. Whenever you break light intensity goes higher. Another development is to adapt it to be used with a dynamo. Have fun.

Bicycle Tail Light: [Link]

Toy cars are something that everybody has encountered during the childhood and even a non-car enthusiast like myself has had his share of toy car playing, racing, garage and road building and all that good stuff. You’d think that something like this would be more suited for kids, but what if you’re a grown-up, you have a small car at your disposal AND possess the knowledge to make some cool hardware improvements? That’s right, awesomeness occurs!

This is a DIY project that enhances a Kyosho Mini-Z car with some incredibly useful features: front, rear and under car lights! The model is actually a VW Golf R32 and, as you can see from the picture, it looks terrific, in true Fast and Furious fashion. The lights are LEDs, two white ones for the front, two red ones for the rear and two bars with 4 LEDs for under the car. The circuit keeping them all on uses a NE555 timer that generates a delay and keeps them on regardless of what the car is doing. Other parts include three capacitors, three resistors and three diodes (you can find a complete parts list and detailed schematics in the link provided, as well as other cool pics).

This is a truly astonishing piece of art, soldering a bunch of LEDs on a toy car, especially with the blue under car lights. If you’re a girl, then I suspect this amazing achievement of science might appear… uninteresting. But if that’s the case, then perhaps an LED Heart of Love would be more appropriate.

Mini-car Light Installation: [Link]

A valuable item in a hardware enthusiast’s arsenal, an ultra violet LED exposure box can be used with good results in making quality PCBs. LEDs are cheaper and safer than usual fluorescent lamps and also have a longer life span, so using high brightness UV LEDs is a great choice for an exposure box. The only downside is the longer exposure time, but this is an acceptable trade-off for most hobbyists.

The Exposure Box described here is a double sided one, with 84 5 mm UV LEDs on each side which need 700mA at 12V (that means 8.4 Watts for each panel). Therefore, the Exposure Box possesses 168 LEDs that add up to 16.8 Watts. It is important that the LEDs have at least 2000mcd brightness, a peak wavelength of less than 400nm and a viewing angle of at least 20 degrees. Other important components are the resistors (you will need 56 of them), the boards on which the LEDs are mounted and a power supply unit (this design uses 2x 160mm x 100mm pieces of Veroboard and a 12 Volt 24 Watt switch mode power supply).

The box itself is made of 6mm MDF, which can be easily shaped to build the case. The walls of the box are glued together and you will also have to drill holes for screws and the PSU connector. Take precautions when soldering the LEDs (check the polarity and don’t look directly to them when powered). You will also need a power switch and wiring to connect both panels to the PSU. When everything is done you could test the Exposure Box to see how it works using a piece of metal with photo resist.

A great project for home use, the UV LED Exposure Box can come in handy when making PCBs. Details on the components, assembly, soldering as well as schematics and other useful information in the link.

DIY UV LED Exposure Box: [Link] – [via]

“Love is a smoke raised with the fume of sighs,

Being purged, a fire sparkling in lovers’ eyes,

Being vexed, a sea nourished with lovers’ tears.

What is it else? A madness most discreet,

A choking gall and a preserving sweet.” (William Shakespeare)

When in love it can be important to find the right gifts for the right occasions. You know, the kind of thing that would bring a big smile to her face (followed by a wet kiss). It can be difficult to even remember to buy a present, let alone come up with something that will be appreciated. But fear not! I have found the solution to the problem and next Valentine’s Day you will be ready to really impress your sweetheart.

The Twinkling LED Heart of Love is a wonderful DIY project that will melt her heart and turn the next 14th of February into an unforgettable event. The Heart of Love is based on an Atmel AVR ATmega168 microcontroller coupled with 20 red LEDs that… blink randomly! The heart itself can be made of cardboard and, although red is the usual color for love-related objects, you can show your creativity and paint it something meaningful to you and your damsel. Like orange, if you’re both Netherlands soccer fans. Or black, if you’re black metal fans (pentagram is optional). Originality is usually appreciated, so go for it!

Tips and tricks: your Heart of Love is awesome and will surely knock her socks off, but don’t start explaining how it’s made, how the microcontroller works and so on, because she might start yawning and eventually fall asleep.

As for me… I think I’ll settle for a big bouquet of red roses, thank you very much.

Twinkling LED Heart of Love: [Link]

When i first saw this project i thought this could be turned into one of those dance steps learning game. The device consists of a 3×3 matrix of buttons. The system memorates the sequence in which you press the buttons and then plays it for you by lighting a LED under those buttons. You can also program multiple sequences. If you don’t press any button for a longer period of time, 4 seconds i believe, the device goes from recording to playback.

It is quite an interesting memory game, and can be transformed into a larger project. Like in one of those smart houses… in case you get lost this device will show you what path you took. On the other hand the algorithm behind this project can be used in beat generators, sequencers, power distribution sequencers and many others.

The Nove Bit as it was called is using Arduino and a TLC 5940 microcontroller. Source code is available for download as well as some instructions on how to build it.

9 Bits Visual Memory: [Link]

When POVs first appeared in advertising panels i used to wonder how are the characters diplayed. Seemed like magic. Later on i found out its because of our eyes’ inertia. Where you needed a large number of LEDs to display a message, now you just need to spin a few LEDs. The rotation speed must be fast enough to display at least 10 frames per second, complex graphics may require a higher value between 15 and 30 fps, movies usually have between 24 and 30 fps.

This project will show you with great details how to build such a device. It is not an easy build, it takes a fair amount of tweaking to get it to work but the results can be spectacular. The developers of this project decided on a modular design, putting an emphasis on interactivity. In the end they came up with a very customizable POV that can display images you upload wirelessly and that you can manage in real time.

The microcontroller used is the Atmega644, leds are driven with the MAX6971 IC and Xbee modules provide the wireless serial communication between the POV and the PC from which you upload the image. A GUI written in Java makes this task easy for you.

The LEDs are place on a different PCB than the rest of electronics, this way it can be easily upgraded to a 3D version. The motor part needs some attention because you will have to carefully balance the LED board, you will deal with a lot of vibrations and the speed of rotation can be quite dangerous. Also there will be some tweaking involved when you will build the brushing system to power up the LEDs.

The motor has a separate power supply so that the main PCB won’t pick up noise. A HALL sensor is used to count the rotations and give the position of the LED board. When real time management of the display is not desired, the POV can use the picture stored in its EEPROM.

In order to power it, you will need a 9v power adapter that can deliver at least 1.1A for the electronics boards. Since the motor is powered from different source, you will need to meet your motor’s requirments. The one used in the project had its own 5V/2.5A power supply.

The cost for building this project is a little higher than 50$ and you can further reduce that if you already have some of the needed parts . All schematics and code are found in the project, below you can see the POV in action.

2D Wireless POV Device: [Link] – [Via]

Interesting weekend project for those who still remember the awesome game or just to awake in your kids the knack for electronics here is the Space Invaders Button.

Fairly easy build, having the ATmega16P at the heart controlling an 8×8 bicolor LED matrix which will display animations of the game’s spaceships. Principle of operation is simple, the rows of the matrix are driven by a npn transistor in an emitter follower configuration.

This transistor provides the boost of current needed when two or more leds from the same row are on. The state of the ports driving these transistors is set on “high” logic level. You can now command each LED to light the color you want by setting the assigned port to “0″.

The ATmega16P will draw a small amount of current and can be powered by a 3V cell battery. The PCB is of small dimensions since SMT components are used, it’s even smaller than the LED matrix and thus hides nicely behind it.

It’s an easy project with little chance of failing and offers a nice lesson on how to handle the ports of your microcontroller. If you are doing this just for fun and you don’t care about dimension than the SMT can be avoided so that you can use the controller for other projects.

Happy soldering!

Space Invaders on a LED matrix: [Link]