YourITronics

If you ever get the message “Entering programming mode.. FAILED!” while trying to program your AVR device don’t start messing with your MKII or upgrade its firmware 10 times or messing with the windows drivers like I did, cause that’s probably not the cause and you risk damaging you’re perfectly working MKII. As it turns out its just a faulty connection between the programmer and the target board. This is not likely to happen if you only use the programmer occasionally but I used it for like 50+ times a day and that meant inserting and removing the little 6pin connector a bunch of times. I removed the old 6pin connector, crimped a new one and its working again, so long live the MKII because it is a great tool.

You might be wondering why would I need to use that many times, well its because I’m working on this new project, a quadrocopter based on an atmega64. The microcontroller was chosen because I had a bunch around and ATMEL was kind enough to send me a ATAVRSBIN1 for my project.

The Inertial One System Board delivers a full 9 degree of freedom sensor platform comprising a 3 Axis Magnetometer from AKM (AK8975); 3 Axis Accelerometer from Bosch (BMA150) and a 3 Axis Gyroscope from InvenSense (ITG-3200) connected through an I2C interface.

At $54 its probably the most accessible 9DOF breakout out there and it comes with great sensors. Unfortunately I only managed to test and fly with the ITG-3200 gyro before I had a crash(you crash allot when developing from scratch a new firmware for a quad). Because of the crash a short-circuit happened somewhere on my board and it messed up the ATAVRSBIN1 sensors.

The ITG-3200 is a very good gyro with low noise so to replace the damaged ATAVRSBIN1 and to continue flying I got a wii motion plus which has the ITG3205 inside(its supposed to be just an OEM version of ITG3200). I haven’t done allot of testing with this new ITG3205 but I did notice some problems on the I2C bus, like sometimes the sensor does not respond , which is strange.(maybe the 400KHz freq is too high for the ITG3205 ?).

Anyway enough for now, I’ll post some updates on the project soon.

I’ve been pretty busy lately working on the latest spectrum analyzer V2.1. Its available in the online shop if you are interested in getting one. Here are some videos bellow with the new blue LED’s and with a half green/half red display.

I have also been working on the new expansion board pictured bellow. The Expansion Board was created to help you create bigger custom displays for the Spectrum Analyzer. The Expansion Board works by switching the signals from the spectrum analyzer with the help of high current MOSFET transistors thus giving you the possibility to output higher voltages and currents for your custom setup. You can read more about it on the forum.

There is also a new assembly guide for the audio spectrum analyzer V2.1 that will be helpful to those that get the kit unassembled.

Since I was working with the ST1S10 for an upcoming project I decided to publish this little calculator. The ST1S10 is one of the cheapest if not the cheapest 3 A, 900 kHz, monolithic synchronous step-down regulator. The high frequency switching allows the use of tiny SMD components so the board space taken by this regulator its pretty tight. The calculator is written in OpenOffice Calc and saved as XLS, so you can open it with both MS Office and OpenOffice. The calculator works for Vout > 2.5V. You have to enter the requested values on brown boxes and you’ll get the result in the yellow boxes. If you notice any mistake in the formulas please let me know.

Download here: ST1S10 calculator

I wanted to test if its possible to have a magnetic-less ethernet connection for an upcoming project. Why magnetic-less? you might ask, well space & weight is critical for my application and since both devices will be present on the same PCB, why not skip the magnetics and reduce space & weight. I have to admit it was allot easier than I imagined it would be. When I first thought about it I imagined I would need to simulate the impedance of the transformer with some inductors, this way before I did any reading on the subject.

Thankfully I had a friend who helped me with some network switches from his junk box. The switches were running ok but its clear that they once had some reliability issues which got them into the junk box, but it was enough for my little experiment. I decided to use the following configuration to test the magnetic-less connection:

• both switches will have the transformers removed from one of the ports.
• computer1 connects to switch1 with standard connection.
• switch1 connects to switch2 with magnetic-less connection(no transformer).
• computer2 connects to switch2 with standard connection.

After the setup is made and link is up I would ping computer2 from computer1 and if the link is ok the result should be obvious.

Now, at this step I still haven’t done any reading on magnetic-less ethernet connections, so I took the bad choice of connecting the two magnetic-less ports wire to wire, “wire coupling” . I knew i had slim chances to make it work, but I said nothing can go wrong. I hooked the two ports, and nothing happened, no link, no LED turned on. Probably somewhere in this process I damaged the two network switches(RTL8309SB) because they were both working before I did surgery on them, and they were both not working when I finished the surgery So directly coupling the two interfaces is a bad idea, it should of ringed a bell the first time I thought about it but it didn’t, I learned it the hard way.

This is where I knew I had to do some reading on the subject. Luckily every manufacturer of ethernet interfaces such as ATT ethernet services has an application note on how to couple them magnetic-less for exactly the same situation that I have: both chips on the same pcb. I picked up another 2 switches and opened them up. Since they both had Realtek controllers I used this application note from Realtek. The app note provides a simple solution, capacitive coupling.

I got to work, once again I removed 1 transformer from each network switch but this time I used capacitive coupling. I soldered wires to where the transformer used to be and I connected those wires to a mini breadboard. The resistors+capacitor pair which sit between the lines on the schematic are still on the board, they’re needed with or without the magnetic transformer. All I had to add was one 0.1uF capacitor on each line. I didn’t even used the pull-up 1.8V as suggested by the app-note. I did the connections as mentioned above, the link activity LED’s signaled immediately, I assigned the computers some ip addresses and success I had the link up & working in no time. I only did a ping test which resulted in under 1ms replies. Perhaps I should of tested the bandwidth too, but I don’t think it suffered from the change.

Now that I know its working I can save space & weight by implementing the real thing.

In this article I continue my project where I needed a constant current source. In the previous article I talked about the first approach the LM317 constant current source which didn’t worked out that well but could of been improved as suggested by some of my readers in the comments. So I decided to use a dc-dc converter and turn it into a constant current source. Since I’m using the TPS54232 in another project and I have it around I decided to use it. This technique can also be applied to other dc-dc converters with a bit of reading on the subject.

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