Teensy3 and LEDshield

While I am waiting for Tindie.com to re-activate the fundraisers I am continuing to play with the hardware and software at hand.

The photo below shows a Teensy3 connected toe the LED shield and illustrates how little is necessary to connect a non Arduino microcontroller to the LED shield. Only three wires – SCL, SDA GND – are required, a 4th one to supply +5V from the LED shield’s on-board switched power supply to the Teensy 3. The Teensy3 is an Arm Cortex M4 powered board and has much more capable I2C hardware allowing it to run I2C frequencies up to 2.4MHz. Most Arduinos only support 100KHz and 400KHz. The hardware on the LED shield OTOH all supports FM+ mode so up to 1MHz. The Arm Cortex M4 is a 32bit microcontroller and can be operated at 96MHz. All for $19. Pretty sweet deal!

I am currently in process of getting the EthernetBonjour and ArdOSC libraries to compile and run on the Teensy3 to allow zeroConf networking and OSC control through TouchOSC from any iPhone/iPad and many supported Android devices. This already works on a Teensy++2 which is an Atmel processor based board and I hope not to run in too many obstacles.



About trippylighting

Mechatronics Engineer

Posted on June 15, 2013, in Uncategorized. Bookmark the permalink. 6 Comments.

  1. Question: Did you removed the TMP421 i2c temperature sensor ? or just moved it on the back side ? because I dont see it on the new design.

    • I moved it to a different place on the board. In the photo together with the Teensy3 on the breadboard it is almost completely hidden behind the terminal strip to connect the LED.

      • ok, I see it now. In the original design it was close to the switching regulator that was supposed the hottest point on the board. When you use the board have you tried to measure the temperature on each component ? You can use a small IR termometer used for small kids – just make sure it can measure up to 100 celsius as most of them measure only aprox body temperature.
        For PCB and assembly you can try to ask for a quote from olimex.com too. They’re in Bulgaria so they might be cheaper than Germany and they make some pretty good stuff.

      • I moved the temperature sensor because I wanted to free up some precious cooling space for the hottest part, the LED chip. I have written a pretty extensive article on the blog in respect to thermal management, so you may want to give that a read 😉

        IMHO the temperature sensor serves relatively little purpose. If you want to measure board temperature you may want to do that during prototype development of the board, but afterwards it makes little sense to continue monitoring the board temperature. If you’ve got problems, then the board design needs to be revised. Obviously there’s not much you can do about that in a production board. You may have to reduce the output, but then you may not reach the design goals of your project.
        The other temperature one would likely measure is the LED junction temperature. The better way to do that and the only way I have ever encountered on any of the LED applications papers and spec sheets is per K-Type thermo couple and the chip sensor on the board does not accept that input. Again Intention was nice but in praxis it serves little purpose.
        A IR thermometer to measure body temperature is also utterly unfit to measure the junction temperature of semiconductor chips. The max junction temp is usually defined to be around 125-150 degrees Celsius ( not Fahrenheit). A thermometer for measuring body temperature will not measure accurately in that range.

        In terms of manufacturing I appreciate the suggestion. However, I am using my own engineering judgment based on 20+ years in manufacturing. My blog partner lives in Germany and having a person closer to the manufacturer is always better than having to deal with problems remotely. Not only does that match my own experience but I’ve been given that advice by the maker of the Teensy processor boards, who have a very successful Kickstarter project with the Teens3 boards. He has another 20+ years in electronics design and I trust his judgement.

        Price, this may come as a shock to some, is secondary to me. Not unimportant of course, but it’s not my primary motivation. For me quality comes first and having something manufactured in my Country of origin, or here in the US where I live has also a higher value to me than price only.
        I had the prototype boards ( just the PCBs) made at Itead in China. cheap! Upon closer inspection I discovered, however, that the drill precision leaves much to be desired. Here precision was clearly secondary to speed.

  2. I read everything here on the forum before posting. I did read the article about thermal management.

    I think the sensor is usefull during the life of the board. The way I am thinking of using them is hidden away somewhere behind a wall in an enclosure. For sure it will not sit on a well ventilated desk. So the conditions will be different. And also will depend on the ambient temperature and the time it is in use (powering the leds) and how much power flows to the led. PWM will probably play a role here to.
    So depending on all this conditions the temperature might be higher or lower. If it is too close to the leds it will probably get worse in terms of temperature.

    I agree that a good design will adress some of this during prototyping, but the sensor is usefull for monitoring during usage.

    So… you’re having the first prototype working. What is the longest time you used it and how much current was it pumping to the leds ? Does any component heat up ? for how much ? (relative to ambient temperature) It would be interesting to have something like a chart.

    I know that the best way to check this would be with a thermal camera. Do you have one ? I don’t …. it’s about 4-5kE from what I hear. So i’m using the next best thing having arround … an IR thermometer. It can read up to 100celsius. It can read my kids temp, the temp inside the fridge and the temp on my core i7 radiator. So far nothing heat up that much … and continued to work long enough for me to measure it.

    Right now I’m working on a SMPS that delivers 2-3A. it has the same basic design as the LED driver: an IC + a coil + a diode + some MOSFETs + a few small smd Rs and Cs. To my amaze the hotest component is the COIL! the case of the coil goes up to 55-65celsius for 1A and 65-75C for 2A in about 2-3minutes but then it rises no more. I cannot measure the temperature of the actual coil inside, it’s for sure higher. but the PCB is far cooler. the components close by measure only 37-42celsius – again is the temperature of the top of the case and not the junction temperature. But still I get an idea about what’s going on. It also have a temperature sensor that is a little further way from the coil (it has a PIC18 microcontroler on the PCB). Comparing what I am reading with the IR sensor now while sitting on a well ventilated desk and what the PIC is reporting, I can use this to guess what’s happening when I will put this in an closed enclosure somewhere. At that point I will not be able to measure anything … I will only be able to read the temperarute reported by the sensor inside the PIC microcontroler. If that will be 2-3degrees higher then is ok. If the temp goes 20 or more higher then is not that good.

    Looking forward for some LED drivers!

  3. Hi doru,

    Let me first thank you for the detailed feedback, I really appreciate it!

    I am glad the sensor is useful to you. Of course where it is located right now it won’t help you much in measuring the board temperature without the additional external components needed (A transistor and two resistors, IIRC). In that case I’d suggest you use some thermal epoxy an glue the measuring transistor to the hottest component, which is the LED driver chip.
    I have not shown this in any photo, but on the underside of the board directly located under the thermal pad of the chip I left an area of copper free so that you can mount a little finned aluminum heatsink. If you do mount the heat sink I believe the best position for the temp sensor is on top of the chip, otherwise the best position is on the underside.

    I have not tested the board under full load as I currently don’t have the means to do it. I had another very interested potential user that was going to connect up to 40W of LEDs to a single shield. I sent him one of the five prototypes I made but I have not received feedback. Your post reminded me and I sent an email to him.
    In my own projects I use only one RGB LED per shield. @700mA that may account for 5-6W of power. At that rate the LED driver or the inductors only slightly elevate above my body temperature as my fingers tell me 😉 I know, that sounds really scientific. After all, I may invest in a little IR thermometer 😉 I agree that checking that with a thermal camera would be very desirable as the is really the best way to verify thermal performance but I don’t have access to such a thermal camera.

    I’ve run the new board for a week with one of my LEDs attached. I’ve been running the “old” board design for 2 years daily for 8-24 hours in one of my LED lighting systems @ http://trippylighting.com

    It is not surprising that the inductors get warm, as the full current flows through them. The only way to battle that is to over-dimension them. Most LED driver spec sheets I have seen recommend 1.2-1.5x the max current that is going to flow through them as the magnetic performance decreases quickly when approaching the specified saturation current and as heat increases. Using the lowest DCR value will battle heat. Unfortunately the lower the DCR, the bigger the physical size of the inductor.

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