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Thread: Power

  1. #1
    Junior Member
    Join Date
    Feb 2018


    What current consumption range can I expect from the 3.2?

    Ditto the colour LCD touchscreen?

    And LCD LED backlight?

    Can the LCD regulator be bypassed to run it at 3.3V?

    What will I see looking into the LCD LED pin? Direct white LED connection?


  2. #2
    Senior Member
    Join Date
    Dec 2016
    Montreal, Canada
    what lcd are you referring to?

  3. #3
    Junior Member
    Join Date
    Feb 2018
    I asked these questions in an email to PJRC, and in place of actual customer service, was told to post them here, where they (Paul?) would answer them. Still nothing. So...I'll start answering them myself. (oh, tonton81, what LCD? Just what I said. The "colour LCD touchscreen". I believe PJRC only sells one of those. Need a link?)

    Q: What will I see looking into the LCD LED pin? Direct white LED connection?

    The web page so helpfully says,"Use 100 ohm resistor". To what? Oh...Teensy VIN I guess. Hmm. Helpful.

    A: Yes, 3V LED.

    Looking at the back of the PCB, there appears to be just one LED. From the front, maybe 2 or 4 (in parallel). Doesn't matter much. Here are voltages & currents (actual specs...hallelujah), comma-delimited for your importing pleasure:

    V, I(mA)


    I wouldn't suggest a direct 3.3V connection, tempting though it may be.

    Without knowing more about the LCD module, it's hard to make any recommendations. I'll probably try running ~50mA, which would take 36Ω to 5V. Though, as per one of my other questions, I think the LCD can run at 3.3V with a regulator bypass, in which case a 2Ω(!) resistor to 3.3V will add some small safety margin. Hmm.

    I don't know what PJRC's problem is. The website sucks, navigationally, and informationally. It's out of date too. I've seen better specs on Chinese sites!

    Oh well, if they go out of business, there are other companies doing similar products...

    p.s., The "DemoSauce" code is running on my 3.2/LCD as I type. I'm truly impressed at what's possible over a serial link. Wow. Even though my first project doesn't require any real display "bandwidth", I'm really glad to see it's there!

    p.p.s., Couldn't get the UTFT demo to even compile... :-( (even after DLing UTFT!)

  4. #4
    Senior Member+ defragster's Avatar
    Join Date
    Feb 2015
    please cheer up mgd - if you make friends on the forum the Teensy world is your oyster. The cost, support and capabilities isn't simulated anywhere I've seen. Emails get lost and Paul is quite responsive on the forum when good info is offered - when others don't pick up first. tonton81 asked a good question that wasn't answered - so I ignored it waiting for it to show up.

    When I wanted to know the current I used my multimeter. A T_3.6 can hit 70 mA which is what I measured last - a T_3.2 is maybe half ( under 40 mA ) that so an LCD can run direct from the power provided from the Teensy. A bing or google search should find that info already noted. The Forum search is very lacking.

    There is a link on the PJRC web page ( indeed generally quite dated with much T_2 stuff still having focus confusingly ) to the PJRC offered TFT ILI9341 that shows connect details - still not known if that or whatever display is at issue.

  5. #5
    Junior Member
    Join Date
    Feb 2018
    Here's one of the demos running.

  6. #6
    Junior Member
    Join Date
    Feb 2018
    That V-I curve is actually pretty weird for an LED. It looks largely resistive (with an offset), not exponential. WTF?

    Click image for larger version. 

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  7. #7
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    The reality of these displays is we buy them from the same Chinese sellers as everyone else. There simply aren't any reliable specs. They also tend to change slightly over time.

    We do test every one here, including touch test on 2 corners of the touchscreen. But other than getting a fully tested display (we do end up discarding several from each batch), it's exactly the same display as if you got it from the many sellers on Ebay or Aliexpress.

    There simply aren't any reliable specs about the power consumption.

    However, PJRC does try to deliver extra value on the software side, specifically the optimized ILI9341_t3 library and quite a lot of testing & development to allow good performance display use while also doing other tasks like playing audio from a SD card sharing the same SPI pins. That's the sort of thing you can't get from those Chinese sellers (or pretty much anyone else in the Maker market...)

  8. #8
    Senior Member
    Join Date
    Mar 2013
    I have a few of the PJRC Touch Screes it has 4 led's so max current is 80mA. You can safely run the back-light from 3.3V, technically if you want the same brightness using 5v as running from 3.3V you can use a 22R from 5V. There is also a built in 3R9 resistor on my LCD's going to the back-light on my screens. The LCD power consumption last I measure it was around 11mA @ 5V. I have run it with 3.3V but don't appear to have recorded the values.

    You can bypass the LCD regulator and run at 3.3V.

    I for one have never had issues with getting help on these forums. As long as you provide details you will get a fair answer.

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