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Thread: Cannot get serial data to work when connecting external power supply ground

  1. #1
    Junior Member
    Join Date
    Oct 2018

    Cannot get serial data to work when connecting external power supply ground

    Hello everyone,

    I'm having an issue with the Teensy LC.
    I've connected an I2C accelerometer (MPU6050) and using teensy as a midi controller.
    For debugging, I just compile in standard mode (usb serial) and it works great.
    However I am trying to connect a RGB "analog" led strip (with R,G,B and 12V wires) with TIP120 transistors as described here :

    It seems to work but as soon as I connect the external power supply ground to Teensy ground, serial communication breaks.
    As soon as I disconnect grounds connection, it works again.

    Is there a way I can make usb serial work using the external power supply ?

    Thanks a lot !

  2. #2
    Senior Member+ defragster's Avatar
    Join Date
    Feb 2015
    Not indicated how many RGB LED's are in use? A photo showing the connections might help.

    Quick guess based on something observed here : that GND wire may be feeding large return current through Teensy to host USB when connected.

    Quick test running fewer RGB's or at lower brightness - or off - might suggest if this is happening. Or use DVM in Current mode to connect those two GND points and see the current passing there - this is how I figured out what was causing the following.

    Saw that once here with 1.5A flowing out GND into PC running an RGB matrix when the external USB power's GND to the display was not as handy as the GND path through the Teensy. When that was happening the USB Upload was intermittent. Added 2nd GND wire from display matrix to External connected power and all was well.

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