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  1. #1
    I have a small problem that I need to sort out. It concerns much of what I have read here on this forum, but doesn't answer the problem that I have.
    I have built a laser engraver and have it working fine using a cloned Arduino UNO and a cloned CNC shield v3. Firmware is GRBL 1.1h
    I have also purchased a Lekn G-sender standalone controller that connects to the CNC shield TX and RX pins plus the 5v and Grd pins.
    Once the controller starts up it can be used to control many things that I would normally do with the computer, and also a SD card holds all the prepared files that are used for the laser engraving.
    The problem is, once the computer USB is disconnected, the system dies as there is no power.
    Also when the G-sender is attached, the computer will not work with the system.
    To overcome the problem with the computer not working, I cut the wires to the TX and RX and added a double pole switch so that I could switch between the G-sender or the computer.
    Still have the problem of the power. If the USB cable is disconnected from the computer, the G-sender has no power.
    Does anyone know if it is possible to add a small 5 volt power supply to the Arduino UNO so that my machine can work completely without the computer? And when I want to use the computer it would be a matter of just plugging in the USB cable.
    I was thinking about using a small step down from 12 volt to 5 volt, so only one power supply would be needed for the machine.
    I am not sure about the dangers of wiring in an external 5 volts, I don't want to blow things up.

  2. #2
    Part of your problem may be a lack of current capacity of the 5V supply from your computer. The square black connector at the bottom of the picture of an Arduino UNO below is an external supply input. The Arduino specification is for 7-12V at this input. Have a look at the Arduino website for detailed information on power requirements.

    Click image for larger version. 

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    https://store.arduino.cc/usa/arduino-uno-rev3

    Kit
    Last edited by Kitwn; 14-03-2021 at 12:42 AM.
    An optimist says the glass is half full, a pessimist says the glass is half empty, an engineer says you're using the wrong sized glass.

  3. #3
    You can plug a USB cable into a charger socket to power the Uno. It just needs 5v regulated at up to 500mA. If you use the DC input socket then 7v min is required to push past the voltage regulator on board 12v max input to keep the reg from frying.

  4. #4
    Quote Originally Posted by Kitwn View Post
    Part of your problem may be a lack of current capacity of the 5V supply from your computer. The square black connector at the bottom of the picture of an Arduino UNO below is an external supply input. The Arduino specification is for 7-12V at this input. Have a look at the Arduino website for detailed information on power requirements.

    Click image for larger version. 

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    https://store.arduino.cc/usa/arduino-uno-rev3

    Kit
    Thanks for your input.
    Maybe I have something wrong with my boards then? I have connected 12 volts to that socket and still there seems to be no power getting to the UNO from there.
    The only way I have been able to get power is by using the USB connection. Power to the steppers is by the connection on the CNC shield power connections which doesn't give power to the UNO.
    The way I understood it was that if 12 volt was applied to the square black connector, it would be reduced down to 5 volts for the UNO supply, but in my case(s) it is not working

  5. #5
    I have a laser cutter kit, which came with a controller that worked (!) using an android app, SCARVE. That piece of junk went in the bin

    My idea is to do similar with a Cronos CNC/Laser GRBL controller (Arduino based) and offline board. I have a couple of spare Toshiba Flashair WIFI SD cards (I use one in my 3D printer), so one will be assigned to the offline controller. It is an easy way to get Gcode to the machine to run it offline.

    I found that when the offline controller is connected to the board, LaserGRBL would not do certain functions, such as upload GRBL parameters to the board or jog the machine directly. To that end I plan a similar multi pole switch to disconnect TX, RX and 5V.

  6. #6
    Quote Originally Posted by cropwell View Post
    I have a laser cutter kit, which came with a controller that worked (!) using an android app, SCARVE. That piece of junk went in the bin

    My idea is to do similar with a Cronos CNC/Laser GRBL controller (Arduino based) and offline board. I have a couple of spare Toshiba Flashair WIFI SD cards (I use one in my 3D printer), so one will be assigned to the offline controller. It is an easy way to get Gcode to the machine to run it offline.

    I found that when the offline controller is connected to the board, LaserGRBL would not do certain functions, such as upload GRBL parameters to the board or jog the machine directly. To that end I plan a similar multi pole switch to disconnect TX, RX and 5V.
    I didn't find it necessary to isolate the 5v or ground lines, just the TX and RX were needed.

  7. #7
    Quote Originally Posted by regpye View Post
    I didn't find it necessary to isolate the 5v or ground lines, just the TX and RX were needed.
    I agree with you, and on reflection it is probably better to keep the SD card powered up for the WIFI connection.

  8. Maybe this answer is a bit late and you have already solved your problems, but I have written an article showing how to power the Arduino Uno / CNC shield combination from a single 24V power supply that should give you the information you need. The link to my article is here.
    The Lekn G-Sender should have the 5V and Ground wires connected to the CNC shield, so that it is powered by the 5V regulator on the Arduino.
    You will not be able to have the TX / RX data lines connected at the same time as connecting via USB as you have found out, as the two data streams will conflict, but you already seem to have overcome this issue with your double pole switch. It should also be possible to use a tri-state multiplexer to automatically switch the TX / RX data lines, using the 5V line from the USB to control the enable pin on the multiplexer. The 74FCT257T (datasheet here) should do it. Hope this helps!

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