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Standalone eggbot

edited January 2016 in Egg-Bot
I'm in the process of adding a raspberry pi to my eggbot, so I can do long jobs like engraving without tying up my laptop the entire time.  So far the setup has been pretty straightforward, but I have a couple of enhancements in mind for the future.

The first is I'd like to reduce the cables by possibly running the pi off of the power from the ebb.  I see LOTS of places to take 5volts off of the ebb, but it's not at all clear to me that it can provide enough current on the 5 volt "rail".

1 ) Does anyone have any experience with powering anything else off of the 5 volt connections and have any advice?

2) The ebb schematic has the note that "IC 5 is only good for 1 or 2 servos worth of power" but there is room for four servos.  I've looked at the datasheet for the voltage regulator and it has left me scratching my head, unclear about how much current it can handle, but I doubt it will be enough for the 700-800 mA I expect the pi to use while still driving both servos. Or am I being too cautious?  I see on the back of the board that it references an alternate power source for the servos, with a place to solder another jack,  but that doesn't help with my goal to reduce the power cables to one.  If I replace that voltage regulator with one that can handle more current, are there other gotchas that would preclude running the pi off of the the servo rail?

3) I see that the alternate power supply connection disappears between version 2.1 and version 2.3, is that because no one used it, or was there something wrong with using it?  Since the connections for the other two servos are still on the board, is there any way to power them with version 2.3? (this is just curiosity, knowing the answer probably won't help my project.)

4) Maybe what I should do is just add my own voltage regulator to change V+ (the power from the barrel jack) to 5 volts for the pi, and leave the servo regulator alone.  But I'm not sure where to get it from short of soldering to the jack itself.  Is there a handy way to tap into V+ on the board that I've missed?

The other thing I wondered about was automating frequent jobs.  When the inkscape plugin talks to the eggbot, is it a two way communication, or could the commands be saved to a file and then played back later?.  i.e. is the conversation like this?
    PC: Here are a bunch of commands.  Do this, then this, then this, then this, etc.....
    Eggbot: OK

Or is it like this?
    PC: Do this
    Eggbot: OK, done
    PC: now do this
    Eggbot: OK, done


  • 1, 2) The 5 V power supply (voltage regulator) on the EBB is there to provide a consistent 5V supply for the servo motor only.  It is a 78M05 regulator, with 0.5 A output current capacity. The servo motor takes almost no current while idling under no load, typically about 125 mA (momentarily) while switching positions under light load, and can pull 250 mA or more, if under heavy load (for example, if the arm is blocked from turning).

    If you are using this as a regular EggBot, then that leaves a maximum of about 350 mA at 5 V available for your use elsewhere. From what I've read, power consumption of a RPi Zero (Or really, an RPi except for the B/B+/2B) is likely to be under that number, so long as you aren't driving any other peripherals from it that draw power. (See , for example). That suggests that this would actually work just fine. However, to truly understand the power consumption for *your* RPi, use a multimeter (or other current measuring device), to measure it under realistic conditions.

    However, you mentioned the " 700-800 mA I expect the pi to use while still driving both servos."  I do not understand (1) where the 700-800 mA figure came from (2) why the RPi would be driving servo motors (rather than letting the EBB do it) or (3) why you have two servos, rather than the one standard that goes on an EggBot. 

    If the total current that you will draw gets this high, you may run into a separate issue as well.  The standard 9 V 1.5 A supply that we ship with the EggBot is enough to drive the EBB, its two steppers, and the servo motor. Typically, these together take less than 900 mA. However, if you are adding enough extra current draw, you might exceed the total current available from the power supply. You could compensate with a larger power supply, and/or consider using a switching power supply (instead of a regulator) to create the extra 5 V that you need.

    3) As you have noticed, the current version of the EBB (v 2.3) does not actually have an alternate power jack location on the back. So far as we know, no one ever used that power jack-- you are the first person that I can recall ever even asking about it -- and we eventually discontinued that option.

    The power (center) pin for all four servo connections is tied together. If your servo motors draw more current than is available on the 5 V rail, then you will need to provide power to the servo motors separately. A common way to do this is to use a servo extension cable, where you cut the center wire, and use that to connect the alternate 5 V power source.

    4) If you have the current (2.3) version of the EBB, I would recommend soldering your wires to the pins of the power jack, where they protrude through the bottom side of the board. 

    "5") The EBB does not store a full print job; it's a line-by-line type of communication. There are some exceptions-- it is possible (but not straightforward) to store a full print job in the *firmware* so that no external computer is needed. You might also consider using RoboPaint to drive the Eggbot.  RoboPaint should be able to run on the RPi, has (beta) support for the EggBot, and a full API that you can use to automate the machine, at a low or high level.
  • Thanks for the quick, detailed, and helpful response!

    I was being pessimistic about the power requirement based on my last pi project which I ended up having to buy a powered USB hub and moving peripherals to it.  But that was with a camera and two wifi dongles plus the occasional USB keyboard.  So, as you suggest, I should just measure the actual amperage of the device with the single wifi dongle I'll be using. 

    I have two servos because I'm using the engraver attachment. I know it's not really a servo, but I was making the (possibly poor) assumption that it had similar characteristics because it plugs into a servo port and is driven from the same rail.   I'm not planning to drive the servos from anything other than the EBB, I was just figuring the highest total load on that voltage regulator would be whatever the PI pulls plus the peak current from the engraver motor and pen lifting servo.

    I guess I should measure the eggbot too and then just add together the two numbers I get.
  • I tried it tonight and it worked!

    At first I connected them via my multi-meter, and while the light on the pi flashed, it never came up on my wireless network.  I observed it pulling between 110 and 170 mA during this time, but the lights on the wireless dongle never came on.

    I had decided that there just wasn't enough current, but took the multi-meter out of the circuit and tried again before giving up.  That time (and every other time I have tested) it worked, the pi booted up and joined the network.  I'm not sure why my multimeter would keep it from working, maybe I'm not using it right.

    Anyway, I'm happy to report that a raspberry eggbot can run with a single power cord.
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