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Connecting a standard servo cable to A-Star 32U4 Robot Controller


#1

A standard servo female connector has ground (-), power (+), and signal, in that order. I’ve used Arduino boards that conveniently had a row of header pins for the I/O (signal) and then a parallel row of header pins for +5V and a parallel row of header pins for ground. Thus one could easily plug in a standard servo and begin programming.

The A-Star appears to be capable of the same thing – almost. By soldering on the appropriate header pins one can obtain a row for I/O signal, a parallel row for “something” and a parallel row for ground.

The A-Star schematic and continuity tests suggest the “something” row is not connected to anything. I suspect this is intentional so that servos can be powered from a different supply if desired.

That said, if I have 5V capable servos, I believe I can jumper some 5V location on the A-Star to a “something” row to achieve what I’d like … simply plugging in a servo connector to the board. Is this description/assertion correct?

Also, I am powering the A-Star from the USB connector via a power bank. The schematic and the pinout diagram suggest I want to jumper the via labeled “5V” next to the Pololu symbol to the “something” row to achieve what I’d like. Is this correct?


#2

Hello,

You are correct that the optional power and ground buses on the A-Star 32u4 Prime are intended to be able to be used with servos. As shown in the “A-Star 32U4 Prime pinout and components” section of the A-Star 32u4 User’s Guide, the power buses are not connected to anything by default. The 5V regulator on the A-Star would probably not be able to source enough current for most servos. If your power bank outputs 5V, you could probably jumper from VIN to accomplish the same thing, but without the current restraints of the on board regulator.

-Derrill


#3

Thanks.

I would like to add additional information and ask additional questions.

I will be powering the RPi from a power bank as well, so in effect, the entire 1A coming from the power bank to the A-Star should be available to the A-Star, if I interpret the User’s Guide Power section 3.5 correctly. Further, since I will not have any other power supply connected to the A-Star, it seems that the power multiplexer will output from the USB connected power bank. The schematic seems to say that 5V is in effect the USB power supply. Is that correct?

I believe roughly 40mA is needed for on-board A-Star use, leaving most of 1A for powering things like servos. The servo I use stalls at 300mA, and use 30mA no load. I should be somewhere in between, so I think that should work. Do you see any problems?


#4

As long as the combined current draw of your servos and the A-Star does not exceed 1A you should be fine. Please note that using the on-board regulator will be less efficient (between 10-20% less) than powering your servos directly form your power bank due to losses with the regulator.

-Derrill


#5

I apologize, but I did not clearly state a question, so it did not get answered.

The User’s Guide Power section (3.5) states “The A-Star 32U4 Robot Controller’s power selection circuit uses the TPS2113A power multiplexer from Texas Instruments to choose whether its 5 V supply (designated 5V) is sourced from USB or an external supply via the regulator, allowing both sources to be connected at the same time and enabling the A* to safely and seamlessly transition between them. The TPS2113A is configured to select external power unless the regulator output falls below about 4.5 V. If this happens, it will select the higher of the two sources, which will typically be the USB 5 V bus voltage if the A* is connected to USB.”

Since I have no battery or other input voltage to supply the regulator, the regulator must put out nothing or 0V. I attach the power bank (which outputs 5 volts) via the USB connector.

If I understand the statement above, the power multiplexer thus selects Vusb (the power bank) as the source of the so-called “5 V supply” for the A-Star. Thus I believe I don’t have to worry about the voltage regulator since it is not being used, and further, the power source for the A-Star and servos would be the power bank (via USB). Are these assumptions correct?

Thanks for your patience.


#6

I am sorry, I missed that you were connecting your power bank via USB. If your are powering the A-Star robot controller via USB, power will not be going through the regulator, so you will not have efficiency losses from it. Please note that the TPS2113A power multiplexer will still limit the current to about 1.9A nominally (though it sounds like that is well above what you need).

-Derrill