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I currently am working on a battery powered Atmega328 design, using 4 AA batteries, at around 6.5 volts. I am using it to control a single digital servo, HiTec HS-5645MG. This is done using a RF reciever, here, I know this is not an ideal receiver, but it is my only option currently. I have to have all of these on the single power supply, due to size restraints.

I put them on a solderable "perfboard," and was able to get it to work well, with just a little interference here and there. I used two ceramic decoupling capacitors, 0.1uF and 1uF.

I am working on designing a PCB for this setup, and am quite new to that area altogether, and especially with such a easy to get interference setup like this. I would like to remove the interference I am currently experiencing.

So, my main questions are:

1) Should I take the input voltage directly from the battery's (6.5v) and go to the servo supply? Then the 6.5 will go to the voltage regulator to 5v. Or should I power both with the 5v?

2) Star grounding, or ground plane?

3) For the decoupling capacitors, since the voltage regulators will have two capacitors already, where and what size should the additional decoupling caps go? For the prototype I just used them as close to the input of the chip as possible.

Thank you so much for any/all feedback. Any other suggestions would be much appreciated. There aren't many resources out there for having servo and MCU on the same supply, but I don't have a choice for this project.

EDIT: There will be very little load on the servo, so it should be at the minimum current draw possible for the servo.

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  • \$\begingroup\$ The datasheet for the servo says it has an operating voltage of max 6V. Stall current is 2.4A. Assuming the power supply connection servo<->AA-cells is good you could calculate the voltage drop at the cells at peak load. Ipeak x Internal resistance of the cells. Checkout : data.energizer.com/pdfs/batteryir.pdf (page2). \$\endgroup\$
    – Dejvid_no1
    Commented Jun 10, 2017 at 18:58
  • \$\begingroup\$ @dejvid_no1 OK great. Added some information about the load to my original post. Thank you \$\endgroup\$
    – Kyle Hunter
    Commented Jun 10, 2017 at 19:00
  • \$\begingroup\$ Realistically you should run your MCU and radio at a lower voltage such as 3.3v so that you have regulator headroom even when well-used batteries are loaded down by a turning (or worse, stalled) servo. Most modern RC systems use 2.4 GHz digital radio chips which are far more sophisticated and noise immune than the part you are considering - and likely cheaper, too! \$\endgroup\$
    – Chris Stratton
    Commented Jun 10, 2017 at 20:22
  • \$\begingroup\$ @ChrisStratton That's true. The issue is that I need 12 channels for the transmitter, and I'm not going to manufacture a custom remote. I would love to use a better setup but haven't been able to find one \$\endgroup\$
    – Kyle Hunter
    Commented Jun 10, 2017 at 21:23

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1) You can probably get away with powering the servos directly from the battery pack. 4x AA cells nominal voltage is 4 * 1.5 = 6V. (You probably measured 6.5 V on the batteries without any load. This is not the right way to measure battery voltage.) Edit: Use a buck-boost converter for the 5 V logic supply.

2) Go with a ground plane and put the servo ground connection as close to the battery ground as possible. Same goes for the servo positive supply. The regulator will deal with the voltage drop.

3) Use the recommended values from the datasheet for the regulator and ICs in your design. Keep loops tight, so put capacitors as close to the pins as you can.

I hope this helps!

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  • \$\begingroup\$ Great thank you. That's about what I was thinking. \$\endgroup\$
    – Kyle Hunter
    Commented Jun 10, 2017 at 18:59
  • \$\begingroup\$ You won't be able to get 5v from a regulator fed by a loaded 4xAA pack except during the early part of the life of Alkaline cells. To actually get 5v reliably would require an extra cell or a buck/boost converter, so it's best not to design for a 5v requirement. \$\endgroup\$
    – Chris Stratton
    Commented Jun 10, 2017 at 20:20
  • \$\begingroup\$ You could use a different radio module which has a 2-3.6V supply range and power it directly from two of those four AA batteries. \$\endgroup\$
    – kva
    Commented Jun 11, 2017 at 6:34

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