Correct power supply for Photon and servo (Pet Feeder)

I look to build a reliable pet feeder with the photon. As many before me I would prefer to use a cereal dispenser to serve portions of the pet food, and a servo to turn the wheel. The dispenser holds up to 2L / 0.5 gal and the wheel does not turn easily.

Similar projects have used servo’s with stall torques ranging from 6kg/cm to 12kg/cm (83oz/in to 166oz/in). As the wheel does not turn easily I would prefer to use a decent servo, preferably the HSR-2645CRH as used in the project mentioned above. Operating voltage is between 4.8 - 7.4V and the current draw at stall is 2A (current draw at no load 140mAh).

I figured that I would need an external power supply in order to power the servo. 2x 18650 li-ion batteries will give a supply of 7.4V 2800mAh which should fit the servos specifications. The system will be stand-by 24/7 and feed a portion two times daily (3-10 seconds sequence). Would the mentioned battery pack be sufficient for some weeks use?

My other alternative in terms of power supply would be an adjustable luxorparts 36W power adapter 5-24V (7V @ 3A) directly connected to the servo. In order to connect this to the photon I would use a IRF520 Mosfet module (to step down to 5V). I have read that the IRF520 needs a heatsink when exceeding 1A, but as the Photon and servo only will be operative 3-10 seconds twice a day I figured that a heatsink would not be necessary.

Which one of the alternatives above would you prefer, the 18650 battery pack or the adjustable power adapter?

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Hi Roheik!

What a fun project! Make sure you build an enclosure to make the feeder tamperproof- I’d hate for your cat’s paw to get stuck in the cereal dispenser.

How attached are you to using that particular servo? There are cheaper options that should work just as well.

I would suggest using a buck converter instead of that mosfet module if you go with option two.

Personally,I would use the battery pack. It should last for a few weeks of low-activity use.

Good luck!

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Thank you for the response Colleen! I will build a case for the feeder as a whole.

If I end up using the mentioned servo I will apply 7.5V 3A power with a wall wart and step down the voltage to 5V 0.5A for the Photon with a TO220 LDO (capacitors at each side).

I figured that I needed a strong servo running at higher voltage in order for the wheel to turn, but I’m not set to use the particular servo. I have looked at the Osepp LS-955CR servo but I’m a bit concerned of using it as it’s not very well documented (e.g stall current draw missing). The Osepp servo can operate at 4.8V (stall torque @ 9,4kg/cm) which is great as I would not need to step down the voltage for the photon. Would you apply the Osepp servo to this setup?

I have found out that the servo should not be powered by the Photon VIN, but directly from the power supply instead, with a 100 to 1000uF capacitor in between to deal with spikes. I will also apply a ~2.2uF capacitor when powering the photon from the VIN-pin just in case of a drop in voltage when starting up or using the servo.

I look to buy a 5V or 7.5V wall wart @ 3A depending on the servo that I will use. I figured that this would be a more permanent and stable solution for the setup.

My only concern at this point that I would be happy if you could reply to:
Does the PWM signal to high-voltage servos require 5V? I have understood that most of the Photon’s GPIO pins deliver 3v3 and not 5V. Could this be a concern for controlling any of the servos?

Thank you in advance!

Hi @Roheik -

Sounds like a nice project, something I have been wanting to do for a while I just do not have the time. Just a random question from my side, have you maybe considered using stepper motors with standard DRV or TCM drivers? similar to what you will find on just about any 3D printer out there.

My reason for asking is simply that this is the route I chose for Augers we mounted under coffee silos to dispense roasted coffee beans. I do not have any recent pictures, but below was in the early stages.

Each silo held 300kg of coffee beans, you can see more HERE. It was one of my first projects ever, so don’t laugh too much :rofl:

Anyway… good luck and enjoy building your project :wink:

Regards, Friedl.