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?