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I would like to make an automatic fish feeder using a low power microcontroller (probably an Arduino Mini Pro, unless there are better suggestions). Something like this. It is basically a drill bit rotated by a stepper motor. I don't have access to an electrical outlet so would like to design this for battery power. It looks like the motor consumes a lot of energy and will likely drain 2 AA batteries in a few days, if not sooner. The stepper motor seems the culprit. Any suggestions on alternate motor or something else?

An AA battery gives me a juice of about 400-600 mAh per cell. So maybe I can double that with more batteries. But still, 800-1200mAh seems insufficient to run this setup for more than a few days. and yes, even if tried high capacity (2500mAh+) batteries, the problem is still with the large energy draw by the motor.

Per my calculations (based on datasheets/info I found online), the stepper motor draws about 170-200 mAh over 1 day, even though it runs for just 5 minutes total. That is huge.

Calculations: The microcontroller will wake up and run the stepper for about 5 minutes per day. As a buffer, I've taken that as 12 minutes or 0.2 Hours.

Total power draw per day (mAh) = MCU power + Stepper Motor

MCU power consumed = Active consumption + Idle consumption
= (4.75mA x 0.2 Hrs) + (0.3mA x 23.8 hrs )
= 8mAh/day
Thus I'm not too worried about MCU power consumption.

Stepper motor power consumed = Active consumption + Idle consumption
= (240 mA x 0.2Hrs) + (5 mA x 23.8)
= 167 mAh/day

Note: the idle current draw for the stepper motor - I can't find any data on actual value. So I've assumed a best case draw of about 5mA when the driver is put into sleep mode (A4988 stepper motor driver)

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  • \$\begingroup\$ Probably a geared DC motor (possibly a serously modified hobby servo) but your question is mostly one of mechanics and no details of the mechanical requirement are given in the question. It's also unclear if you will need an encoder and feedback control. \$\endgroup\$ – Chris Stratton Jun 18 at 13:01
  • \$\begingroup\$ @ChrisStratton thanks for taking the time out to reply. I'm no expert and since this involved moving just a drill bit, I presumed that would be self explanatory. I can try answering your question re: feedback control : I'm ok with the motor not having external control on how much to rotate, as long as I can experiment via the Arduino and lock in a desired level of rotation and number of turns etc that would occur once everyday. \$\endgroup\$ – SamuraiJack Jun 18 at 15:16
  • \$\begingroup\$ It's quite possible that the power drain of the original project isn't the actual motor operation but the idle draw of the motor driver and or Arduino. Low power design is hard. And Youtube is not a valid mechanism of engineering documentation. \$\endgroup\$ – Chris Stratton Jun 18 at 15:18
  • \$\begingroup\$ Alkaline AAs are good for around 3000 mAh. | Stepper needs take zero energy when not activated - the driver board can be fully powered off - this takes maybe a single transistor to achieve - so 50 mAh/day. | Guestimate: 5 minutes seems extremely log. 1 minute should be ample for even a large tank. But even at 5 mins at say 60 mAh/day total = 3000/60 = 50 days. Yes? \$\endgroup\$ – Russell McMahon Jun 19 at 5:04
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A geared brushed DC motor could be more efficient than a stepper motor. For example this

210:1 Micro Metal Gearmotor HPCB 12V

on 4.8 V draws less than 60 mA and does about 1 revolution per second.

enter image description here

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