I’m having trouble deciding how to power an Arduino Pro Mini via batteries.
Before getting into the issue, I’m going to describe the situation.
I have access to a locker -which is not on my property- and I would like to automate it, I have permission to do it, using a RFID reader, specifically the famous RC522 which, once triggered, would make an Arduino move a servo attached to it in order to open the lock mechanism.
The opening/closing process would take place 2 times a day. One in the morning and one in the afternoon. And normally it would take approximately 1 min each, so 2 min in total/day. After the process ended, the Arduino would enter a Deep Sleep mode, waiting to be woken up by a push button which acts as an interrupt. So, in a nutshell, 23hr 59min in Deep Sleep mode and 1 min of work per day, 24/7 365.
Now it the problem comes. I’ll explain myself.
In order for this contraption to function properly, the Arduino needs to handle both 5v and 3.3v voltages since the RC522 works at a voltage of 3.3v with a maximum input of 3.6v whilst the servo works at 5v. (I know that there are 3.3v servos but after buying one and testing it, I’ve come to the conclusion that this kind of low-power servos don’t have enough torque to even move a normal door latch). Furthermore, I need the design to be compact, so I thought about using an Arduino Pro Mini for the job since it offers both 5v and 3.3v versions.
I DO NOT have access to a power outlet, which means that the device has to be battery operated, so that adds to the list of problems to solve.
So, now the problem is to use both 5v and 3.3v voltage values in the same board while maintaining a low power draw from the battery.
If we take a look at this image regarding powering the Arduino on batteries for a long period of time we can see that removing the voltage regulator saves a lot of energy, so I was planning on doing that, but if the regulator is removed, the pins will output the same voltage that it’s being given on the input, if I’m not mistaken, which means that, if I use an 18650 battery as the power supply, the RC522 will burn when the battery is fully charged at 4.2v.
So I thought about using three AA batteries in series, with a voltage of 4.5v to power up a 5v Arduino and so the servo (That has a working voltage of 4.8v -good enough-) without the voltage regulator and also using a step-down module to get the voltage to 3.3v for the RFID chip.
Obviously, in order to save power while the system is asleep, I’d use a couple of 2n2222 transistors to cut the power to both the RFID chip and the servo while they’re not being used.
PROBLEMS TO SOLVE
- Deciding the type of battery that should be used. (I have access to a lot of 18650s batteries, but I’ve heard that they’re not the best in terms of continuous work for long periods of time since they tend to self-discharge)
- Determining whether it’s better to get a 5v Pro Mini or a 3.3v one.
- Knowing how much battery life I can expect, preferably more than a year.
I would appreciate your help and ideas. All ideas are welcomed. Feel free to recommend different types of batteries, etc…
Thanks a lot in advance,