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Using an 18650 battery to power a 3.3v MCU, with servo

ntewinkel

Hi All,

 

I would like to use an 18650 Lithium battery to power my project. These standard batteries have a nominal rating of 3.7v (I think) but will provide 4.2v when fully charged, and I'm not sure how low it gets, though probably around 3.5 is a good stopping point.

 

So this seems to be alright for ATtiny chips, which appear to be happy from 1.8v to 5.5v. I'm guessing I could just use the battery directly for those.

 

But I was thinking of using a 3.3v MSP430 chip (from a "value line" G2 Launchpad) to conserve power (it seems they can sleep really well). It doesn't like anything above 3.6v. Easy-peasy, I can add a 3.3v voltage regulator.

 

But the project I'm thinking of has a servo, which from what I can tell needs over 4 volts to work (or am I wrong on that)?

 

Anyway, here is the project: Charlie and the Treat Dispenser

 

Right now I'm using one of those USB cell phone chargers that contains an 18650 battery and it produces 5 volt to run the Arduino Pro Mini and the servo.

However, it only lasts a day - and the servo stops responding if we stay up really late image

 

So I was hoping to make the whole thing more power efficient. Partly just to see how and if it can be done, as really I suppose I could plug that sucker in image

 

My thoughts about the existing project power usage are that:

1) I'm losing a lot of power just having that charger up-converting to 5v

2) The Arduino pro mini is on all the time, using up power that way.

3) The servo rarely gets used - the unit is usually on standby.

 

If I could set this up to run on the MSP430 launchpad, I can make it sleep most of the time, and wake when the button is pressed. The only time it needs to be awake is to handle that button press to dispense the treats, so that setup should work charmingly well (in theory at least).

 

So the question is - what is a decently efficient way of doing this? (I don't need "most" efficient)

It needs to power the chip (Arduino Pro Mini or trinket, ATtiny, MSP430 are what I have here) and the servo.

 

I'm thinking that the voltage regulator will suck up some power (but how much, and is there an efficient way to hook that up?), and up converting to 5v will suck up power.

Maybe the up converting circuit to run the servo could be placed on the other end of a transistor that gets switched on when needed?

 

edit, ps: I currently turn off the servo by using "servo.detach()". It still does have power supplied to it at all times, but it doesn't do the buzzing thing that the servo was doing if I kept it attached. I could try using a pin to switch a transistor to the servo power line to see if that helps at all.

 

Thanks!

-Nico

  • 0

    Hi Nico

    I faced the same problem some time ago and I used a board based on the NCP1402 (like this one https://www.sparkfun.com/products/10968 )

    This has an efficiency greater than 80%, which I think is quite good. However, I don't know i 200mA are enough for your purposes

    To save battery, I would for sure add a transistor to switch on and off the servo. As a matter of fact, small jitters in the PWM signal make the servo motor to continuously adjust its position, thus draining battery power

     

    Cheers

    Ambrogio

  • 0 in reply to amgalbu

    Thanks Ambrogio!

     

    That step-up board sounds like a good idea.

    Do you know if the step-up board uses much energy when there is no draw from the items connected - i.e., chip is sleeping and servo is off?

     

    I noticed, after writing my original post, that the 3.3v regulator probably won't work with the 18650 battery because it needs 1 volt above 3.3, which is higher than the maximum 4.2 the battery provides.

     

    I suppose the other option is to combine 2x 18650 batteries and then use a 3.3v regulator for the chip and a 5v regulator for the servo. That seems like a lot of battery for the project though.

     

    ps, detaching the servo in code stops that jitter you speak of, so I don't think the motor runs, but I think it's still drawing a bit for the servo circuitry.

     

    The other thing I just realized a few minutes ago is that the power bank I'm using has a very old power-tool 18650 battery that might need an upgrade!

     

    Cheers,

    -Nico

  • 0

    Hi Nico,

     

    For your regulator you might consider one of these Buck - Boost DC/DC switching Regulators. They are much more efficient than the linear regulators. Also I do not believe you have enough room between your 3.6V batter and 3.3V application to fit even an LDO linear regulator. I tried to tie a link from Newark but the site was down a few minutes ago so here is a partial Data Sheet. The supply can be from 2 Volts to 30 volts and the output locks onto 3.3 volts if that is the fixed output you have chosen. When Newark is up again you can search their stock and download the entire data sheet.

     

    John

     

    image

  • 0 in reply to jw0752

    To add to John's comments - to get the most out of a Lithium battery depends a bit on load current and the make of battery but you need to go down to some where between 2.5 and 3V.

    The simplest way to get what you need is to step up to 5V and then down from 5V to 3.3 with a second switcher - look at parts from Linear Technology (but beware difficult packages and quite pricey parts, admire the lovely performance image and easily best of breed simulation support.)

     

    MK

  • 0

    Maybe I'm not understanding your requirements, but couldn't you use the switch to bring up the power?

     

    If you have it so that it biases on a FET to supply power from the battery to the USB charger thing, with the Arduino taking over and holding the power on once it has booted, the Arduino can then do whatever it does and finally drop the power when it has finished. It would be a very simple circuit and would save you all the bother of engineering a switching supply and getting the processor to sleep, etc.

     

    Perhaps not so much fun though.

  • 0 in reply to clem57

    The cat by the switch

    with a bit of a twitch

    unless there's a hitch

    or a sudden glitch