Low power servo button pusher

Hi - sounds like a great idea!

Are you considering using one of the Bluetooth Espruino boards? Basically as long as you stop sending signals to the servo motor when you're not going anything, the power draw of the microcontroller should be pretty low as it'll just be asleep.

The difference between hardware/software timers won't really be an issue for you since when it's moving the servo will be drawing way more power than the microcontroller.

You can run those servos off a LiPo battery too - you won't get as much torque as if you ran them from the full 5-6v, but it should be enough.

I just checked with a FITEC micro servo here and it looks like when they're just sitting there not doing anything they draw about 3mA - so off a 2000mAh battery that's 670 hours -> about a month. I guess other servos (more expensive ones?) might draw less power though.

If you want to get it to work for longer off a battery you'll need to think about turning the servo motor on and off, so you only turn it on when you need to send a signal. If you do that hopefully you could get a year or more out of it.

The difference between the 3.3v voltage of the microcontroller and the ~4v voltage of the LiPo for the servo makes that slightly more painful to turn the servo on and off than it should be though. Normally you'd just use an FET to switch the GND wire, but you can't do that because it'd put 4v on one of Espruino's IO pins - instead you need to switch the power pin of the servo.

So you have a few options...

• The easy way is just to use a relay module (http://www.espruino.com/Relays) - but that's a bit big and clunky.
  
• You could use a MOSFET based solid state relay, like https://uk.farnell.com/vishay/vo14642at/relay-mosfet-spst-no-2a-60v-th/dp/2396108
  
• You can use a device called a 'High Side Driver' like this: https://uk.farnell.com/infineon/bsp452huma1/ic-smart-high-side-pwr-sw-sot223/dp/2215478 but it seems it can be hard to find ones that are big enough to solder nicely
  
• You could use a 'half bridge' motor driver (basically a high side driver plus a low side driver) - these are be more easily available on breakout boards, but might themselves draw some power when idle
  
• You could make your own 'high side' driver using two FETs and some resistors.
  
  

Hope that helps!