Turtle Power!

A headless Turtle really needs to be tail-less too.  It’s hardly free to roam with a mains lead shoved somewhere indelicate, is it?  It’s been a little tricky and so costly getting this to work, so I’ll share with you the final magic ingredients…

Lithium Ion battery 7.4v 2.2AH – why?

  • Even without monitor, keyboard and mouse, RPi boots and then dies without a 3.3v and 5v supply even though the 5v isn’t used for much outside the GPU
  • Lithium Ion cells produce 3.7v, so you need to get a couple in series to get >5v required
  • I bought 2.2Ah batteries to give me > 2 hours running the Turtle between charging
  • So 7.4V 2200mAH it is, but I don’t want to just warm up the ecosystem with the extra  voltage over 5v, so…

switch mode regulator is needed as the most efficient way to convert 7.4V to 5V at up to 3A without producing 7W of heat –  the 5V powers the stepper motors as well as pieces of Pi and they can draw > 1A each if they’re in a mood.

Then I need to take the 5V 3A DC, and tap off a 3.3V @ ~700mA to run the bulk of the Pi – that I used a Low Drop Out regulator as the voltage drop was 1.7V and the current < 0.7 =>  1.2W heat generated – the regulator doesn’t even start to get warm.

Put that lot together and this is what you get – a well stuffed breadboard, to the extent that I bought many heat sinks and inductors while trying to find a combination that would fit on the breadboard.

A headless RPi

TurtlePi, powered from a Lithium Ion rechargeable 7.4V battery with just WiFi to the outside world

P.S.  A little logical thought (too late) meant I realised the 3.3V regulator was unnecessary. The 5V regulated from the battery was fed via the GPIO pin 2 to the RPi 3.3V regulator, thereby feeding 3.3V back to the breadboard via GPIO pin 1.  Which is lucky because I was running out of breadboard lines, but this frees a couple.


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