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…
A 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.