efficient 12v -> 5v switching regulator

[gregcope]

Thanks Tom.  I get it now.  I thought coding would be the complex bit.

So another idea that you have alluded to is to only use the main power boat supply when required.

For example this Pololu regulator has an enable pin: https://www.pololu.com/product/2850/specs

Could I power the system from a MightyBoost/Lipo, and enable the pin/12v step down when I need to charge the lipo?

[TomWS]

Sorry, I totally missed this.  I think you're close to a good solution.  Comments below:

<snip>
The GPS has an enable PIN that I should be able to leverage.

Yes, I noticed this as was going to suggest that this might be a good alternative to a separate transistor switch.  It will consume more power than a pure transistor solution would (23uA, I believe), but you can't beat the simplicity.

Most Modems also have a hardware on/off pin that requires pulsing for 2 secs HIGH.

When "off" not sure if either have parasitic draw to monitor that pin.

I haven't had a chance to look at the GSM radio in any detail, but I think you understand the issue if you turn power off to any device attached to your processor.  If there is NO voltage to keep the peripheral's inputs biased, each input could be a relatively high load to the processor.  The best thing to do in this case is either set all your outputs to the device to LOW before (or as) you power off or do a pinMode change to input with PULLDOWN.

I could fit larger batteries.  Although I wanted to run off AAs :-(  The parts are all looking to be quite small bar the battery.  The GPS is tiny, the modem not much bigger.  Ie looking at a stack of PCBs not much bigger than a cube of SD cards or something the size of a cigarette packet (maybe thiner).  Three AAs would double the volume.  C or Ds tripple.

I think you're getting close to being able to run from AAs but I think you'll need 4 of them to get to >5V for your GSM radio.  You can get two 2XAA battery holders and tap off the middle to get the 3.3V+ you need for the low voltage parts and then the 'top' of the battery stack for your >6 volts supplying power to a low drop out VR with enable to power your GSM radio.

I was originally hoping (hence the post) to run off the boats 12v.  If the power draw is low ie 5Ah/yr then we could assume the boat boat batteries are charged by another source (either solar, or alternator, or trickle charger) as the owner will have to deal with that anyway (ie battery drain/condition over longer periods).

Hence back to the Original idea where;

Boat 12v-> efficient step down reg -> 5v MightyBoost + backup Lipo -> system

However the challengette is that if the Step down switching regulator is not really efficient at the 0.1ma range, we will lose more power there than anywhere else.  ie a 4Ah/hr consumption becomes allot higher.

Thanks again.

You almost have it with this last model.  Consider this, not LiPo as your 'backup', but LiPo as your main low voltage/low current supply.  Then, the switching regulator from the 12V battery (like the one you just posted), which wants to run with a multi-100mA load, is simply periodically switched on to the input of the 'safe' charger (I'm not sure that MightyBoost is what you want, but there are several other choices available from Sparkfun et al) for the LiPo.

You control the supply to the charger by enabling the switcher when the LiPo falls to around 3.9V-3.8V (as monitored by your Moteino), the LiPo charger will fast charge at 200mA with relatively high efficiency.  Then, when the 'charge' signal from the LiPo charger goes off (indicating full charge) you turn off the switching regulator.

Finally, I would NOT use a boost circuit (or this switching regulator) to produce the 5V you need for the GSM radio.  Boost circuits are notoriously noisy.  You want a very clean supply for the radio so, given that the duty cycle is so low, I'd bite the bullet and just use a linear regulator with enable control to convert from 12V to 5V. 

Whatcha think?

Tom

[gregcope]

Thanks again TomWS

You almost have it with this last model.  Consider this, not LiPo as your 'backup', but LiPo as your main low voltage/low current supply.

Yep - got that - most of the time it will run off the efficient 3.7v -> 3.3v.  And only use the 12v regulators for a boost.  Ie most of the time stay in the (must more) efficient zone.

Then, the switching regulator from the 12V battery (like the one you just posted), which wants to run with a multi-100mA load, is simply periodically switched on to the input of the 'safe' charger (I'm not sure that MightyBoost is what you want, but there are several other choices available from Sparkfun et al) for the LiPo.

Any reasons not to use the MightyBoost?  Or are you suggesting going simpler and just ordering a lipo charger?

You control the supply to the charger by enabling the switcher when the LiPo falls to around 3.9V-3.8V (as monitored by your Moteino), the LiPo charger will fast charge at 200mA with relatively high efficiency.  Then, when the 'charge' signal from the LiPo charger goes off (indicating full charge) you turn off the switching regulator.

Should be easy - just more logic to include.

Finally, I would NOT use a boost circuit (or this switching regulator) to produce the 5V you need for the GSM radio.  Boost circuits are notoriously noisy.  You want a very clean supply for the radio so, given that the duty cycle is so low, I'd bite the bullet and just use a linear regulator with enable control to convert from 12V to 5V.

Need to think about how I power the GPS/GSM parts.  That should not be hard mind. 

Whatcha think?

Yep - nearly there - more complex than I wanted, but should be very efficient, which is a key / critical goal.

[TomWS]

<snip>
Any reasons not to use the MightyBoost?  Or are you suggesting going simpler and just ordering a lipo charger?

<snip>

I think the MightyBoost is overkill (and therefore higher power consumed) for what you need in this particular case. 

As I see it, all you need is a basic charger between your 12V->5V switching regulator and your low power load (Moteino, PIR, GPS).  Something like this would work: https://www.sparkfun.com/products/10401, however there are two 'annoyances' about this kind of implementation:
1. The Charge Status pin doesn't have a convenient connection for your Moteino to monitor (nobody seems to bring this out),
2. Even if it did, they use (as everyone else apparently does) the MCP73831, which isn't open drain (like the MCP73832) and therefore drives the signal above 3.3V so you can't hook it directly to the Moteino input.

I looked around the web last night for the 'ideal' LiPo charger for your case, but didn't find a better one than this one: http://www.jayconsystems.com/micro-usb-lipo-charger-mcp73831.html which is a bit easier to incorporate into a breadboard than the Sparkfun module although either could be used. To use this, you need to solder a wire to a pad on the PCB (Pin 1 of MCP73831 or cathode of the LED) and use two resistors (4.7K & 10K) to divide the signal voltage to below 3.3V.

To provide 5V for your GSM radio, as I said, IMO you should use a linear regulator with an enable control.  The source for the regulator would be 12V (or something >=6V) and the enable connects to a GPIO on your Moteino.  Let me know if you need help finding something like this.

Finally, considering the number of I/O you're consuming for this project, coupled with the need for serial ports to talk to some of the devices, you might consider moving to the Moteino Mega.  This has two serial ports and a few more I/O.  It also has 4x the RAM of Moteino, something that might come in handy as it will free you to concentrate on something that works, rather than including the problem of limited memory resources...

Tom

[gregcope]

Thanks Tom for all your replies and help.

Will have a look around and report back.

PS I had already ordered megas as already knew I was out of interrupt and Digital ports!!!