Good evening!  A few weeks ago I asked for some help reviewing a schematic that I put together and I am hoping that I could get a quick review of some changes that I made.  Basically I just need someone to take a look at the voltage regulator and LiPo charger circuits on the right hand side of the schematic and tell me if anything looks off about it.  Thanks in advance for your assistance!


GPS_LOGGER by jg1996business, on Flickr

Not hard and there are plenty of off the shelf solutions for this.  Many of the wireless modules are 3.3V so you need to do a bit of level shifting but other than that the modules are readily available.  I use these:

http://imall.iteadstudio.com/wireless/bluetooth-and-zigbe/im120723009.html

but you can buy them in shield form from this same vendor which handles all of the level shifting for you.

The bluetooth shields that I linked to only require two pins, TX and RX.  You write and read to it using Serial.print and Serial.read.  Pairing and communication between bluetooth devices is handled by the shield.

I use one of these:

http://imall.iteadstudio.com/development-platform/arduino/shields/im120417010.html

on an arduino that is hooked up to the internet via an ethernet shield to transmit instructions received over the internet from my smart phone to another arduino with one of these:

http://imall.iteadstudio.com/development-platform/arduino/shields/im120417006.html

It allows me to control a relay board with 8 relays on it from anywhere in the world (as long as I have cell phone coverage). 

The way you have drawn that, the Cathode and Anode are at the same potential so no current will flow through the LED's.

Thanks!  I will use that one instead.  Probably easier to solder as well.

Yes you are.

Boo!  Oh well.  What about something like this:

http://www.mouser.com/ds/2/405/slvs520b-128772.pdf

I was planning on using the TI TPS62237 regulator to supply 3.3V from a LiPo battery, but I just noticed on the data sheet on page 3 "Recommended Operating Conditions" that minimum supply voltage should be Vout + 1V which would mean my battery should be 4.3V which is higher than a fully charged LiPo battery.  Am I reading that correctly?  I linked to the datasheet below.

http://www.mouser.com/ds/2/405/slvs941e-197967.pdf

Thanks everyone for helping me work through this!

that's 19 days eaten by the voltage divider.  Too much?

Okay perhaps I'm over thinking this.  I was under the impression that it would drain much quicker.  Voltage divider should be fine then.

 Or are you not planning on actual battery cut-off, just some kind of a minimum current draw mode?

Lefty

That's the idea.....put everything into deep sleep mode and have a pin change interrupt that will wake everything back up again when it is time to turn back on.  Its just a small GPS logger that my wife can wear on a lanyard around her neck when she runs.  I'm trying to make it really small (my goal is for it to all be on one board that will fit inside of an "altoids smalls" tin along with the battery.

It will read 255 (or whatever the max value for 10-bit resolution is) until the voltage falls below 3.3V.

1023.

The internal clamping diodes will protect the pin as long as I supply enough current limiting to it.  Is this right?

That is my understanding.  It is also my understanding that protection is obtianed by the diode conducting.  Which would be a constant drain on the battery just like the voltage divider (until the battery drops below 3.3V).  Which brings us full circle to my earlier question.  Is the voltage divider that much of a drain to be a problem?

I don't really know.  It's a 400mAh battery.  I hate the idea of it just draining down until the battery protection circuits kick in even when the device isn't being used.

basically it would just look like this and I would rely on the pin protection diodes to limit the voltage to Vcc+.3


example2 by jg1996business, on Flickr

Well if you would return to your first idea of powering the 328P directly from the battery below is a proof of concept sketch that Coding Badly and I worked on a couple of years ago that allows a sketch to directly determine the actual value of the voltage applied to it's Vcc/Avcc pins. It uses the known internal band-gap reference as a way to 'back calculate' what the applied Vcc must be at any given time.
Of course that still leaves you with a means of how you are going to disconnect the battery once you have performed all the orderly shutdown tasks.

But I'm not running it directly from the battery.  The mega328 will get a regulated 3.3V from a step-down/boost converter. I've shown this on the schematic above on my original post. That means I can't just read the internal band gap voltage to get the voltage at Vcc because Vcc (assuming the boost converter is working properly) will be regulated to 3.3V regardless of the battery voltage.

Just giving you an alternate look at the universe.  

Actually I would love to run it directly off the battery (which I think is what you are saying) but I am going to be running the GPS module and an SD card off of the same supply and they are only 3.3V tolerant (at least according to the datasheet absolute maximum ratings).  I figure running the mega328 at 3.3V gets me around having to do a bunch of logic level shifting for the communications.

Based on this... http://www.atmel.com/images/doc2508.pdf ...if the resistors are large enough, the processor will not be damaged.  I assume that, when the voltage is above 3.3, the clamping diodes on A0 and D2 would conduct thus defeating the intent.

Does the voltage divider have that much of an effect?  Is a few milliampere drain going to cause a problem?

Based on your link, it appears I could just bring the battery plus side to the analog pin directly through a current limiting resistor and read the voltage.  It will read 255 (or whatever the max value for 10-bit resolution is) until the voltage falls below 3.3V.  The internal clamping diodes will protect the pin as long as I supply enough current limiting to it.  Is this right?