[HH] low battery indicator

[Greg London]

ah, well, I got the impression that minimum number of parts was the chief  
requirement, and I probably added minimum current as number two.

if time and effort is high on the list then all the overhead of a  
microcontroller isnt justified.

Connected by DROID on Verizon Wireless

-----Original message-----
From: Tom Metro <tmetro+hhacking at gmail.com>
To: hardwarehacking at blu.org
Sent: Sat, Mar 31, 2012 19:36:37 GMT+00:00
Subject: Re: [HH] low battery indicator

Greg London wrote:
> If you want something that specific, and as few parts as possible,
> you might be better off trying something like a tiny microcontroller.

Throw a micro at it is often a good option these days. If the circuit it
would be replacing was any more complex, I'd gladly trade off some
software development effort for reduced hardware assembly effort.

But in this case the circuit is so conceptually simple - a comparator
and an oscillator - that I'd be more tempted to go in the other
direction and just use discrete transistors.

Like most of my projects, it is being pursued to address a practical
need, and not just for the fun of it, so I'm optimizing for least effort
at a reasonable cost. (My first pass was to look at off-the-shelf
automotive battery monitors. But I didn't find any that behaved the way
I want.)

I can probably prototype the circuit (comparator or op-amp version) on a
solderless breadboard in under an hour, and build the final version on
some perf board in another hour. Any micro option would require a bunch
of overhead time to get up to speed, plus programming hardware in some
cases, so not the best trade off.


> First thing is I'd probably talk to your architect and see just how
> important that blinky pattern really is.

:-)  I have a specific use case in mind where it will be useful. I need
an indication that can easily be observed at some distance (20 ft), and
is easy to distinguish the states, gives an active indication when
things are working, and a more attention getting indication when the low
battery condition occurs.


> http://www.atmel.com/dyn/resources/prod_documents/2535S.pdf
>  
http://www.instructables.com/id/Tiny-AVR-Microcontroller-Runs-on--a-Fruit-Bat 
tery/?ALLSTEPS

I've ran across this (or at least other AVR-family) micro before and
have some bookmarks set on it:

http://www.avrfreaks.net/
http://shop.tuxgraphics.org/ (lots of kits that have Ethernet, including
a host watchdog:
http://tuxgraphics.org/electronics/201102/eth-network-watchdog.shtml)

Feature-wise, it seems to be like a successor to the PIC.


> you might even be able to PWM the LED so it looks like its on all the time
> through persistance of vision, but actually saves some battery juice.

Yup. A micro would also permit making the blink rate proportional to
voltage.

Both of those could be fairly easily done with a VCO, but by that point
the micro circuit would start showing an advantage in simplified hardware.

But neither of these are requirements for my application.


> Or, get a bi-color LED...

I'd be more apt to take the lazy approach and use a pair of LEDs - a
green one wired to one comparator to light when the voltage is good, and
a blinking red LED wired to another comparator (used as a
buffer/inverter) that illuminates when the voltage is below threshold.

Or if I wanted to pay $14 for an LED I could get a programmable "smart" LED:
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2123645_- 
1

that could do slow fades across the RGB spectrum when OK, and rapidly
blink red when in low battery state. :-)

(I think the only control mechanism it offers is I2C, so if you need a
micro to switch from OK to bat low states, then it isn't very practical.)


> Just a thought.

Thanks for the ideas.

 -Tom
_______________________________________________
Hardwarehacking mailing list
Hardwarehacking at blu.org
http://lists.blu.org/mailman/listinfo/hardwarehacking

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.blu.org/pipermail/hardwarehacking/attachments/20120331/ada72ced/attachment.html>