[HH] low battery indicator

[Greg London]

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

;)

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

http://www.atmel.com/dyn/resources/prod_documents/2535S.pdf

The schematic they have in step 5 here is pretty bare bones.

http://www.instructables.com/id/Tiny-AVR-Microcontroller-Runs-on--a-Fruit-Battery/?ALLSTEPS

It's the chip, a cap on the power pin, a resistor and LED.

The chip has some features that might be useful for low battery detection:

– 4-channel, 10-bit ADC with Internal Voltage Reference
– Programmable Watchdog Timer with Separate On-chip Oscillator
– On-chip Analog Comparator

Maybe wake up once a second to check the voltage, use the ADC and the
internal reference to figure out the external voltage. Then either pulse
the LED on then off, or leave it on, then sleep for another second or so.

You'd need a low current voltage regulator and two resistors to divide the
battery voltage down to something the ADC could sample. But it would be
programmable, so you could blink any pattern you want.

Low Power Consumption
– Active Mode:
• 1 MHz, 1.8V: 240 μA
– Power-down Mode:
• < 0.1 μA at 1.8V


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. I
hate running LED's all the time on battery projects. The milliamps really
suck. And you're usually only going to glance at the LED once in a blue
moon anyway. So, if you PWM a 10ma LED at 50%, it's not as bright, but
it's also only drawing half as much power.

Or, get a bi-color LED, and hook it up between two output pins fo teh
chip. Then you can PWM it green for good power, adn pwm it red for low
battery.
Either way, you'll be draining one-tenth the power you would when the LED
is constantly on. Just pulse it once every second or two. (depending on
how important the architect says that blinky pattern is)

looks like digikey has the microcontroller in stock for just over a buck.

http://search.digikey.com/us/en/products/ATTINY13A-PU/ATTINY13A-PU-ND/1914671

Just a thought.

Greg



> I'm looking for a low battery indicator for a 12 V battery that
> perpetually illuminates an LED when the voltage is good, and blinks the
> LED rapidly when the voltage drops below a threshold.
>
> Do you have a favorite circuit for this sort of thing?
>
> My first thought was to use a 555, or if necessary the dual version,
> where one timer gets used as a comparator and the other provides the
> oscillator to blink the LED.
>
> Although the 555 has a comparator, it doesn't have an absolute voltage
> reference. The reference is proportional to Vcc, so that means you need
> to run the chip from a regulator, and the regulated voltage would have
> to be safely below the low battery voltage you want to sense. That's not
> unreasonable if monitoring a 12 V battery, where you might want your low
> point to be something like 11.5 V, so you just run the 555 on 5 V.
>
> Here's a 555 comparator circuit:
> http://home.cogeco.ca/~rpaisley4/LM555.html#13
>
> Workable, but not particularly elegant.
>
> Another 555 variation:
> http://gorum.ca/lvdisc.html
>
>
> Next thought was to see if there were any readily available low battery
> monitoring ICs. That led me to the ICL 8211:
> http://www.eleccircuit.com/low-battery-voltage-indicator-by-ic-8211/
>
> which is not so common. It led me to Maxim, which has a whole family of
> ICs that work as battery monitors, but also include other functionality,
> like watchdog timers or microprocessor resets, because they are intended
> for use in cell phones and the like:
> http://www.maxim-ic.com/datasheet/index.mvp/id/3994/t/al
>
> It seems unlikely that one of these chips would provide the illumination
> pattern I'm looking for without adding additional chips.
>
>
> Next I ran across the LM393 Low Power Low Offset Voltage Dual Comparator:
> http://www.national.com/mpf/LM/LM393.html
>
> It comes with two comparators per package, and one can be wired up as an
> oscillator. They're cheap ($0.25) and widely stocked:
> http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_24281_-1
>
> Unlike the battery monitoring ICs, it requires an external reference
> voltage, but that's cheap and simple: either a zener diode, or a
> reference IC, like the LM336Z:
> http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_23771_-1
>
> which goes for $0.39 and comes in a convenient TO-92. The LM393 runs on
> 2 to 36 V, so it wouldn't need a regulator for my application.
>
> But the big advantage to the LM393 seems to be its low power (0.4 mA),
> which is fairly irrelevant for my application where I'm going to be
> perpetually driving a 10 mA LED.
>
> So I'm wondering whether rather than mail ordering an LM393 I should
> just use a more common LM741 op-amp, which I probably already have.
> Plenty of low battery circuits built with op-amps:
> http://www.reuk.co.uk/12-Volt-Battery-Low-Indicator-LM741.htm
> http://electroschematics.com/4004/low-battery-indicator/
>
> Opinions?
>
>  -Tom
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