[HH] low battery indicator

[Tom Metro]

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