low battery indicator idea

[Paul Perry (Frostwave)]

I was ROFL after seeing this on a pedal site
"6. Battery monitor built in - when battery runs low LED-lights gets dim."

but then i thought... you know those 'expanded scale' car battery meters (that show only the voltage from say 10 to 15v for better resolution over the scale of interest). They work by having say a 10v zener in series with a volt meter optimised for 5v.
If you put a 6.5 say zener in series with a led and a suitable resistor, you would have a useful indicator. Whether they do this in the pedal in question I dont know..

[petemoore]

There's the little two tranny voltage dependant LED lighter...pretty simple and LED lights when battery is below preset level. [I used a 100k trimpot on the board].
  Nice thing to add to a box but also draws a little current itself.
 Is it possible to see the LED dimming in brightness when the battery voltage falls below a say ~8v by using the right LED and resistor combination or some other trick?

[Boofhead]

Whether they do this in the pedal in question I dont know..

Essentially that's what Boss and Ibanez do.

[DDD]

There is a very nice Schmitt trigger schematic using Sziklai configuration (pnp and npn BJTs). If the trigger's threshold is a part of a battery voltage, you may arrange its functioning in the following manner:
1. When the trigger is OFF, BOTH transistors are closed and LED is off. Current consumption is about 10-50 microamperes, hence you don't waste battery energy.
2. At some low battery voltage the trigger flips to ON, both transistors are open and they collector current feeds LED - it's time to change battery.

By adding one or two electrolytic capacitors you'll transform the trigger to "threshold" multivibrator with close to zero quiescent consumption.

[R.G.]

The LM3914 and some LEDs makes a great expanded scale voltmeter. You can program it with a bicolor LED so that the LED is one color when the voltage is above X, the two-at-once color when it's between X and Y, and the other color when it's below Y, so you get a "good, worry, bad" indication. I designed a car battery "gas gauge" based on this once. Worked great.

On the other side, seems like what you really want is an indicator that leaves the LED on solid when the battery is good, starts blinking when it gets low, and blinks shorter and shorter on times as it gets toward empty.

...hmmm... how was it you spell "PIC Microcontroller"?

The eight pin DIP version of the 12F508 could do the entire job of reading the battery voltage, and intelligently controlling the LED for the above job. It would take a 12F508 ($1.40) a 78L05 (0.40) and some resistors and caps. Trivial programming, should take about twenty minutes to hack up the code.

[toneman]

hey RG, where do U find 3914's???
i thought they were obsolete....
i've been leaning towards a pic for a batt mon,
with a bunch of comparators in front.
i need 18 batt-mons
LMK where U get 3914s....(reasonably priced)
thanx
tone

[brett]

hey RG, where do U find 3914's???
i thought they were obsolete....

They're still in stores here.

But when I've attempted to use them, I get all the leds flashing.  I *think* that it's rubbish coming through on the supply line.  There's some stuff in the application note about de-coupling using a large cap (10uF?).

Anybody else had had bugs with 3914/3915s ??

thanks

[Paul Perry (Frostwave)]

$2.70 at Digikey, I believe.