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Battery-low detect using Voltage Detector and RC

I thought I had a really clever way of implementing 3-volt battery-low detection with only 3 parts: a 2.7-volt reset detector, a resistor, and a capacitor. The idea was to periodically raise and output which will would be filtered by the RC filter, and then feed that to the reset detector. By timing when the reset detector declares the filtered voltage to be above 2.7 volts, I could infer battery voltage. The trouble is those reset detectors (Microchip MCP 130 series) have a built-in additional power-up delay that swamps out the effect of my RC filter.

Does anyone know of any tiny 3-terminal device that acts like a self-contained voltage detector but without any delay? If so, then this method could still be salvaged. I plan on performing an in-circuit calibration on each piece, and then custom-flashing the PIC to compensate for variations in threshold voltage and RC values.

-Robert Scott Ypsilanti, Michigan

Reply to
Robert Scott
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What would be wrong with periodically lowering the output and timing when the reset detector declares the voltage to be below 2.7 volts?

Reply to
nospam

By golly, it works! In a way, it's an even more sensitive measure of the battery voltage. The only disadvantage is that the time period is much shorter, and therefore harder to measure with accuracy. But on my target system, a 3.6-volt battery supply takes about 60 usec. to trigger the reset detector at 2.7 volts using the RC filter I've got now. My 4 MHz PIC can measure time to within 1 usec, so I guess I'll go with it. Thanks for the suggestion.

-Robert Scott Ypsilanti, Michigan

Reply to
Robert Scott

Hello Robert,

Just in case you get hit with inaccuracies due to internal delay or the tolerance of the capacitor: Check out the National LMV431 or the TI counterpart. That is a very precise chip in SOT-23 and you could build something around it.

Regards, Joerg

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Reply to
Joerg

I will definitely look into it. As for inaccuracies, I expect to deal with most of them with a system calibration. I will flash a test program into the completed module after it is potted. This test program will determine the correct time delay for a threshold battery voltage, and then reflash the application program based on the results of the test. I won't be relying on any precise part tolerances.

-Robert Scott Ypsilanti, Michigan

Reply to
Robert Scott

Hello Robert,

Sounds good, as long as the tempcos of the contributors are within your spec.

Regards, Joerg

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Reply to
Joerg

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