6/10/2023 0 Comments Low battery indicator![]() A single flash every second or so is probably good. You will change your sample time to be faster if desired when the indicator is supposed to be flashing. The reason for using the MOSFET to drive the LED is that the capacitor will charge up to the battery voltage, which is significantly higher than the PIC operating voltage. The trade-off is how often you flash the LED vs how bright it is. The energy stored in the capacitor gives you a really nice bright flash of light but the average current consumption can be very low. What you would do is to use a large-value resistor to charge up a capacitor, then use a small MOSFET to connect the bottom end of the LED to ground through the appropriate current-limit resistor. They aren't very accurate, but they don't have to be because you are using separate accurate voltage reference.įlashing the LED can be extremely power-frugal if you use a capacitive-discharge technique. You can get suitable regulators with Iq down in the low uA region. You will power the PIC from an extremely-low quiescent-current regulator. I would also add a bypass cap to the output of the voltage divider. The voltage divider that brings the battery voltage down to the proper level for the comparitor needs to be high-value resistors so a to minimize the current consumption of the divider network. The pin is never allowed to be Output and Hi. The i/o pin is either set as Input or as Output and Low. ![]() ![]() What I mean by that is that you will need a voltage divider to feed the input of the comparitor, You can use a PIC i/o pin set up as open-drain with a series resistor to shift the sample voltage down. You can use another i/o pin to shift the sample voltage to add hysteresis. That way, the reference isn't consuming current when not needed. If you are using an external voltage reference, power it from one of the PIC i/o pins. You would need to choose an appropriate voltage reference to feed the comparitor. Some small PICs have both a decent voltage reference and comparitor while many more have only a comparitor. The PIC watchdog timer is good for this sort of thing - the time period isn't accurate but it doesn't have to be.įirst: you need an accurate voltage reference and a comparitor. Because this is a simple battery dead indicator, you should be able to get away with taking a sample every few seconds. The approach would be to have everything that consumes current powered OFF until it was time to grab a sample. Something like one of the small PIC 16F family would work well. The easiest method involves throwing a small microcontroller at the problem. You can reduce the current consumption dramatically with a few simple tricks. I'd be looking to make a LED blink when the battery is low.
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