10/18/2023 0 Comments 9v low battery indicator circuit![]() If the circuit has a low-current 'sleep' function, it may not be possible to use the circuit unless you use an ultrabright LED that will be bright enough with no more than half the current drawn in sleep mode. To be safe, I'd recommend a maximum current of about 2.5mA. ![]() For example, if your circuit draws 5mA at 5V, the LED current must be less than 5mA with a brand new battery (typically about 10V for a nominal 9V battery). The resistor (R1) should be selected such that it doesn't pass more current than the circuit following the regulator draws. Once the battery voltage falls to ~7V, there's not enough voltage to turn on the LED, so you know that it's time for a new battery. Your circuit gains just one LED and one resistor. Because the LED current flows into the load, it's not wasted, and no additional circuitry is required. Once the voltage falls below the LED's forward conduction voltage it goes out, indicating that it's time to replace the battery. Provided the battery voltage is greater than ~7V (for a 5V output) the LED will get current via R1. Most common 3-terminal regulators will function down to the point where the input voltage is only about 2V greater than the output.įigure 1 - 'Zero Power' LED Battery Monitor It has to be greater than the minimum input-output voltage differential of the regulator. The basic circuit shown here can be adapted for almost any voltage, but you need to verify the LED forward voltage. Mostly, I'd suggest an 'ultrabright' LED, as these can provide more than enough brightness even with as little as 0.5mA. Standard voltage regulators can only source current, and if your load only draws a very low current, if the LED current is too high the regulator's output voltage will rise. ![]() There is one caveat - the circuit being powered must draw more current than the LED at any battery voltage. It's a clever idea (thanks to B Fraser), and despite a lengthy search I found nothing similar elsewhere. 5V) that's powered by a battery, which will typically be a standard 9V battery for small, low-power applications. It only works when you have a regulated supply (e.g. However, where most LED battery indicators draw current that does nothing other than illuminate the LED, this idea puts the LED current to use in the circuit being powered. Although I've called this 'zero power', that's not strictly true, since anything that draws current and has some voltage across it must dissipate power. ![]()
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