Simple 6v battery guard circuit4/14/2023 Unless it's necessary, I'd prefer not to have components under the HD44780 display, which is spaced around. I was pretty good at building and fixing, but not on designing circuits.Įliminating the LM10 DIP-8 I was having trouble with in favor of a LM431 T0-92 gives me a bit more space on the PCB, but I'll still need to see if I should go axial or radial on the capacitors. I'll hold off ordering the LM341 until I hear from you since I want to get new capacitors too, most of the ones in my parts bins are NOS from when I was repairing pro audio/lighting and arcade video games back in the 90s. I also forgot the last closing parenthesis in the formula for the low voltage set point so i revised the schematic and so also added a note about the LED color/forward drop.Īlso note that the upper range limit is quite high because there is no lower resistor in series with the pot. Some LEDs "first light" current might be very low too, so i wanted to avoid that as well hence the diode. I was going to mention that blue or white or some greens would work without the diode but didnt get around to it If the LED turns 'off' with low voltage then it doesnt matter if the battery runs down to zero as it will still show the true battery condition. With the LED coming 'on' for low voltage, there's always the chance that the battery will run way way down and thus the LED wont light up so it doesnt indicate what we really wanted it to indicate. Yes, the diode is there for the typical red LED used with low voltage monitors, but as i was saying in another post most of the time the under voltage circuits are built to turn the LED 'off' when there is an under voltage so that if the battery runs down real low we dont have to worry about not being able to turn the LED on. It's not made to regulate voltages below 2.5 volts so the cathode may not go lower than 2v minimum for any connection. Oh yeah that's one pesky little downfall about the LM431. They make blinking LEDs already but im not sure if you can get that low of a duty cycle from one of those. I used this idea with a refrigerator monitor to reduce battery current draw and get the batteries to last for 2 years, so it make a BIG difference. An LED running at 10ma always draws 10ma from the supply (battery), but an LED at 10ma that blinks with a 50 percent duty cycle only draws 5ma average from the battery, and at 10 percent duty cycle it only draws 1ma average. The resistor is still sized for the LED.Īt 12v and an LED with forward voltage of 2v nominal and nominal current of 20ma, a resistor of 500 ohms would be necessary, but i would drive it at less than 20ma so maybe 750 ohms or something like that as the life of the LED goes up sharply with lower than full rated current. To get the LED to turn 'off' with low voltage the LED is placed in series with the LM431 (cathode) instead of in parallel with the LM431. This logic might be a bit better because if the battery goes really really low then the LED wont turn on anymore anyway. The OP wanted a circuit to turn the LED 'on' when the voltage goes under a certain limit point, but the more usual under voltage monitor turns the LED 'off' when the voltage goes too low. You brought up a good point about the dieing battery.
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