Charge indicator circuit, Will this be okay.

[rickey1990]

Hi, I am currently wanting to make a 12v battery low voltage indicator.

I have been experimenting in Lush circuit simulator and would like some feedback on if you see any issues with the design.

Kind regards,
Rick

[ledtester]

I don't think the 50k resistor does anything for you -- you can just remove it.

Also, you should have a resistor between the base of the PNP and the collector of the NPN.

Finally, you can increase the size of the LED's resistor to cut down on current consumption. Modern LEDs can be bright enough at just 1 or 2mA of current.

Here's the circuit with these changes.

Falstad link:

https://falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWcMBMcUHYMGZIA4UA2ATmIxAUgpABZsKBTAWjDACgAlcMFEPK1rxp4oomgJTQaoqjARsALiBQJC4QlVzSwG0RBYwaOvNmyEshPCWxI5qgijDYaKbAUgQwcEABMGAMwBDAFcAGwVFEDdtGmkjNTBY0RYQJjkURyIwDAR+R0gMXjToQh0ESjMMQgywMnIvKj8gsIiAJ1odZXwOtRU1Kko4Nh1eaOVCqLNx3l4mkPCmUIYfcBkoWHYAd3BiXh0qNBF9qDZ2rT5NSG0UEQF4SDZt854RZ6SHp6vwKqivzD3HlE0NNftJ-idtvFlKoeiCPrDwWNwfCoS9YfwIejLtIMewAOagkHnc6yQGo3RQw7gNgADxAxCOGW+9B4SG0CRQADUADoAZwAWgwAHYMVqnIECJI2PbvZDwQHS8BSvAxbQK4GJaRvaTw7DAxFTZHiqnHPrqWRdeXOEAAL2FopYe3A0AwiWleFqNwakBQzHIB1g9yD9xV4ABNp2Mu0u3pvVt9tajsBggu3zUuOTMeICRjGPhntT5wz22y6YEudJJZjxwLmpOQA



The current draw at 16V is just 6.2mA now.

[mikerj]

Using a TL431 design for your low voltage sensor will yield superior voltage stability and clean switching for negligible cost penalty.

[MikeK]

Also, you should have a resistor between the base of the PNP and the collector of the NPN.

And, rickey1990, just to be clear why ledtester said this.  Look at the current path that goes through the PNP and then through the NPN.  There's no limiting resistor.  As in, full current from the power supply...and damage to those transistors.

[Benta]

If you want a LED to turn on above a certain voltage, this is the easy way to do it (threshold around 12.5 V):

[rickey1990]

Okay thankyou 👍, well the reason for not putting the resistor in was because I was basing it off a Sziklai pair, which didn't have a resistor.

[rickey1990]

Cheers matey. 👍. I had a look and settled on two 1.5k resistors. Yours make sense. Having lower current. I want to experiment with making it like a night light but just bright enough to see a small corner of the room. Then when the battery is low if shuts itself off.