Low battery indicator, setting minimum voltage

[JackSkellington]

Hello, I never tried this thing, I just got it on the breadboard.



I'm using a 9v battery and it works, turning the pot passing a spot the LED light up brightly, and if I turn it back it light off.

But I don't understand how I can set the minimum voltage. If I want the LED light up at 8v how I can measure it?

[danfrank]

Use a reverse biased zener (1 watt) in series with the LED. You will have to experiment with zener values in order to find the correct one that will turn the LED off at 8 volts. I'd try a 6.2 volt zener first with a red LED. Good luck and please report back with your findings.

[Zerro]

This vehement works automatically like switch, which will set LED diode ON, whenewer power supply fall under some level, without some another zener diodes. For 9V battery it is about cca 5V. So, set at power (sensor) pins 5V, and move with trimmer, until it starts to shine. After power is upon this level, cca 6V, LED will be set OFF automatically. Thus it will shine always, when power is low at 5V.

[JackSkellington]

Use a reverse biased zener (1 watt) in series with the LED. You will have to experiment with zener values in order to find the correct one that will turn the LED off at 8 volts. I'd try a 6.2 volt zener first with a red LED. Good luck and please report back with your findings.

I don't use a lot of zener diodes, I don't want ot buy much of them just to try it. I thought this kind of circuit was allowed to set the low battery level.

This vehement works automatically like switch, which will set LED diode ON, whenewer power supply fall under some level, without some another zener diodes. For 9V battery it is about cca 5V. So, set at power (sensor) pins 5V, and move with trimmer, until it starts to shine. After power is upon this level, cca 6V, LED will be set OFF automatically. Thus it will shine always, when power is low at 5V.

Do you mean I can set it at the voltage I want?

I tried to put a 10k pot between the positive of the battery and the 9v of the circuit to tame the power, and see what happen if I low the voltage. Indeed, the LED light up. But I'm not sure it's the right thing, I afraid that the trimmer (or pot) of the original schematic could mess this setting.

I thought to this kind, too. It use just one transistor: TL431.

Or this: https://www.youtube.com/watch?v=T8_7KMBYVEY

I need something I can set the minimum voltage, else it will be a bit useless.

[Zerro]

Yes, you can set it. That second schema use reference voltage source TL431. It is not transistor, but it works similar way here.

[JackSkellington]

Thanks, ok.
I'll buy a TL431, and I'll see well how I could set the right voltage for me.

[Zerro]

That first schema is sufficient. Only to set trimmer.

[JackSkellington]

I ordered the TL431, the circuit is smaller, so I prefer it, anyway.

For the two BC547 transistor schematic I can't get how I have to set the trimmer to make the LED light up at the voltage I want.

I tried to put a pot in line to the battery and the 9v to decrease the voltage of the circuit. Then I set the trimmer until the LED turned on, but maybe this mess up the voltage, too. Not sure.
I need some instructions I can't find anywhere.

[Zerro]

This first circuit works this way: set at power pins power value, which means discharged baterry. For 9V baterry is it about 5V. As 5V is set at pins, you set trimmer for this value, to start to shine LED. Whenever value will be higher, first transistor will be opened, and second transistor will be closed. When power is going down to 5V, as you set, transistor hasn't got high enough power to be opened - it will close itself, and this will open second transistor with LED. That value is depending only at your setting that trimmer. Then only connect this with power pins of your machines you built. At 9V it will be closed, at low value it will be shine.

You only must choose value of baterry here, that will be recognized by this indicator as low one. Nothing more.

BTW you can use any common transistors you have now in your drawer.

That second schema may be good too, you only must set that trimmer the same way as described upon. Try both to consider what will work bettter.

[JackSkellington]

Ok, I'll try to do something.