Need help making crude battery indicator light

[mantisxgp]

I need to make a crude battery indicator light for an airsoft gun witch does not have a micro controller just both +/- wires running directly to motor. I would like it to use as little power as possible. And have it come on when the battery is fairly close to being depleted.

 

[MikeHalloran]

Of course you understand the logical fallacy of having a 'low battery' light come on when the battery is dead; where do you get the energy to run the light?

Assuming you solve that problem or are okay with a 'battery good' light, the only simple way to do such a thing is with a voltage comparator... but that's only effective on lead-acid batteries. You didn't tell us what battery chemistry you are using.

For actual money, you can buy IC chips that are designed to monitor batteries; see the Digi-Key or Mouser catalogs.





Mike Halloran
Pembroke Pines, FL, USA

 

[OperaHouse]

One option is to use a LM431 amplified zener. This has a voltage reference of about 2.5 volts. Make a voltage divider to the reference pin for that value. Add an extra NPN transistor and tie the base to the zeners + output with a pull up resistor to + supply. The emitter has an LED and a diode in series to the - supply. This will insure transistor doesn't turn on till the base goes well above 2.5 volts. A review of the application notes should make this clearer.

 

[OperaHouse]

If this is a budget project, the LM431 can be found in many old PC power supplies. We all have a couple of them around or you can go cruising trash night.

 

[mantisxgp]

ok sry i did not give enough info earlier when i posted this. i play to use a separate battery to run a low battery light in the stock of the gun. haveing a good battery light would defeat the urpose mainly becausei would like to be notified before the guns stops operating. but LM431 is a good idea. im gonna have to do some hunting soon thanx.

 

[OperaHouse]

No need for a seperate battery. There is always enough power to run a LED and the purpose is to identify prior to a battery being not useful. One problem you will have is that battery voltage drops when being used due to internal resistance of the battery. Any circuit will consume some power. That can be a problem if a battey is stored for a long period of time or it is a small AH battery.