LED battery backup power

Thread Starter


Joined Oct 13, 2008
I found your site from a web search for electronics help on a model railroading project. I know very little about electronics, but would like to find out how to implement a small simple power switching circuit. I have 30 LEDs in an N scale carousel that are currently battery powered. The 30 LEDs operate on 3 volts and drain the two 1.5 volt batteries fairly quickly at shows. I have tried using external power for the LEDs, but the pickups lose contact when the carousel turns and cause the lights to flicker. I have tried using capacitors to eliminate the flicker, but while they keep the LEDs from going out completely, they cannot provide enough current to keep them from dimming and still causing a flicker effect.

Since fresh batteries can power the LEDs at 3 volts without dimming, I would like to know if there is a simple means of powering the lights with external power and only switch to the batteries for the occasional momentary power loss and then switch back to the external power without any flicker.

Any help or direction would be greatly appreciated,
John Hudson

Alright i'm only a novice/beginner at electronics myself. But i'd guess that using larger valued caps might help. If you use a larger capacitor, you'll get a larger time constant. Which basically means that the capacitor will charge/discharge more slowly and be less sensitive to changes in voltage over lower frequencies. Therefore, it will be able to provide you with more constant current.


Joined Apr 20, 2004
If you are using alkaline AA's, get a charger and go to MiMH instead. They have lots more capacity.

Also, consider reducing current through the LED's. That might mean changing 30 resistors.


Joined Jul 17, 2007
Beenthere meant NiMH batteries.

Capacitor voltage drops off very steeply, where battery voltage remains relatively constant from fully charged through fully discharged.

Consider routing the LED current supply up through the carousel aka turntable aka roundhouse turning point (axis). If you limit the turntable's travel, you could even use solid wire.