Dazatronyx :: Low battery LED fade out

Low battery LED fade out

Most circuits can run off a battery. A 9 volt battery will hold its voltage nice and steadily until it is almost flat when it will begin to drop by a couple of volts and then become unusable. Most circuits won't perform differently when the voltage has dropped to around 8V, but some will. Then, sneakily, your circuit will just not work properly at all and it may happen without warning.

With this simple trick you can have the LED fade out when the battery starts getting low, giving you a bit more warning.

Observe the following:

Low battery LED fade out

Using the simple method on the left, the 1K resistor limits current to the LED. This also works as a bit of a regulator. For modern clear LEDs, the minimum resistor value could be above 220Ω and around 50K for a very dull illumination. 1K is a reasonable and safe value to protect the LED (modern or old style) from damage while getting nearly the brightest output. Around 10K-22K may be more reasonable for the high intensity clear LEDs without blinding the viewer too much.

The method on the right is a slight improvement while only adding one more component. The reversed zener diode stops 6.2 volts from passing through the circuit. So 9V minus 6.2V gives us 2.8V, which is nearer to the forward voltage drop of the LED. A small 220Ω resistor is still used for current regulation.

By the time the battery drops to 8V, the zener will limit the circuit to 1.8V. The 220Ω resistor will make the LED pretty dim. When the battery is almost flat the LED just won't turn on at all.

The order of the resistor, zener diode, and LED can be swapped any way, just not the polarities. You don't need to buy an expensive zener, just get a low wattage one.

N.B. - A 6V2 zener diode usually works for me with most LED types, but not all LEDs have the same forward voltage drop. If it is too dull, then try a smaller value zener diode. Similarly, if it is too bright use a higher value diode.