# Efficiently powering multiple LEDs from higher voltage than used for microcontroller

#1

Hi to all, before start topic thank you taking time to read. There are several of questions are similar to my question, but I can’t get answer, so the situation is following: I need to control 12 leds powered by 6V (4 AAA batteries). I am using the PIC18F25K40 microcontroller, which will be powered by 3V (2 AAA batteries). I can’t tie the knot because of different power sources. Sufficient IO pins can be used for individual addressing leds.

Because I will rely on persistent vision, the LED will be powered one at a time, so I expect it to be well connected with a single limited-flow resistor.

To turn on an LED http://www.kynix.com/Detail/738157/LED.html, the corresponding IO pin will be set to Output LOW in order to sink current. To turn off an LED, the IO pin will be set as an input (high impedance). Schematic shown below.

The issue is that the PIC documentation says that any applied voltage to the pins must not exceed Vdd + 0.3V. With the LEDs off, a voltage of about 3.8V will placed on the pins. (6V - 2.2V = 3.8V). 2.2V is the forward voltage of the LEDs, and Vdd is 3V.

In order to address this, I’m adding a series diode to the current limiting resistor, in order to drop the voltage placed on the IO pin to be below 3V. (Shown in schematic).

My questions:

Will this circuit function as intended (over the range of battery life 1.0V-1.5V)?
Will a Zener diode be better for producing a definitive voltage drop?
Are there better ways to accomplish this? (The PIC needs to remain powered by 2 batteries. Other parts of the system require 6V, so can’t route the remaining 2 batteries to be in parallel with the first pair.).

The use of 4 batteries to power the LEDs is to spread the power consumption, as well as to ensure that the LEDs can still be powered even when the individual batteries deplete to below 1.1V. Otherwise 2 x 1.1V will barely meet the 2.2V forward voltage. Please note that the 6V is required for other parts of the system; not just for the LEDs.

Haven’t been able to find Zener diodes of 1V. Using standard diodes will provide a 0.7V drop, which doesn’t leave much margin for preventing damage, especially if the initial battery voltage is a bit higher than 1.5V each.

PCB space is VERY tight. Strong preference to keep part count to a minimum. ie. no room for a handful of transistors or additional power regulators.

Thanks all device!

#2

Umm, did you really mean to post this here? Its not entirely related to what the forum is for (Synthesizers/Mutable Instruments products)

#3

I think it’s cool, I would have looked a bit at it and answered if I wasn’t so tired.

#4

There is an almost identical post here: https://electronics.stackexchange.com/questions/346055/efficiently-powering-multiple-leds-from-higher-voltage-than-used-for-microcontro

But the users name is different and there are no links within the post either…
Suspect or not, it looks like there are some good answers over there.

#5

You could use a transistor as a driver/switch for the LEDs.