Can I somehow use one PWM output to set various LEDs to different brightness?

Question

I would like to build a project in which 15 groups of LEDs are set to a brightness level. I have an Arduino with digital PWM outputs (which can mimic a range of voltages.) How can I do this?

One idea: connect each LED to a capacitor and use a transistor array to charge these capacitors with a voltage which will produce a controllable brightness in the LEDs? I was thinking that I could charge each capacitor with a different voltage, then re-charge them every 10th of a second of so to maintain the set level of brightness.

If so:

Will I need to use a small capacitor to turn the initial PWM output into a voltage?

How big will the individual LED capacitors need to be to maintain illumination?

How often will the capacitors need to be charged?

How will I need to use resistance in the circuit?

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

This is not exactly what you asked for, but if you want to control multiple LEDs with a single signal, I would look into WS2812 (and similar) family of LEDs and LED drivers. They only need one wire signal(plus supply rails) to control a very large number of LEDs (the more LEDs, lower the "framerate" you get).

Any "diy" implementation of a single PWM to drive multiple LEDs with independent brightnesses would require much more than a "transistor array" and capacitors. (although, yes, an IC is an array of transistors, but what I mean is that doing it from scratch might not be viable solution).

If you are willing to use more than one signal, you can look into ShiftPWM (based on 595 IC's), or into i2c based port expanders, such as PCA9685.

TLC5940 might also be useful.

Answer 2

Yes, it is possible to connect each LED to a capacitor. And use a PWM to control the power going into the capacitor/LED. But we don't understand why you think you need any capacitor to implement brightness level? LEDs are typically directly driven and use the human persistence of vision to integrate the brightness. That allows for faster control of brightness.

Answer 3

Will I need to use a small capacitor to turn the initial PWM output into a voltage?

NO, you don't need any ANY size capacitor to control the brightness of LEDs.

How big will the individual LED capacitors need to be to maintain illumination? How often will the capacitors need to be charged?

You don't need any capacitors at all. So you don't need to charge any capacitors.

How will I need to use resistance in the circuit?

You need resistance in your circuit to limit the current through the LEDs. Else they will self-destruct.

We typically connect the positive end of the LED (or string of LEDs) to a positive power supply, and then use a transistor between the negative side of the LED(s) and circuit ground. Then you can drive the transistor from any kind of logic-level output without having to worry about how much current the LEDs require.