AVR Freaks

Agree with the above.

The LEDs and their current limiting resistors would be better placed on the high side of the transistor, with the transistor switching to ground.

An NPN transistor will work as the switch with a base resistor.

I might suggest changing the transistor to a small NFet, (2N7000, for example).

It will be a more efficient switch, and will burn up less energy as heat within the transistors.

As an NFet it doesn't need a series "Base", (Actually Gate), resistor, but one often puts a high value resistor from the Gate to Ground to hold the Gate low as the micro powers up, and the I/O pin is initially an input, and isn't actively driving the transistor.

Certainly it is possible to put a Fet in the Vcc or the Ground line, to turn all of the LEDs an and off together.

That requires a little bit of design work to drive a PFet high side driver connected to Vcc from the micro's I/O pin.

You haven't stated if the Vcc for the LEDs is the same as the Vcc for the micro or not, (e.g. is everything powered by +5V, or are the LEDs powered by +12V, and the micro by +5V, etc.?)

Another option, of course, is to put the Led and its resistor on the high side of the transistor, and the transistor connects to Ground.

Now put a Nand gate between the micro's I/O Pin and the Transistors, one Nand gate for each transistor.

The micro drives one input to the Nand, the PWM signal drives the other input to the Nand.

Now the PWM will control the LED's on time, and the micro is essentially a Master On/Off for each channel.

That would be an old school hardware approach to solving your problem.

Although wiring up a large board with 30 LED drivers would be a fun hardware project, as mentioned above LED driver chips make this low level hardware driver approach obsolete.

Finally, with a "fast enough" micro, one could do the PWM for each channel in software, and just use the simple transistor driver for each channel, without implementing a hardware PWM dimmer driver.

Lots of options.

JC

I don't know the PCA9685 but a regular Maxim MAX7219 can control 64 LEDs in an 8x8 matrix.   And control brightness.

All with 3 GPIO pins (or use SPI bus).

David.

30 leds- you could multiplex these and probably avoid the port expander. Other possibilities is to use a device like a max7219 which is probably much the same cost as your port expander - and save on the thirty transistors/resistors. There’s also a chinese chip used in a lot of the 4digit displays you see on alibaba, the TM1637. Only usd $0.1909 for 1 at lcsc.com. Not sure if it does individual leds, so read the datasheet.