Addressable LEDs at 3.3V - Page 1
Forum section: electronics / projects

author: aeberbach
author_rank: Regular Contributor
timestamp: March 31, 2020, 04:24:26 am
content: I am working on a keyboard design and the processor hasjust aboutenough GPIO pins to decode the keyboard matrix - 87 keys - and everything else. As it is a nRF52840 it has a lot of GPIOs as well as supporting USB and bluetooth and I will add battery management, probably MCP73831 same as the Adafruit board I am prototyping with. I would like to use a single lithium cell where it is charged if the keyboard is connected by USB at the same time as it powers the keyboard. If connected by bluetooth the battery will charge but mode of communication will not switch over unless a key sequence is used to selected between bluetooth/USB comms. Entering bluetooth pairing should display some indicator too as should low battery. I would like to use a RGB LED to indicate different things.A conventional single RGB LED would required 3 GPIOS. A regular keyboard uses three LEDs. So three GPIOs to control them if using plain LEDs or 5 if using just a single RGB and two basic ones. Using addressable LEDs they would still require power and ground but could all be driven by a single GPIO. Unfortunately the addressable LEDs I can find can work at 3.3V but don't really like to - they are said to stop working when using more than a certain number but nobody says exactly what that number is, probably it depends in battery charge state. In any case they are somewhat dim at a lower voltage.These things may end up requiring far too much current to be worth including but it would be a nice option to have.Maybe I have enough GPIOs to use 5 for keyboards, maybe only 3 but it's good to use less.

author: ademuri
author_rank: Newbie
timestamp: April 01, 2020, 08:30:11 pm
content: Do you need the LEDs to run off of 3.3v? You could instead run them off of the battery. The spec for the typical WS2812 LEDs is that the signal must be at least 0.7 * VCC, so assuming a max battery voltage of 4.2v, you're safe (0.7 * 4.2 = 2.94 < 3.3v). One downside is that this type of LED has a quiescent power draw of ~0.7mA.If you're worried about GPIOs, you could also use a shift register or LED driver chip to drive standard LEDs.

author: aeberbach
author_rank: Regular Contributor
timestamp: April 05, 2020, 09:48:49 pm
content: Thank you! I will definitely have to look into the quiescent current, that could be a killer when running from battery.

author: Buriedcode
author_rank: Super Contributor
timestamp: April 05, 2020, 10:35:54 pm
content: You may struggle as the forward voltage of the green and blue dies is often >3.3V. They may work at a lower voltage but add to that any possible constant current driver dropout these devices use to drive the LED's (to get consistent brightness from all three dies) and they could be substantially dimmer. With that said, I suspect they just use resistors as I've found the colour to vary significantly with varying VDD.I would say 3.7V is the minimum these should run at. If its just for indication, rather than specific colours then 4.2 down to 3.7V should be OK, and would satisfy the VCC*0.7 requirement for the data line.As a more expensive option, you could use a small 8 pin micro to drive an RGB LED. Less quiescent current and more control - at the cost of more complexity, code to write etc..