Attaching LEDs direct to 3V CR3032

I have questions about attaching LEDs directly to 3V CR3032 batteries.

These https://shop.pimoroni.com/products/5mm-rainbow-led-pack-of-10 are suggested as an option for use with https://shop.pimoroni.com/products/business-beasts-led-craft-kit The description for the beasts says to buy 3V CR3032 but the description for the rainbow LEDs says:

In our testing, these LEDs will work happily at either 3.3V or 5V.

So given that 3.3 is more than 3 is swapping in a rainbow LED actually going to work? If 3.3V wasn’t mentioned I’d just assume it would given the rainbows being suggested as an option.

(I’m replying to my own post because I have two more questions but couldn’t put them in the first post because as a new user the forum won’t let me put more than two links in a post.)

Would I be correct in assuming that https://shop.pimoroni.com/products/led-5mm-pack-of-10 are the same LEDs that ship with the beasts so will work when attached direct to a 3V CR3032?

And what about https://shop.pimoroni.com/products/led-10mm-pack-of-5
I realise the answer is probably in the data sheets which I don’t know how to interpret.

The Rainbow LEDs will have a logic chip inside them so can operate at standard “bus” (PSU) voltages of 3.3 and 5v. A fresh CR3032 might have an open-circuit voltage of over 3.0v so will work as well.

Normal single-colour LEDs however must not be connected directly to a power supply because, without a current limiting resistor, the LED will just pop like a fuse.

The only exception is when using a CR3032 as a power source - the internal resistance and limited power capacity of these batteries will usually safely limit the available current for a LED. Adding a small low-value resistor in series will provide some additional protection.

NB driving a LED directly off two AAA or AA batteries will also blow the LED as there’s much more current available to “pop” the LED.

The forward voltage of the LED will vary slightly based on what color it is. That being said its right around 3 v for the 5mm LEDS. They mention 3.3v simply because that is the logic level the Raspberry Pi uses. And the Pi also has a 5v bus, thus the mention of 5v.
The more voltage you apply the brighter they will glow, up to a point. They have a max voltage, that if you exceed it, will burn out the LED. They should work on the 3v battery, thats the battery in the battery powered tea lights I have here at home. The ones that replace the little candles. The LED will just slowly dim as the battery runs down. Once the battery voltage goes low enough the LED will stop being forward biased and go dark.

Now that’s useful to know as it’s exactly the sort of thing I’d likely try.

Forward voltage means voltage required to make the LED light up, right?

I’m sure that’ll be very useful info once I know what a 3.3V logic level and a 5v bus are ;)

Yes the forward voltage is what you would measure across the LED when it is forward biased and conducting (lit up). Apply a voltage lower than what is required and it won’t conduct and won’t lite up.

The logic level refers to the voltage that constitutes a logic high 1 or a logic low 0.
On a Raspberry Pi a logic 1 is 3.3V and a logic 0 is 0 volts.
Some devices like my Arduino UNO us 5V logic where a logic high is 5V. Feed 5V into the Pi’s GPIO and you will damage it.
The Rasperry Pi has a 5V power supply output and a 3.3V power supply output. Also know as a bus.

One other aspect I’ve seen a lot of is when folks are calculating the necessary dropper resistor for a LED they might look at the spec sheet and go “humm, 30ma MAX so lets say 20ma” and calculate accordingly.

For info 20mA is 50 ohms per volt to drop but have you seen just how bright a LED glows with 20mA flowing through it? You could light a room with some of the high-efficiency LEDs at that current.

For newer LEDs you can get a perfectly usable glow with a mere 2mA - sufficient if you are just using it as an on/off indicator - that requires a dropper resistor of 500 ohms per volt to drop.

For a 3.3v power supply rail with a 1.8v red LED that’s 3.3-1.8=1.5v x 500 ohms = 750 ohms (or more)

That is useful info, thanks. :D

Well I got some of the rainbow LEDs, stuck one to a CR3032 and I’m pleased to report it flashes merrily away. It looks quite festive wrapped in a post-it note and balanced upright in a little bulldog clip.