I have a project in mind but I am unsure best way to wire the LED’s (parallel/ series or both)

Hi all,
I am trying to build a board (using perfboard only) which is a little harder than I thought it would be.

So the idea is, with a battery (Not 5v mains) power source.

I have a PP9 large 9v 5000mAh battery (not the small standard PP3 9v 500mAh) as they hold much more life in them unlike the standard PP3 battery.

I have just over 100 LED’s 3mm (Red) and I have arranged them so they spell out a name (using multiple LED’s for each letter)

My issue iswhat is the best safest way to wire these LED’s so that they don’t kill the battery straight away and can be switched on and off when needed.

I know this is probably a big challenge I set myself, which is why I am now stuck.

Any kind of help would be appreciated

kind regards
Paul

One option would be to put 4 LEDs in series (dropping voltage around 7.2V to 8V) with a small resistor. Then you parallel 25 sets of those 4 LED strings.
Assuming you draw 10ma per string, 25 strings will draw 250ma. Battery would last, at most, about 20 hours assuming the voltage drop is not too much such that the 4 LEDs in series drop out due to low voltage.
If you can reduce the current per string, battery will last longer.

paul_b_78 said:

can be switched on and off when needed.

Click to expand...

Are you asking about individual control of each of 100 LEDs, so that andy of them can be turned on and off individually and randomly? If so, that is not a trivial task, and usually is done under computer or microcontroller control.

OR - are you asking turning on and off pre-wired groups, like all of the LEDs for one letter?

ak

For the little 3mm LEDs, you can put 4 in series with a 470 ohm resistor to get 5mA per strand. Repeat 25 times and you'll draw 125mA. Check for brightness and use anything from 220 ohm to 1k ohm for the brightness you want. Don't go lower than 220 ohms because the 3 mm LEDs are usually limited to 10 mA.

Remember, guys, a "9V" dry battery will drop down to 6V over its discharge life; it will be down to around 7.5V with roughly half its useful capacity remaining.

Four LEDs in series will be fine when it is brand new but will dim long before the battery is anywhere near dead.

I'd suggest a little boost regulator board set for eg. around 12V and use six LEDs in each chain, with a 100 Ohm series resistor.
The brightness can be adjusted by changing the regulator output voltage up or down slightly.

A boost reg such as one of these would likely be reasonable; you can get similar things on Amazon:

It introduces a small inefficiency, but being able to get all useable energy from the battery and lower resistor losses would give it a massive advantage overall, I think.

rjenkinsgb said:

Remember, guys, a "9V" dry battery will drop down to 6V over its discharge life; it will be down to around 7.5V with roughly half its useful capacity remaining.

Four LEDs in series will be fine when it is brand new but will dim long before the battery is anywhere near dead.

I'd suggest a little boost regulator board set for eg. around 12V and use six LEDs in each chain, with a 100 Ohm series resistor.
The brightness can be adjusted by changing the regulator output voltage up or down slightly.

A boost reg such as one of these would likely be reasonable; you can get similar things on Amazon:

5Pcs 2V-28V Output 2A DC-DC MT3608 Step Up Boost Power Supply Regulator | eBay

Find many great new & used options and get the best deals for 5Pcs 2V-28V Output 2A DC-DC MT3608 Step Up Boost Power Supply Regulator at the best online prices at eBay! Free delivery for many products!

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It introduces a small inefficiency, but being able to get all useable energy from the battery and lower resistor losses would give it a massive advantage overall, I think.

Click to expand...

The OP was already having trouble with a switch and some LEDs, I didn't want to introduce more complexity for the sake of a bit more efficiency. I'm guessing this is a one-time-use gadget.

You guys have been fantastic, thank you.
I did wonder if I needed a boost regulator (or buck converter).
I have one of these laying around spare so will make use of it.
I’ve just realised, one of the letters will have less LED’s (which obviously isn’t a problem as I can wire these LED’s so that the quantity is the same throughout until I get to the last set of lines which will end up having less LED’s.
Could I just change the last resistor to a different value or have I got to try and get all the LED’S to be equal (which I think will become a problem since I cannot see anywhere in which I can adjust the letters to make way for the equal quantity of LED’s without making the letters look o

Again really appreciate your support guys.
Kind regards
Paul

Apologies Analogkid I missed your question originally...
The plan is to have them all turn On & Off at the same time.
And I want to make the most use out of the newly purchased larger PP9 battery.
Kind regards
Paul

paul_b_78 said:

Could I just change the last resistor to a different value or have I got to try and get all the LED’S to be equal (which I think will become a problem since I cannot

Click to expand...

The simplest option to guarantee equal brightness is hide the "spare" LEDs somewhere or black them out, to keep all the strings the same size. That allows the brightness of all LEDs to track equally with any adjustments to the voltage.

Or:
You can use a higher resistor value to set around the same current as with the full strings, but ideally you should play around with the rest of the setup and decide the final brightness (voltage & resistor values) first.

Then measure the voltage across the normal resistor, and measure again across both the resistor and however many LEDs will be omitted from the "short" chain.

Increase the resistor value for the short chain by the ratio of those two measurements, or as near as possible, to get the same current in that short chain.

The downside is that the full strings will be rather more sensitive to voltage variations than the short string, which is why I suggest setting the final brightness first.

This is all great advice and I have learned a lot.

I couldn’t agree more with Charlesli

I am now in the process of actually building my project.
Thank you to all of you that left replies

You can always use LDO as a current source to aid in brightness matching
of LED strings. That then eliminates variation due to drive current variations
of individual strings. Leaving led to led process variation the principal remaining
error contributing to brightness matching.

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Regards, Dana.