# Help with Multiple LED, Multiple Switch Circuit

Discussion in 'The Projects Forum' started by circuit_newbie, Nov 4, 2014.

1. ### circuit_newbie Thread Starter New Member

Nov 4, 2014
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1
Hi everyone, I recently got an idea for project for a friend's child, but Im afraid I'm a bit out of my depth. The project is based on this one here: http://www.instructables.com/id/Childs-Toy-Light-Switch-Box/

What I wanted to do is do something similar, but turn it into a baseball scoreboard that would have 7 switches and 7 LEDs for tracking Balls, Strikes, and Outs (3 Green, 4 Red).

These are the Red LEDs I wanted to use: https://www.sparkfun.com/products/10632
These are the Green LEDs I wanted to use: https://www.sparkfun.com/products/10633

The plan was to have 1 main battery power source and 7 sets of Switch/LED/Resistor wired in parallel. However, I am having trouble figuring out exactly what power source and resistors to use.

My thought was to use 4 AA batteries, but would that be sufficient (as a rough goal I was looking for about 4 hour run time if all 7 LEDs were on, but I'm not sure how practical that is)? Since parallel circuits drain a lot more power, do I need to have 1 battery per Switch/LED/Resistor or can I use a single power source? Should I be using a 9 volt battery instead of AAs?

The other problem I was having was figuring out what resistors to use. All the resistor calculators I have found don't seem to accommodate what I want to do as they don't allow to for multiple color LEDs or the fact that anywhere from 1 to 7 LEDs might be on at the same time. How would I do the calculation taking those parameters into account?

Finally, as this will all be housed in an enclosed wooden box, do I need to be worried about the heat it produces and put in some vent holes?

2. ### wayneh Expert

Sep 9, 2010
12,086
3,025
Four AAs should be fine. Worst case, all 7 LEDs are on. Let's say you run them at 15mA. (It's wise to stay below the rated max current. They'll still be plenty bright and will last longer.) That means a total of 7 x 15mA = 105mA. Good AAs are rated at ~2400mAhr, so you should get about 2400/105 = almost 24 hours. I would not use a 9V battery.

The resistor calculators don't consider parallel strings because they don't need to. One string has little to no effect on the others, just as plugging a lamp into your wall socket has no impact on other devices in your home. Of course the battery voltage will sag a little as more LEDs are lit, but the online calculators don't include that. They assume a constant supply voltage.

Heat is no concern for this project.

FWIW, you could use 2 red LEDs in series instead of just one, and use a lower ohms resistor. The green ones require more voltage and I don't think you can rely on getting two in series to work. You might try it if that seems like something you would like.

Last edited: Nov 4, 2014
3. ### circuit_newbie Thread Starter New Member

Nov 4, 2014
3
1
Thanks wayneh!

4. ### wayneh Expert

Sep 9, 2010
12,086
3,025
Your welcome. Be sure to come on back and show us how it turned out. I think my grandson might like something like this although, at 20 months, he's starting to get fairly skilled at using any iPhone he can get his hands on.​

Nov 4, 2014
3
1