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
    3
    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?

    Any help you could provide would be greatly appreciated.
     
  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
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    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.​
     
  5. circuit_newbie

    Thread Starter New Member

    Nov 4, 2014
    3
    1
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