LED Switch Project

Discussion in 'The Projects Forum' started by alanderego, Nov 25, 2011.

  1. alanderego

    Thread Starter New Member

    May 16, 2007
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    Hey everybody,
    I am working on a project for work.
    It is a test box that connects to a device that monitors temperature.
    I have a diagram of it below.
    It is a simple device that uses 2 "D" batteries and some incandescent lamps.
    The thing about this box is that when the switches open and close (most times 2 lamps are on, but in a worse case 3 switches...Never all 4 switches), the lamps brighten and dim and it also eats batteries.
    I want to make something using LEDs which will not change intensity as the switches change state.
    I have been reading Bill Marsden's Index Page (thanks Bill for the post!), specifically Chapter 2: Current Limiting.
    Figure 2.2 looks like something that might work for my application, but I am not sure if it can handle an open circuit like that.

    Any thoughts?

    Thanks in advance,

    Alan
     
  2. DickCappels

    Moderator

    Aug 21, 2008
    2,664
    634
    That circuit should work fine. The current through the LEDs will be about 40 ma -is that ok for the LEDs?
     
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,176
    397
    Do you have a LED color pref? With all red the 2 Ds will last forever; with white an additional battery is needed, say 3 ea AAs. A single LED per SW should be OK & verry possibly no V reg required with only 20 mA per led.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    You won't be able to use Bill Marsden's schematic with just a 3v supply.

    You need to give us more information about the LEDs you want to use.

    If this piece of gear you're testing is turning on lamps to control the temperature, then replacing the lamps with LEDs won't work very well, as the LEDs won't produce much heat.
     
  5. alanderego

    Thread Starter New Member

    May 16, 2007
    6
    0
    Thanks for the replies.
    I plan on using some plain old red LEDs (they should be 20mA).
    The power supply is not an issue. I'll get something to work with it.
    This is for shop testing, So I can use a wall wart or batteries.

    SgtWookie, The device is used to measure temperature then closes a dry contact. The test box does not see any heat.

    Right now, the only concern I have is if the circuit can handle 4 open legs and one closed leg.

    Thanks again for the help... This is very confusing to me!

    Alan
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    "plain old red LED's", eh? Well, the old style were 1.7v @ 15mA. If these are modern red LEDs, they will be around 2v @ 20mA.

    At any rate, you really won't need much current through them in order to see them light up, especially the newer super-bright LEDs.

    If you use 100 Ohm resistors in series with the LEDs, you will be OK with a 3v supply. That's using one resistor per LED.
     
  7. alanderego

    Thread Starter New Member

    May 16, 2007
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    0
    "Plain Old LEDs..." yes, poor choice of words...

    Thanks SgtWookie for the info, but can I just add 2 more "legs" to the circuit without changing R1 and/or R2 (say using 3vdc)?

    Alan
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    I'm sorry, I guess that I was not clear enough.

    Here, I have drawn a schematic equivalent to what you posted before, only using LEDs instead:

    [​IMG]

    You can continue adding more switches, resistors, and LEDs off to the right if you wish - or remove some.

    I show green LEDs, but you really need to use LEDs that have a Vf of 1.7v to 2v; which usually means red LEDs only. The 100 Ohm resistors will allow ~10mA current if the batteries are fresh and the LED Vf is 2v @ 10mA.

    The LEDs and the batteries should last much longer than when incandescent bulbs were used.
     
  9. alanderego

    Thread Starter New Member

    May 16, 2007
    6
    0
    Thanks SgtWookie for the diagram...
    I'll wire it up and post my results...

    Thanks to all for your input.

    Alan
     
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