Need Help with my first LED circuit.

Discussion in 'The Projects Forum' started by Creatively Deranged, Dec 2, 2011.

  1. Creatively Deranged

    Thread Starter New Member

    Dec 2, 2011
    Hello all! I've got a super simple LED circuit that I've wired up as my first foray into DIY electronics. Unfortunately, it doesn't seem to be working quite right, and I'm not sure where I went wrong. It lit up alright for a bit, but it seems to have stopped. I checked my wiring with a multimeter, and everything seems to work alright, but no light. I have a feeling I picked the wrong resistor and burned out my LED, but I don't know for sure.

    I'm hoping someone who knows what they're doing can advise me so I can move forward rather than being confused and discouraged.

    Here's what I've got going on (If I've left out any necessary information just ask):

    The circuit is running on a 3v lithium coin cell battery (cr2032). Following the positive lead, it goes through a small tact switch, then to a 68 ohm, 1/2 watt, 5% tolerance resistor. Next it hits the LED. Here's the details off the package:

    Intensity: 80mcd (typical)
    Wavelength: 660nm (typical)
    Viewing Angle: 154 Degrees
    FW current: 20mA
    FW Supply: 1.9v (typical), 2.4V (max)

    Coming out of that, there's a second tact switch, and then into the negative of the battery.

    Hope someone can help me out!
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    The resistor seems to be high enough to be safe, as even with only 1.9V Vf, with 3V input you will only get about 16mA.

    One possibility might be that you have wired the switch(es) wrongly, so that the battery is shorted out instead of disconnected to turn it off. That would ruin it very fast. Why are you using two switches?
  3. LoganFife

    New Member

    Feb 7, 2010
    a quick and dirty way to test your LED is to use your multimeter.
    set to the diode setting if you have one, or the lowest resistance setting, touch each leg with a probe and see if it lights. (check both ways to be sure - it will only work one way round).it will probably be very dim (if its working), so look closely.
  4. SgtWookie


    Jul 17, 2007
    You may have damaged one of the tactile switches. Some of them are rated very low for current, and will easily burn up.

    If you measure voltage across the switch when it's supposed to be open, and no voltage across the switch when it is closed, then it is probably OK. However, you have two switches in series. You will need to press both buttons at once while checking the voltage across them.
  5. Creatively Deranged

    Thread Starter New Member

    Dec 2, 2011
    So I've been fiddling with it a little more, and every once in a while it'll light up again. I'm starting to think it's a loose connection somewhere. Considering how little I've done any soldering, I wouldn't doubt it's my issue. To address the other possible problems:

    SgtWookie: Checking the voltage on the switches seemed fine. Checked the packaging, they're rated 50mA and 12Vdc.

    LoganFife:I tested the LED (after looking up the user manual online and figuring out how to do it) and it DID dimly light in one direction.

    Adjuster:Incorrect wiring is one I worry about a bit. There was a stunning lack of instructions available for the little surface-mount tact switches I bought so I sort of trial and errored them until it worked. Lets see if I can describe it here. They look something like this one:
    With three tabs on one end and two on the other.
    I've got the circuit through the one furthest left on one end and furthest right on the other. The only forseeable problem I could see here is if the switches are directional, in which case I probably have one backwards.
    The reason I have two switches in the circuit comes down to the purpose of it- I've got this set up on the cover of my sketchbook, simply for decoration. Two switches makes it harder for it to accidentally turn on in my backpack and drain the battery.

    For what it's worth, I'm attaching an image (sorry for the quality, I've only got my cellphone camera) of the circuit. Also note, at the moment I've cleaned off the glue on the wire between the resistor and LED and it seems to be working now. Not sure why that is.

    Thanks for the help guys! I clearly chose the right forum to post on. :D