2x AA, Flashing LED and Resistors - is my maths wrong?

Discussion in 'General Electronics Chat' started by BrentD, Mar 3, 2013.

  1. BrentD

    Thread Starter New Member

    Mar 3, 2013
    EDIT - Sorry, I meant to post this is the Projects forum - MOD's please move if necessary.

    Hi all, new to the forum (and electronic circuits) and would really appreciate some help. It's a really basic project but I'm sure I must have the maths wrong somewhere.

    I have an alarm system which does not have a constant flashing LED on the bell box and am wanting to add one.

    Length of battery life is my primary concern, but the most I can fit in the bell box is 2x AA's. So, I have a 6000mcd "ultra bright" flashing (approx. 1.5Hz) red LED (20Ma typical, 1.8v - 2.4v forward voltage) running off 2x Panasonic Evolta AA's (so 3v and circa 5200mah total).

    So, after reading up, resistors are the easiest way to go for battery life. The only resistors I currently have are a 1k and a 10k from another project kit, so I soldered them both onto the positive line in series, not expecting the LED to light - however, it does! It's MUCH dimmer than normal but is night time visible at 20 feet which is fine. According to my maths the resistors limit the current to around 0.1Ma which is why I am confused - from what I've read around the net, LED's don't generally light below about 1Ma. And further calculations reveal that the batteries will possibly last up to almost 3 years which I find difficult to believe.

    Is all of this possible?

    And apologies for the long post but there's hopefully enough information for help.
    Last edited: Mar 3, 2013
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    LEDs need a minimum voltage to be forward biased and conduct. More current past that point results in more light.

    The materials and design of LEDs has improved by huge amounts even in the past 5 years, so the "standard" of 15mA for an LED makes a new one, especially the "super-bright" types, be blinding. This also means that they will produce visible light at very small currents.

    If you only need an indicator that you look at (vs. use as a flashlight), you can run the new ultra-bright LEDs at far lower currents (the minimum voltage must still be available for the forward biasing). The resistors limit the current, an LED can be approximated by a dead short over X volts (the Vf of the LED), so Ohms Law can be used to calculate the resistor size for the amount of current, or for the current you'll end up with only having a fixed resistor size.

    If the LED is a "Self-Flashing" type, or has any other effect, such as an RGB LED that cycles through colors, then the requirements change a little bit. There will be both a minimum voltage and a minimum current, since there is an integrated circuit inside the package (often visible as a black dot on anode or cathode inside LED). This IC may have resistors to limit some current in addition to providing other features. If you put too large of a resistance in series, the IC doesn't get enough current to work with.
    BrentD likes this.
  3. crutschow


    Mar 14, 2008
    If one battery is 2600mAh (which I think they likely are) then two in series will also have a capacity of 2600mAh.

    Yes your projected battery life is many years (If you include the fact that the LED is only on for a short duty-cycle the calculated life may be longer than the batteries shelf life). Makes you realize on little power a modern LED takes.
    BrentD likes this.
  4. spankey666


    Nov 30, 2011
    your bell box should have 12volts supplying the circuit board inside, siren, back up battery etc. why bother with batteries ? just run it off the 12v supply
    BrentD likes this.
  5. BrentD

    Thread Starter New Member

    Mar 3, 2013
    Thanks very much for the replies guys.

    Yes, it's a self flashing LED which I failed to mention, sorry. I knew about the IC which is why I was even more surprised that it would light with an apparent 0.1Ma.

    Yes, I'm sorry, 2600mAh, not 5200mAh - doh! And yes, it does!

    I've shied away with messing with the actual bell box as I know little about electronics and don't want to destroy it. It's wireless and runs on 4x D batteries (so 6v?). I've just changed the batteries after just over 2 years of life. They are 15000mAh so I guess a 0.2mAh LED will have little impact on their life? (I've decided I'd like it a little brighter than the 0.1 mAh provides I think). Is it a case of just soldering the LED wires onto the contacts with a 27kohms, 1/8W (to get the 0.2mAh) resistor in series?

    Apologies for all the questions but this does seem the best solution if things are that easy!