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My first ever project......your thoughts please.

Discussion in 'General Electronics Discussion' started by Craious, Feb 1, 2016.

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  1. Craious


    Feb 1, 2016
    Hi there guys n gals.

    I've just delved into making a circuit with flashing LEDs and it seems to only last a short period, 48 hours, was wondering if I could ask someone to see if I've made a beginners boo-boo please.

    I followed some youtube vids and worked out my resisters at 300ohm using

    LED forwarding voltage is 2.5V to 3V - I used 3V

    Power supply 9V battery


    LED is 2mA so 6V/0.02A=300Ohm

    Next reciter up is a 330 Ohm.

    So I created a parallel circuit with each LED having its own resister.

    | |
    |_______resister_______LED _____|
    | |
    + ----------------|-----------resiter--------------LED---------|--------------- neg

    Total 5 LEDs.

    Have I misunderstood how it's done please?

    My reason for doing this is that I plan to light my scratch build models and eventually introduce speakers & motors, but baby steps at the mo!

    Any thanks for any suggestions.
  2. Craious


    Feb 1, 2016
    Oops my little diagram didn't come out properly so here's a pic of my circuit.

    Attached Files:

  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    2 mA = 0.002 A -> 6V/2mA = 3kΩ.
    With 300 Ω the currrent is 20 mA
    5 LEDs with series resistors in parallel = 100 mA.

    A typical 9V block battery has ~500 mAh capacity. Your battery should last ~ 500 mAh/100 mA = 5 hours.

    A few notes:
    1. Your circuit is very inefficient. Of 9 V available, only 3 V are used to create light in the LED, 6 V are wasted as heat in the resistor.
      Use 2 LEDs in series (-> 6V), drop 3 V across the resistor -> 3 V / 20 mA = 150 Ω. Use three of these strings in parallel.
      Total current consumption is now 3*20 mA = 60 mA -> battery lasts ~8.5 hours.
    2. Have a look at our ressources.
    3. The correct symbol for an LED is a diode with an arrow pointing away from the LED. It is also polarized (haas anode and cathode).
      The symbol you used is commonly used for incandescenent lamps and is mesleading.

  4. Colin Mitchell

    Colin Mitchell

    Aug 31, 2014
    Flashing LEDs like to drop a voltage a little bit higher than 3v. You can get all sorts of flashing LEDs on eBay that blink and flash and flicker and change colour.
  5. Craious


    Feb 1, 2016
    Harold, thanks for the info. I know I'm in at the deep end but I'd never even considered the longevity of the battery in the circuit! How naive is that - I just assumed it would last a long time not 5 hours. I assumed that the larger the V the more life it had, but the V is the push around the circuit isn't it, doh.

    So really I could use a lesser battery next time around. I'm assuming too that the series part of the new circuit would only require the one resistor on that arm with the two LEDs. I'll take a look at the resources again but I find them a tad overwhelming as I'm not the brightest spark in the box! Point on the symbols noted - cheers. Thanks for the reply again, appreciated.

    Colin Mitchell - I looked on eBay and did get the bits from there as they're the cheapest in case I fluff it all up! Again, at this stage of my learning curve it's mighty overwhelming knowing what to look for! Interesting to hear about the flashing like "to drop a voltage a little higher than 3V." Does that mean 4V or 2V - bit flummoxed with the terminology of 'drop a voltage' and then 'higher than 3V"? Sorry to be dense.

    Cheers guys.
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