Easy question on flashing LED circuit

Discussion in 'The Projects Forum' started by dogs2u, Dec 18, 2008.

  1. dogs2u

    Thread Starter Member

    Dec 18, 2008
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    How to build a cheep circuit with 3 GREEN leds that flash. Most important is battery life. The longer the better, as I'd like to run it 24 hours a day.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Scroll about halfway down in the attached PDF file for a novel solution.

    The inductor will be your challenge.
     
  3. Wendy

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    Mar 24, 2008
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  4. SgtWookie

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    Jul 17, 2007
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    If our OP wants them to flash, my proposal won't work.

    It's simple, but for a steady light.

    However, a 556 with 1/2 controlling the RESET input of the other half could accomplish what the OP's looking for.

    Want to tackle that, Bill?
     
  5. dogs2u

    Thread Starter Member

    Dec 18, 2008
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    Look at this circuit but it only runs for 196 hours on the battery. I was looking for something that ran at least 3 months. I don't care if the leds flash together or at random. A flash a second is OK.
     
  6. Wendy

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    Mar 24, 2008
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    Dang Wookie, why didn't you point that out before I started mine? :D No problem, I'll draw it up, from what I've seen either approach will work. I might even have to try that circuit sometime.

    To dogs2u, I'm one of several that draw schematics around here, so what I've just signed up for is to do the Joule Thief with slower flash rate using either qty 2 CMOS 555's or a CMOS 556. This will extend the life, how much I don't know.

    An alternate is to put 3 LEDs w/ a resistor each in parallel on my design, and use C or D cells. I don't know how long mine will last either, but the circuit I'm using as a insperation can flash a single red LED for more than a year using a single D cell (look up the spec sheet for a LM3909, which is an obsolete chip).
     
  7. Wendy

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    Mar 24, 2008
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    OK, here are the schematics from the site Wookie suggested redrawn, using a CMOS 555 and 556...

    [​IMG]

    Parts List
    U1,U2 - Two 555s or one 556
    Q1 - 2N2907A or equivanent
    Q2 - 2N2222A or equivanent
    D1,2,3 - Green LED
    C1 - 1µF Capacitor
    C2,3 - 0.01µF Capacitor
    C4 - 470pF
    R1 - 1.2MΩ
    R2 - 150KΩ
    R3,5,7 - 1KΩ
    R4 - 1MΩ
    R6 - 10KΩ
    L1 - > 200µH Coil, High Q


    Basically, if we cycle the lights on 1/10 of a second once a second it should extend the battery life, maybe up to 10X (though I doubt it would be that much). I think I'll breadboard this one too, I'm curious how well it will work (never been built yet).

    BTW, I recalculated the freq of the Joule Thief oscillator at 146Khz. My bad.
     
    Last edited: Dec 22, 2008
  8. dogs2u

    Thread Starter Member

    Dec 18, 2008
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    Bill,
    Couple of questions, but first I want to thank you for your quick response. The Vcc power will be for a 1.5v battery, right? Also the flash rate of 1/10 per second... does that mean the leds will flash 10X per second? If I could gain longer battery life I'd be OK with a slower flash rate. Again thanks in advance for your time.
     
  9. dogs2u

    Thread Starter Member

    Dec 18, 2008
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    Found this circuit as a replacement for a LM3909. Sounds to good to be true. Could I get away with adding 3 leds to it?
     
  10. dogs2u

    Thread Starter Member

    Dec 18, 2008
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  11. Wendy

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    Mar 24, 2008
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    Not that I can figure, though it would work great with one LED like it suggests. Someone might be able to come up with a kludge that would work.

    The advantage of the Joule Thief is it would boost voltage as a consequence, and does exactly what you describe. Usually multiple LEDs have to be in series, or parallel with a separate resistor for each. They claim the JT uses a 1½ battery, more on that later. My modification is 1 Hz (one flash per second) with a duration of 1/10 of a second, though this can be increased.

    Someone might come up with something better, and it won't hurt my feelings if you wind up using something else. I was already researching this project when you asked, a case of perfect timing. I haven't built anything yet, but I will. I'll post another schematic showing how to use 3 LEDs with my basic flasher.

    The joule thief boosts voltage in super efficient mode from what I can see. There is a single transistor version floating around for a single LED (you can google it). My designs require two 1½V batteries, since the 7555 is speced at 2V minimum, but I was going to see if I could cheat on mine much the same way the JT design does. The main reason for 2 batteries is modern LEDs drop a slightly higher voltage, but create much more light for the tradeoff. I figure your green LEDs will probably take around 2½V each.

    If you like the way I drew my schematics access my blogs linked to all my posts on the post header, I offer the package as a freebie.
     
  12. zac

    New Member

    Dec 15, 2008
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    Yes, you could add 3 LED's to that in series. But it would make sense to up the voltage to say 4.5v? You could pick up a cheap AA battery holder.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    1) Green LEDs generally have a higher Vf than red LEDs. Just one green LED would require a higher trip point to be set.
    2) Placing three in series would more than triple the requirement of the existing circuit's output capabilities. This would require the tripling the output stage.
    3) The ZTX869 transistor is expensive in small quantities, IF you can find it. Even in large quantities (1000 or more) they're around $0.42/ea.
    4) I don't know of a possible substitution for a ZTX869 offhand.
     
  14. Wendy

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    Mar 24, 2008
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    I have breadboarded it using my schematic with the CMOS 556, it works. It is a LOT brighter with 3V than 1½V, no surprise there. To me the surprise is it works with 1½V at all. This also suggests it will last a very long time with 3V, those batteries will be drained beyond most before it quits. I bought a selection of inductors from Tanner's, the first one, 220 µH, worked like a charm.

    I measured 83.3Khz out of the JT oscillator, and eliminated C2 and C4 (which might contribute) just for convienence.
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    I made a 3V Ultra-bright Chaser project with red LEDs that made its LEDs chase around and around forever and it was boring. Then I made a 6V Ultra-bright Chaser project with blue and bright green LEDs that chases the LEDs around a few times then a pause with darkness before chasing again.

    The LEDs are each lighted for only 30ms and the pause is about 2 seconds so the average battery current is very low. With the brightness set to max two AA alkaline cells last for months. It still works fine when the battery voltage drops to 2V.

    It uses a 74HC4017 and a 74HC14.
     
  16. Wendy

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    I'm a blinky light person myself. I like using the 4017, I've found you can cheat on it a bit too. It will take 10ma on the outputs, which I know is not recommended but simplifies the design somewhat.

    A lot of people miss than you can wire LEDs to create more than just a boring string, by having something like airport chasers going down a runway. I did something like that for a friends auto ashtray way back when, it really helped in the dark.
     
  17. Wendy

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    Mar 24, 2008
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    [​IMG]

    Parts List
    U1 - CMOS 556
    Q1 - 2N2907A or equivanent
    Q2 - 2N2222A or equivanent
    D1,2,3 - Green LED
    C1 - 1µF Capacitor
    C2,3 - 0.01µF Capacitor
    C4 - 470pF
    R1 - 1.2MΩ
    R2 - 150KΩ
    R3,5,7 - 1KΩ
    R4 - 1MΩ
    R6 - 10KΩ
    L1 - > 200µH Coil, High Q

    I'm trying an experiment here. I drew up the breadboard to see if it makes things easier to actually wire.

    [​IMG]

    If I use a smaller C1 it will look like this...

    [​IMG]
     
    Last edited: Dec 23, 2008
  18. dogs2u

    Thread Starter Member

    Dec 18, 2008
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    Bill,
    I'm getting ready to follow the design of your latest (very well drafted)design. I have a couple more questions.

    1. C1... 1uf cap, does this need to be an electrolytic, at what voltage?
    2. L1... Inductor; High Q; Ind 220uH; Tol +/-10%; Cur 45mA; SRF 6MHz
    Will this part meet the requirements?
     
  19. Wendy

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    Mar 24, 2008
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    Since we are talking 3V max for the cap almost anything would work, and the inductor is overkill (it will work fine). Funny, I've been kitting myself tonight for the same thing. I'll upload all three drafts (they changed as I found parts). If you have XP just click on the image after you save it, and use Windows Picture and Fax Viewer to print it. It comes out very neat.

    It is possible you will run into a problem with the 1µF cap. If it is a bit leaky the 1MΩ may not charge it properly. You may want to charge and discharge it a few times off the batteries before using it. Odds are this won't be a problem, just a heads up.

    Are you going to use 1½V or 3V?
     
  20. dogs2u

    Thread Starter Member

    Dec 18, 2008
    18
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    Settled for a Tant at 35V for the cap. I will be using 3V for the battery, if I understand from your other post that if they are wired in series the battery will last a long time. One more thing I ordered ultra bright leds with a 2.2v forward voltage. That should be ok, right?
    Thanks in advance for your time
     
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