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Bad Idea: Put 9V Flasher LED on top of 9V Battery??

Discussion in 'Electronic Basics' started by NoMailPlease, Jun 16, 2004.

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  1. NoMailPlease

    NoMailPlease Guest

    I bought a 9V flasher LED and put it on a 9V battery. 5 days later, the
    battery is dead. Did I just get a bad battery, or, should I have put a
    resistor (or something) in the circuit?

    All I wanna do is have a flashing LED. It was working great until the
    battery got drained. If there are more components required to make this
    thing last longer, can you please point me in the right direction?

  2. Rich Grise

    Rich Grise Guest

    Well, if you have a 600 mAH battery, and your LED draws 20 mA, then
    you'll get 600/20 = 30 continuous hours' operation.

    Apparently, you lucked out! :)

  3. The flasher must have a 25% duty cycle...

    Somebody posted a link to am EDN 'fleapower' indicator that would work
    for years. However, it only flashes once every 10 seconds, and that
    for 20ms. Its a low battery indicator.

    Bob Monsen
  4. NoMailPlease

    NoMailPlease Guest

    You guys are breaking my heart. I thought I'd put this LED on top of the 9V
    and it'd work for 6 months..

    Now, is it even possible (with a better circuit) to make a LED flash (say
    every 1-3 seconds) that would last for months using a 9V battery??

    If I got a lower voltage/mA LED, would it be worth the trouble or would I
    still be looking at a life of only a few days?

    Also, I bought a long life Energizer 9V to replace the dollar store one that
    only lasted 5 days. I'll let you know if the Energizer is worth 4 times the

    Thanks for all you help
  5. Joe

    Joe Guest

    You could try a 6 pack of AA's, but you would probly need some other
    circuitry to control the flash time, duty cycle and current better. What is
    the application?
    I doubt it will last 4 times as long, but it's worth a try since you already
    bought it. 9volt batteries are only rated for about 500mAH. A typical LED
    uses 20mA. If you do the math, you can get an idea of what you would need.
    Alkaline AA's are rated at about 2000mAH. Maybe it would run on 6volts? Have
    you tried that?

  6. NoMailPlease

    NoMailPlease Guest

    I've always had a car alarm and in my new car I figured I'd save the $300
    and just put a flashing LED on the dashboard. There was a blank knock-out
    panel on the dash, so I bought a flashing LED and mounting grommet. I fed
    the battery connector to the coin box and this is where I plug the LED into
    a 9V battery.

    The whole thing works great for at least 5 days until the battery dies. (I
    leave it flashing 24 hours per day.) I've seen articles about attaching
    flashing LEDs to the car battery, but given my tiny knowledge of
    electronics, I'd rather keep this circuit separate.

    My dream now, is to build a better circuit that would allow the flasher to
    live for several weeks. I never knew that 9V batteries had such short
    lives. If I were to create the most efficient LED flasher circuit possible,
    would it be worth the trouble. Or, would I gain just a few more hours out
    of the whole thing?

    If it's worth the trouble, do you know of any plans to build a flashing LED
    that runs on AA's, etc??

    Thanks for all your help!
  7. You may want to consider tying into your car battery. You can get
    cigarette lighter adapters, which will give you 12V. Use that to power
    a 7809 voltage regulator (which you can get anywhere electronics parts
    are sold, like Radio Shack, for example.) Use the output of the 7809
    to power your flasher.

    Your flasher will probably work with 12V with a 150 ohm resistor in
    series; it's probably worth a try.

    The other possibility is rechargable batteries.

    Bob Monsen
  8. NoMailPlease

    NoMailPlease Guest

    That sounds alot safer. I tried using 2 AA batteries to power the flasher
    LED and it works fine. I'm very interested to see how long the AAs work vs.
    the 9V..

    I also ran across a flashing circuit that's supposed to last a year at If I keep going through
    batteries or don't get brave enough to attach to the car battery, I might
    just build it and see how it performs.
  9. I like the second one. Its simple, but it will last for a long time.
    If you use two duracell batteries, it should last well over a year.
  10. John Miller

    John Miller Guest

    Standard disclaimer: use at your own risk, etc.:
    Because you mentioned that you were relatively inexperienced in electronics,
    the expression, "attach it to the car battery" is a little scary. It
    sounds like you might be thinking of attaching directly to the battery,
    which requires both fusing and the knowledge of how to design and install
    it safely (without risk of burning the car or yourself).

    The right place to tie in is at the car's fuse block. That's the
    distribution point for electrical power.

    You'll want to find a circuit with a small-value fuse (e.g., 5 amp) that is
    energized with the key off, and find a wire on the fused side of that
    circuit (when the fuse is pulled, it's dead). Then you can just tap into
    that wire using one of those 3M insulation displacement connectors (no
    stripping, cutting or soldering). Voila! And you're protected against
    accidental short circuits.

    John Miller
    Email address: domain,; username, jsm

    In good speaking, should not the mind of the speaker know the truth of the
    matter about which he is to speak?
  11. NoMailPlease

    NoMailPlease Guest

    Umm, it all sounds so easy when an electrical engineer describes installing
    the LED into the car's electrical system... With my sad grasp of
    electronics, I'm gonna have to stick with the lame 9V battery powered

    With the new Energizer battery, the LED has lasted over 5 days now. If I
    get tired of replacing batteries, I'm going to get a hold of this LM3909
    replacement: Their
    datasheet says that it'll flash for upto a year on a C cell. I'm going to
    use it to build this circuit

    If the spec sheet is relatively true, I expect this thing to flash for at
    least a few months.

    Thanks for all the help you guys have given me for this lame little project!
    I only wish that I'd taken some electronics courses in school. It all comes
    back to haunt you I guess.

  12. Actually, last night, I just built the flasher specified here:

    However, instead of a battery, I powered it using a 1F 5V
    'supercapacitor'. Its been flashing for about 15 hrs now on a 5V
    charge. If you get a solar panel from here:,
    and use it to charge the cap, I suspect the circuit will flash forever
    without a battery at all...

    Bob Monsen
  13. There are LEDs that achieve what used to be "normal LED brightness"
    (that of older red GaAsP on GaAs LEDs at 20 mA) at a milliamp or less.
    Better versions of green LEDs with wavelength in the 520's of nanometers
    get that bright at .3 to .5 mA. 600 mAH with a 10% duty cycle means .5 mA
    10% of the time for 12,000 hours, or about a year and a half.

    - Don Klipstein ()
  14. Dollar store batteries are not alkaline, and so are worth maybe around
    200 mAH or maybe even less...

    And I want to keep reminding people that there are now a large and
    increasing number of brands, part numbers, etc. of LEDs that achieve at 3
    mA or less, even as low as .3 mA, a brightness that most LEDs of 20-25
    years ago (and some current models with such old technology) required 20
    mA to achieve.

    - Don Klipstein ()
  15. NoMailPlease

    NoMailPlease Guest

    Well, well. Guess what happened today? The Energizer 9V stopped flashing
    after 5 days. Here's the sad part. The dollar store battery lasted just
    as long as the Energizer! Makes me wonder why I spent $4 on the Energizer.
    The dollar store battery brand was named "TuffGuy" (I'm serious) just in
    case you wanna stock up..

    I'm still waiting for the flasher on top of the 2 AA batteries to die. I'm
    curious to find out if they last longer than 5 days. We're at day 3 now.

    I like those .3 mA LEDs you're talking about. Do you or Bob have any idea
    what kind of mA the LM3909 style flasher ICs use up? That solar cell idea
    is pretty cool.


  16. If they are both alkaline, then there isn't going to be much
    difference in their capacity. I tried to find non-alkaline batteries
    at a dollar store recently; no luck. The store I visited only had
    alkalines. I was looking for AAs, and for a buck, I got 4 of them.
    Cheaper than costco.
    Suprisingly, AA batteries can supply current much longer than 9V
    batteries. An alkaline duracell AA battery has almost 3 amp-hours, as
    opposed to an alkaline duracell 9V, which has between 500 and 700 mA
    hours. The total energy in an AA is less, due to the lower voltage.
    The LED is still flashing using the capacitor. I hooked up that 3V
    solar cell, and it seems to have kept up up the voltage.

    And, the LED isn't one of Don's fancy LEDs. Its just a blue "high
    brightness" led bought at Halted in San Jose last year. A high
    brightness LED that takes 3mA instead of 30mA would last 10x.

    Regards, and please, start posting replies at the bottom of the
    message... :)

    Bob Monsen

  17. Yep, that's about how I would do it. But if you are scared to do the
    installation yourself, have your car shop do it for you. Takes only 5
    min so their time is a lot cheaper than a new car if you get it wrong...

    IIRC, that sort of dummy burglar alarms are even commercially available
    (so you are not the first to think about that). Whether they would scare
    off a professional car thief is a different matter.

    If you want to learn more about electronics, a real car burglar alarm
    would be a nice project to start with. Some simple circuits come to

    a) Somebody entering a car or driving it would cause movement. A
    pendulum could be used to detect that. Circuit is very simple.
    Disadvantage: Usually some kids find out quickly that shaking the car
    causes the alarm to go off, and use it to annoy the neighbours.

    b) A car ignition uses high current, causing the voltage at the car
    battery to drop. This can be detected electronically. Not too
    complicated, and probably quite tamper-proof.

    c) Detect opening of the doors, using the switches for the internal
    light. Very simple, but no real obstacle for professionals.

    d) Detect the presence of humans inside the car by ultrasound. Circuit
    is a bit more involved, an can sometimes cause false alarm after
    temperature fluctuations.

    Have a look at the electronics section of your local library for a book
    on the subject. You may also be able to get such a system in kit form
    from an electronics dealer.
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