There Once Was An IC Dedicated To Blinking An LED

Today you can buy flashing LEDs; a simple two-lead component that requires only a power supply to produce even flashes of light. They look for all the world like any other LED, though embedded in the plastic dome is an integrated circuit to do all that flashing work.

There was a time though when a flashing LED was something of a big deal, so much so that National Semiconductor produced a dedicated chip for the task. The LM3909 boasted the ability to flash an LED for over a year using a single C battery. That part is now long out of production, so [Dillon] has implemented the LM3909 circuit using discrete components on a small PCB designed to take pins and fit the footprint of the original.

Why on earth might a reborn LM3909 be of interest to him, you ask? Well, he wasn’t able to make a 555 flash the LED from a coin cell, and a friend mentioned this chip which piqued his interest. The internal schematic is in the data sheet (found in the files section of his project), so he was able to implement it relatively easily using common parts. It still requires an external capacitor just like the original, but there is space on-board should you wish to put it there.

He’s produced a video we’ve placed below the break showing the device in action, proving it to be a drop-in replacement for an original. Recreations of classic chips using discretes are nothing new, we recently brought you a reborn PSU regulator chip made in 2014. An while you’re playing around with coin cell batteries, may we direct your attention to the Coin Cell Challenge.

Thanks [Drew Fustini] For the tip.

56 thoughts on “There Once Was An IC Dedicated To Blinking An LED

    1. The LM3909 specification of operation with battery drop down to 1.15V is tough to match. An ATTiny micro can flash an LED with just a resistor and be had for 50 cents Qty. 1 (2023) – but it’s only spec’d down to 1.8V.

    1. I had one that I bought in Tandy (UK name for Radio Shack) in the early ’80s. In around 2005 I helped my six year old son to make a Dalek out of a yogurt pot, with a miniature red led on a stalk. He did it all, including the soldering.

      He loved it, and took it to school for show and tell. His teacher told him off and confiscated it, saying it was dangerous and could hurt a child. She must have confused it with a ‘real’ Dalek.

      To this day I resent this. Instead of encouraging him, and talking to the children about technology, she made him feel like he’d done something wrong, and stole his toy (and my classic chip) They learned loads about acorns and leaves, though.

      I should have complained. That was my error.

  1. He *says* it’s a 1.5V battery, but it’s really a 3V lithium cell. That kind of takes all the challenge out of getting 1.6V to drive the LED, doesn’t it? Does it work with a AAA cell like the original ‘3909 did?

    Some time around 1980 I potted one in epoxy with a D cell. Ran for years. Lost it in a move. Might still be blinking for all I know, but I doubt it. They weren’t actually that efficient.

    1. I did exactly the same thing to about 3 or 4 chips/D cells. One of them got stuck into the peephole of my dorm room. If you stood in front the the door and looked closely, you could see it blinking in the external lens.

    2. I have a 3909 that I have been keeping going since the ’90s when I bought a couple from RS. I need to change the AA battery about once a year it seems. It still sits in a crappy chunk of pcb I tossed it in, but have never gotten around to replacing it. But hey It’s been more reliable than any woman!

      1. So I’m alone! Good to know, I’ve had at least one blinking around here since circa 1983, perhaps before, when I was in high school. Keep one going on my night stand.

          1. My neighbors have deployed a considerable number of those already. Interestingly, they illuminate when the sun is NOT shining on them. Some kind of magical power source inside that turns off during the day, somehow.

          2. @Leithoa
            It was a joke, son.
            There’s no need for a photodiode or CdS cell here (especially since it wouldn’t be RoHS then). No, there’s a perfectly adequate photodetector present already, in the form of, duh, a solar cell. The BoM of a typical garden-variety solar-powered garden light is just four or five components: Solar cell, LED, the storage cell (battery), a custom IC, and sometimes a resistor. It’s that custom IC that does the magic part of telling when the sun goes down by detecting the lack of output from the solar cell. No toxic CdS required. Though, ironically, the cheap ones use a NiCd cell…

  2. Self contained blinking LEDs are nothing new. I recall seeing some in the 80’s, possibly even the late 70’s. The reason to use the special blinker chip was when you couldn’t get the size/color you wanted in a self blinker.

    1. The LM3909 was designed to do a couple of things that made it unique for it’s time. It could flash an LED with a forward junction voltage that was over 1.5V with a single 1.5v cell. Also, if you took the time to look the chip up, some cells outlived their shelf life when flashing with it. The chip was designed to have a full time flashing LED built into flashlights that did not discharge the batteries so you could locate the flashlight in the dark. It was a pretty neat piece of silicon for it’s day. It had nothing to do with the size or color.

      1. Word.
        Somebody here finally get it. The 3909 was a single chip charge pump that could light (and blink) an LED when the power source was below the forward bias voltage of the LED. Doing this with out a transformer (in the late 70’s) was f’ing Magic. By the way, in that time you had red, yellow and the “newly invented” green LED colors to choose from, and they were all dim, and by today’s standards they really sucked.

    2. I’m not sure if it was Don Lancaster, but ISTR an blurb in Radio-Electronics magazine that said a self contained blinking LED could “drive” 50 regular LEDs (in series with it?) to flash.

      1. I don’t know about 50 in series — those self-contained blinkies let the smoke out around 14V, so that would limit the number in series to around 6, unless you get creative with a voltage-limiting resistor ladder (which might be a single resistor in parallel with the blinker LED).

    1. ” … used in Pink Floyd Pulse album box … ”

      It certainly was. Oddly, when the battery expired on mine, it managed to take the IC out at the same time. I was surprised that there was enough energy in the system to achieve that.

  3. my grandpa installed one if these chips inside a ceramic owl he kept on his mantlepiece. it had two tiny red leds poking out of the eye holes. it was powered by two big d cells i think. it was just a cool little art piece that he improvised one day and its visibility and simple fascination (in the lates 70s/80s) probably got me interested in hacking… an interest that has yet to diminsh :)

  4. My father and I built a business around this chip from 1980-1983. We came up with a very simple circuit board (milled, not etched) which held the components, and it was strapped to a “D” size cheap carbon battery. The battery got wrapped with a sticker promoting or commemorating…(whatever). Sold about 340,000 of them over 3 years to various Florida tourist traps, hotels, doctors offices, arcades… The ones we built in ’80-’81 were still blinking in ’88-’89. I still have several tubes of chips, and I still make one on occasion as gifts.

    1. Doctor Wizard? I think we grew up together in Orange Park. Did you have strobes on he corners of your mortarboard for graduation?

      Anyway, one of my very first projects was a 3909 blinky soldered directly to a nicad cell from an old rechargeable toy.

  5. Not mentioned above …

    The human eye responds to peak intensity rather than average intensity. The LM3909 can be configured to modulate the flash with a 2kHz clock to further reduce power consumption while still appearing the same.

      1. Everything has limits, a 0.1% duty cycle will look dimmer than a 100% duty cycle.

        If you play with PWM, you’ll see that as you drop the duty cycle, the apparent brightness of the LED goes down a lot more slowly. For a given apparent brightness, you can use a lot less power by strobing the LED a bit brighter and only having it on 25% of the time.

        This is how modern LED flashlights got to be so efficient the bulb is only lit a small fraction of the time. And you can tell it’s blinking by pointing the flashlight at a wall and moving it around fast.

  6. For specific purposes, this device was way better than a generic LM555. Trying to make a 555 emulate it is just silly. The 555 is an analog timer (building block). The LM3909 was specifically built to be an efficient LED flasher. It was a sad day when the last one was made.

  7. Maybe he should have rolled his own capacitor too.
    Theoretically that should not be too hard and standard caps have a precision of 20% around the value anyway so you don’t need to be that precise either.

  8. Just because it uses the same “schematic” at the original IC, I wonder if the home brewed one is as efficient in its power usage. I’m thinking about such parameters as beta, leakage current and such.
    Will this one flash an LED for two years?

  9. LM3909 chips are still available. I always loved getting chip data books back in the day because they were usually jammed full of interesting and sometimes off-the-wall applications.

    1. Available from China for about $5, but worth it for quick assembly. I made a few art pieces in the ’70’s w/3909’s. One was a heartbeat in a pillow- I discovered that with a scavenged small speaker instead of an LED, the output did a Thump-thump sound each cycle- quite like a stethoscope.

  10. I had an electronics book from Radio Shack by Forest M. Mims (best as i recall, spelling excused) back in 80s that showed a circuit using the 3909 as an oscillator driving an 8 ohm speaker directly. By playing with capacitor and resistor values I made a very convincing siren. Lots fun :)

  11. the stuff you could do with this chip…. a very low current tracer for PCB, a trombone using a high Z speaker and a variable volume enclosure, various sirens…. Look it up in the old NS databooks/app notes. I always thought this chip was a creation of some student working for them for a Summer, as it did not seem to be very serious at all, just plain (?) fun!

  12. The 3909 is one of my favourite ICs ever. I used to make blinking jewellery with them in the 1980s.
    I have 3 left , if anyone knows where to buy them cheap please let me know.
    I wish someone would make them again in an SMD package.
    It also has one of the funniest application reports with other uses than flashing LEDs, such as a continuity checker, water seepage alarm, electronic music instruments, RF-oscillator and a radio !
    Google National Semiconductor AN-154 or go here:

    1. Hi everyone. This seems to be a somewhat active board for such an old thread…

      I’m flummoxed. I’ve purchased an M3909 with the intention creating a flasher for model trains. I’ve wired up a 220uF capacitor on lead 2(positive) and lead 1 (negative). Input is 1.5 volts on pin 4/5. I can measure the output voltage in a couple of different ways. If I probe 1 and 8, I get around .3 volts. If I probe 6 and 8 I get variations of about .9 to 2 volts. However, if I put a load on 6/8 I get no light from an LED (expected as they need 3 volts, correct?) but I also get no output from a 1.5 volt incandescent. I’m no electronics wizard, so could certainly use a little help!



      1. John, what documentation do you have about the LM3909? You seem to know enough to be aware of polarities for the electrolytic capacitor, and of course the battery. for the simple flasher, pin 1 and 8 should be connected together as well, LED (polarity is important here as well) between 8 (cathode) and 6 (anode), or if using an incandescent, it goes between pin 5 and 2. I am referring to AN-154 from National Semiconductor.

        this chip is hard to blow, so most likely you have a missing or an erroneous connection, or you bought a fake chip? Just go back over your circuit to see if you can find a mishap, and do check that the led is functional.

        By the way, you can get sound effects as well out of a LM3909, so persevere!

  13. Google “Joule Smasher LED Flasher” – testing supports 10 years from a single AA battery. You’ll need the quotes though or the less efficient Joule Thief will dominate your results.

  14. Used 3909s from Radio Shack back in 1976 to put blinking Red LED eyes into a couple of blow mold skulls as decorations for a Halloween party we threw while I was in college. Just dug into my old parts organizer and found I still have three of them left.

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