Making EOT With a LM3909

Pete Steinmetz Aug 12, 2019

  1. Pete Steinmetz

    Pete Steinmetz TrainBoard Member

    737
    4
    22
    I have 3 LM 3909 flashing circuits.

    I will run them off a CR 2032 3V lithium battery.

    My question: What value capacitor do I use to get a one second on, one second off cycle?

    I looked hard and found threads with links to circuits for the 3909 , but the threads were so old the links were dead.

    Please, no suggestions about Ring Engineering or other commercially available FRED/EOT. I am going to build one.
    Flashing LED's don't have the right flash rate.

    Thanks,

    Pete Steinmetz
     
  2. wvgca

    wvgca TrainBoard Member

    145
    47
    14
    lm3909 is pretty well dead, and it's worth 10 times what a 555 is ??
    oh, by the way, flash rate of 1 hertz at 3 volts would be 300 microfarad, polarized,. on pins two and eight, from the application guide for the LM3909
     
    Last edited: Aug 12, 2019
  3. Pete Steinmetz

    Pete Steinmetz TrainBoard Member

    737
    4
    22
    I have the data sheet and saw the 300uf capacitor would get the flash rate I was looking for. 300uf in 3 and 6 volt are not available. I was hoping someone might have a work around.
     
  4. wvgca

    wvgca TrainBoard Member

    145
    47
    14
    oh, okay ...
     
  5. wvgca

    wvgca TrainBoard Member

    145
    47
    14
    you do realize that capacitor voltage rating really doesn't matter, except the lower ratings are -usually- smaller physical size ? as long as it meets minimum voltage rating ..

    by the way, digikey have aluminum electrolytics in both 3 and 6 volt
     
  6. Point353

    Point353 TrainBoard Member

    1,844
    564
    31
    330μF is a standard value, but the flash rate will be about 10% slower.
    If you're using a 3V battery, choose a capacitor with at least twice the voltage rating. 6.3V, 10V, 16V are some standard voltage ratings.
     
  7. Pete Steinmetz

    Pete Steinmetz TrainBoard Member

    737
    4
    22
    Digikey has no stock of 3 or 6 volt. I looked there and Mouser, and Newark before I asked here. No stock.
     
  8. wvgca

    wvgca TrainBoard Member

    145
    47
    14
    uh.. i use digikey.ca, and it shows stock, not many, but some ?? relatively expensive, over a buck each ..
    i didn't bother checking the others ...
    parts come from the us though, on digikey ... you could be looking at something different though, lol
     
  9. Pete Steinmetz

    Pete Steinmetz TrainBoard Member

    737
    4
    22
    Found a 330uF 10 volt from Vishay. In stock at Digikey. Going to try this and see what the flash rate turns out to be.

    Thanks for the help,

    Pete
     
  10. wvgca

    wvgca TrainBoard Member

    145
    47
    14
    330 will give about 10% slower flash rate at 3 volt ... if your batteries are 3.3 volt, then it will be about one hertz
     
  11. wvgca

    wvgca TrainBoard Member

    145
    47
    14
    the lm3909 is very much voltage dependant, you will notice a slow down as your battery goes from fully charged to almost drained ...
     

Share This Page