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One-Shot Timer with Long Delay

Gregg Jager
Prodigy 40 points
Other Parts Discussed in Thread: TPL5110, CD4541B, LM3407

I'm looking for a one shot timer with a long delay (~4 hours). I've been looking at the CD4541B. To use this chip I need to set the frequency to about 4.5Hz. The equation for setting the frequency in the datasheet is declared for f between 1kHz and 100kHz, however the datasheet also says it can operate down to DC?  How does the frequency equation (f = 1/(2.3 x Rtc x Ctc) change for low frequencies?  I assume the k value will drift somewhat.  

I've also been looking at the TPL5110. What are the limitations and availability for this part?

  • 0
    TI__Genius 9860 points
    Hi Gregg,
    For extremely low frequencies, that equation may not be valid but it would be a good design start. As these parts were acquired, we do not have any data other than what is available in the datasheet. I would suggest using a variable R or C device to test its behavior at low frequencies.

    As for the TPL5110, I am not aware of the schedule but I can find out an estimate.

    What is your full applicaiton? We might have other devices that can be used as well.
  • 0 in reply to C. Ryan Land
    Intellectual 420 points
    Two thoughts:
    1) Use two CD4541B chips, one in free-run mode (MODE = "1") to provide 4.5Hz to the second one on its external clock input. The second CD4541B would then run as a one-shot.
    2) If you don't necessarily need the operating voltage range (3 to 18V) of the CD4000 series, think about using a small microcontroller like PIC12Fxx series to generate the long pulse. These devices have internal oscillators and wouldn't need any external timing components, however they only work up to 6 volts.
  • 0 in reply to C. Ryan Land
    Prodigy 40 points

    Hi Ryan,

    Thanks for your response.  I am using it as an auto-off for an LED light.  The output of the CD4541B is tied to the enable of a driver suing the LM3407. 

  • 0 in reply to Gregg Jager
    Intellectual 420 points
    The EN input of the LM3407 is a 5V logic signal and in fact has an absolute maximum rating of 7V. So it would seem that a PIC micro would be ideal for doing the timeout since you don't need (or want) higher voltage on this pin. For a low pin count application you can get a PIC micro with an internal oscillator good for about 1% accuracy in a small TO-23 size package for about 50 cents in small quantities.