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LM555: lm555 - need to use as a monostable single shot timer

john stohlman
Prodigy 10 points
Part Number: LM555
Other Parts Discussed in Thread: CD4060B, TLC555

Hey from john

We are trying to use the LM555 as a single shot timer- please see attached pdf  for the official TI pdf (pg12) as a monostable single shot timer triggered by a momentary PB

we need 3 hours so are testing with 20 m ohm resistor and 470 uF cap to get a little less than 3 hrs.but cannot get this to work-

specifically- what do we do with pins 4, 6, and 7  ??  can anybody help?

we want the operator to push the PB and walk---- and have this reset on it's own according to the time constant. t in equation 6-- thanksPG-12-LM555.pdf

  • 0
    TI__Guru**** 152026 points

    Hello John,

    See the maximum time section of mono-stable FAQ , leakage and bias current is key. There is also an up to 2 hours alternate device listed.

    See the minimum frequency section of a-stable FAQ , leakage and bias current still key. There is mention of adding a ripple counter that can extend the time (lower the frequency). Pulse can start with a ripple counter reset release and the end could be stop the LM555 timer making it mono-stable.

    Pin 4 is always high (unless a reset is desired)

    Pins 6,7 are tied together in monostable mode (also in the FAQ)

    The CD4060 could count for 3 hours and the LM555 could latch the start and end of pulse. This is likely the best choice.

    What method is best for your application? What is your supply voltage?

  • 0 in reply to Ron Michallick
    TI__Guru**** 152026 points

    John,

    Here is a full circuit, LM555 (Latch) + CD4060B (clock/counter) for a close to 3 hours output high pulse.  I have a start switch and also added a stop switch (it is not a pause switch)

    The output time will be 8192 times longer than the period of CD4060B pin 9. For evaluation, this will be the fastest way to check the circuit.

      

    If you want to pursue a 555 timer only option. Switch to TLC555 (lower leakages) monostable mode and try C = 1000uF and RA = 9.76M, however, I like the circuit above better.