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One more short question on 555. I am required to say what is the maximum possible duration of the output pulse (or signal). I looked it up online (including the datasheet for 555), but found no such parameter. What does it depend on and what is that maximum value (if it exists)? Thanks!

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    \$\begingroup\$ Indefinite. Just let it change and never change it back. \$\endgroup\$ Jun 18, 2016 at 7:48
  • \$\begingroup\$ Can you specify, please? \$\endgroup\$ Jun 18, 2016 at 7:49
  • \$\begingroup\$ Specify what, how to not change the signal? \$\endgroup\$ Jun 18, 2016 at 7:50
  • \$\begingroup\$ Is this for homework? Tell your instructor it's a silly question. \$\endgroup\$
    – uint128_t
    Jun 18, 2016 at 7:53
  • \$\begingroup\$ I think maybe my instructor means what is the maximum possible period of the output signal. \$\endgroup\$ Jun 18, 2016 at 8:03

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In theory you can generate pulses that last minutes , even hours! But you have to accurately measure the values of resistors and capacitors that you use in your project with NE555. To understand what it depends on, you should understand the working of NE555 as astable oscillator. There is a capacitor charging and discharging, and the period depends on how long does it take to reach a particular value of voltage. If you take a large capacitor and a big resistor, it could take hours.

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  • \$\begingroup\$ Yes, but does it have some kind of limitation? \$\endgroup\$ Jun 18, 2016 at 8:04
  • \$\begingroup\$ @MartinNikovski: Not in the chip. \$\endgroup\$ Jun 18, 2016 at 8:05
  • \$\begingroup\$ No there are not theorical limitation. The limitations come when you consider the real world and all the possibile noise coming from external circuits or from the power supply, or the temperature related issues... These problems could affect your circuit, varying your time constants. \$\endgroup\$
    – Simus994
    Jun 18, 2016 at 8:18
  • \$\begingroup\$ Try plug in 100Farads and 100Giga Ohm in the timing equation of the 555 ic, you will have a ball park then. As said by other comments, this is simply a ridiculous question. \$\endgroup\$ Jun 18, 2016 at 14:30
  • \$\begingroup\$ @soosaisteven: Keep in mind that input bias currents will limit the resistance you can use. But you can still create arbitrarily large capacitances. \$\endgroup\$
    – Dave Tweed
    Jun 18, 2016 at 14:32
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long enough that a CD4060 is a better solution.

Depending on which version of the 555 you choose in excess of about half an hour, but above about 60 seconds a CD4060 starts looking pretty good.

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