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!
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.