While assembling the 555 timer based circuit below I noticed that 1) the intended delay on pin 3 only works if the switch on pin 2 is closed before attaching the circuit to the 9V power supply.
Source: circuitdigest
2) If the power supply is already attached and pin 2 is opened, closing it will produce the desired turn on delay on pin 3, which is fine.
My first question is whether scenario #1 is indeed the expected behavior in monostable mode (or perhaps a flawed construction on my part). The second question is whether it is possible to provide a delay also in the case where pin 2 remained closed but the power supply would be removed and reattached later on.
PS: I have looked into a similar question but was not convinced it can help in this case.
Best Answer
Without the switch pressed, the TR input is open, hence undefined. With an undefined input, you can't expect any specific behaviour.
Maybe you intended R1 to be a pull-up? As drawn, it has no function.
What voltage do you expect on pin 6 when you apply power? As drawn, it is undefined. A diode could be added to discharge C1 to the power rail.
And as HandyHowie remarks, the LED needs a current-limiting resistor.