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In the standard monostable circuit, the 555 triggers on the negative-going edge. With Vcc = 9 V, the Trigger input has to sit above 3 V in the idle state, then go below 3 V to trigger the output. But ... The 555 is not a normal monostable. If the trigger input still is low when the circuit times out (in around 2 s in your case), the output will stay high ...

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