Hi,
Check out this circuit, hope it will be suitable for your need. Just as you requested I have to modified a monostable circuit which I have worked and simulated it to see if it be good for your need. The stable state of the circuit is ‘high’, when the momentary key is held down, it goes low for a preset period determined by the value of C1 and R2 (change these values for your desired period). When the key is pressed and released before the preset period, the output goes back to its stable state (i.e, high).
Modify it to fit into your system.
good luck,