I have designed this monostable (one-shot) vibrator using the 555 timer, yet I am getting this type of error on Pspice for the output voltage only. Where have I gone wrong?
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.
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 until the Trigger input goes high. If the narrow part of your Vtrigger signal is the high part, the circuit will not work because the low part of the trigger signal (2.45 s)is longer than the output period (2 s).
You want Vtrigger to rest high, and go low for 50 ms.
If the trigger signal cannot be adjusted, a solution is to add a differentiator to the Trigger input. This is a series capacitor between Vtrigger and the Trigger input, and a shunt resistor from the Trigger input to GND. The time constant should be shorter than 50 ms so it can recover curing the 50 ms Vtrigger period.