demultiplexer

So I have a circuit whereby a PWM signal is being sent to a demultiplexer, and the demultiplexer sends the signal to a MOSFET which takes it and allows a current to flow through the nixie tube. The higher voltage value in the original PWM signal, the more the nixie tube will light up. My issue...

Hi! Can you help me please? I don't understand why the circuit doesn't work, but it works when the output of timer 555 is also connected to the oscilloscope, although it's the same thing.