Why does a Schmitt trigger circuit form a square wave at its output for any input wave?
A Schmitt trigger is a comparitor circuit that is basically an on-off switch. It's either on or off (which we call high or low). Once a threshold at the input is exceeded, it turns on, or goes high. Below the threshold, it turns off, or goes low. Drive a Schmitt trigger with whatever you want. It responds in only two ways; high or low. It has no other output states.