RC vs Autonomous PWM Switch Circuit

A Schmitt Trigger can be used to differentiate high and low voltages in a noise-resistant way. The output voltage will be set high if the input voltage exceeds a certain threshold, will be set to zero if the input voltage falls below a certain threshold, and will not change if the voltage is between the two thresholds.

A basic design is shown described in detail here. The threshold voltages for switching states are the ratio of the two resistors attached to it multiplied by the voltage of the power supply connected to the op-amp.