A failsafe mechanism which will only drive a heater (the lamp) if PWM current is detected through the fan (the DC motor).
If either switch above the sense resistor is open, no current will flow and the comparator output is low. If the fan's path is uninhibited, current flows through the sense resistor and the comparator outputs PWM.
A smoothing capacitor on the output maintains the output gate above the threshold voltage if the fan is on, switching the load on.
