A signal converter circuit includes a frequency detection circuit configured to determine whether an external control signal is a PWM signal. A PWM frequency converter circuit is configured to detect, when a PWM signal is input, a duty cycle of the PWM signal and to generate a first digital signal. An AD converter is configured to generate a second digital signal based on an input DC voltage or a voltage attributable to a variable resistor to digital data using a maximum AD convertible input voltage as a duty cycle of 100%. An output signal generation circuit is configured to generate, based on the first or second digital signals output from the PWM frequency converter circuit or the AD converter, a PWM signal with a duty cycle based on that first or second digital signals.