A unity gain second order Sallen-Key low-pass filter is used to filter a PWM signal from the MSP430™ LaunchPad™ to a stable analog voltage value. The voltage value can range from 0 V to 3.3 V depending on the duty cycle of the PWM signal. This low-pass filter output voltage goes to a high-current buffer amplifier circuit which drives the electrochromic mirror.

The resistors and capacitors were chosen such that the cut-off frequency is set to roughly around 315 Hz and the Q factor to around 1/2. The low cut-off frequency allows only the DC component of the PWM signal to pass through the filter. The Q factor being close to 1/2 sets the system in a critically damped state which limits oscillations at the output and results in a faster response with minimum overshoot.