A method has been developed to measure the swing period of a simple pendulum automatically. The pendulum position is converted into a signal frequency by employing a simple electronic circuit that detects the intensity of infrared light reflected by the pendulum. The signal produced by the electronic circuit is sent to the microphone port and recorded as a 16-bit wave file by common software. The wave file is then processed to obtain the signal period as a function of time by timing all zero crossings. From the obtained signal period as a function of time, an average value of the period is calculated. Using the calculated average period, it was found that the gravitational acceleration is (9.77 [plus or minus] 0.03) m s[superscript -2]. Noting that the G-type La Coste & Romberg G928 gravimeter obtains a gravitational acceleration of 9.78 m s[superscript -2], the present method offers very good accuracy, with a percentage error of about 0.1%. (Contains 6 figures.)