The Atwood machine is the equipment consists of two masses connected with string or rope on a pulley fixed. In the Atwood best practical implementation by using a stopwatch timekeeping is done manually lead time calculations are inaccurate because it depends on the precision of the practitioner. Manufacture of electronic equipment by utilizing light-sensitive sensor that can automatically record the time interval in experiments using the calculation time is expected to minimize these inaccuracies. This electronic equipment uses the principle of the switch transistor, where the transistor is in two circumstances, namely the state of saturation and discontinuity. Specifications of this electronic circuit when the infrared light is not blocked by an object, the resulting output voltage of 11.07 volts and when the infrared light is blocked by an object, the output voltage drops to 1.2 volts using input voltage of 12 volts. From the research could have made an electronic device that can be used to record time lapse automatically by using a light-sensitive sensor based phototransistor. Series that has been created can be utilized in the best lab Atwood. The result of the calculation of the acceleration of gravity by using the self-timer on the plane practicum Atwood is 9.791 m / s2.