The photogate housing, shown in Fig. 1, was made with plastic (PVC) water pipe parts, as suggested by Griffin and Stakleff.8 The housing requires some ½-inch PVC pipe, two ½-inch PVC MIPT plugs, two ½-inch PVC slip x FPT elbows, and two ½-inch slip x slip x slip tees. All the PVC parts add up to $3.72 at Home Depot. From the ½-inch PVC pipe we cut three pieces: two are 3 cm long and one is 13 cm long. A 5-mm diameter hole is drilled in the center of the PVC plugs, and then the LED and the phototransistor are put in place inside. The wires are prevented from moving with the help of two 5-mm tall slices of bottle cork stuck at the ends.