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We have developed a wearable sensor node with a low power and high detection rate by using sequential control of multiple infrared (IR) modules. Conventional sensor nodes are not practical in terms of size, sensing performance, and working hours. Therefore, we devised a name-tag-size (73 Ã 98 Ã 9 mm) sensor node, which captures face-to-face interactions within 2 meters and within an angle of 60Â°. The sensor node weighs 62 grams and works for more than twenty hours with a small 5-gram Li-ion battery. The sensor uses the beam-scan technique, in which four infrared modules, placed horizontally on the node, are controlled to be on and off sequentially, and this operation is done synchronously with other nodes. The beam-scan technique enables low-power operation with a consumption current of 7.2 mA and 21 hours of operation. We had tested the sensor node in a field trial that collected sensor data for six months from 20 people and had demonstrated that this technique is practical. Feedback from sensing data reminded us of the importance of meeting frequency and this improved our work habits.