“Prototype Social Distancing Reminder Using HC-SR04 Sensor At The Payment Counter Via A Smartphone: Prototype Pengingat Social Distancing Menggunakan Sensor HC-SR04 Pada Antrian Loket Pembayaran Via Smartphone”. Procedia of Engineering and Life Science, vol. 1, no. 2, July 2021, https://doi.org/10.21070/pels.v1i2.952.