Consider this use case: two friends have gone to a crowded concert and need to stay together in the crowd, or find each other if they are separated. Phones can do this, however, they can get lost, stolen, lose battery, or lose signal.
This was the final project completed in an Industrial Design class called Interactive Product Design, working in a group of three. We used Arduino to prototype a pair of wristbands that would help partners stay together, while also being artistic and lively. This was a big learning experience for me, since I previously had little coding experience and had never used Arduino.
When the wristbands were in close proximity and the wearers moved their arms (dancing at the concert) the LED strips in the bands would strobe in different colors. When the wristbands were further apart, they would help the partners find each other by changing color based on if the wearer was getting closer or further from the partner, mimicking a game of "hot and cold".
We used an accelerometer to make the bands strobe, RFID to wizard-of-oz whether the wristbands were together or apart, a vibrating motor to inform the user with haptic feedback while they are finding their partner (this is partially redundant, for those who are colorblind or did not notice that they had been separated), and RBG LED strips sewn between two layers of fabric.