Various example embodiments are disclosed. According to one example embodiment, an apparatus may include a motor, a rechargeable battery, a plurality of visual indicators, and a microprocessor. The motor may be configured to spin a disk upon which is mounted a plurality of magnets. The rechargeable battery may be configured to supply power to the motor. The microprocessor may be configured to monitor a voltage level of the rechargeable battery and to light a number of the plurality of visual indicators. The number may be based on the monitored voltage level.