ST500 is electric aromatherapy machine with 200ml plastic bottle inside, the power supply is plug-in type. There is a button to open the door on the top of the machine, so it is quite convenient to refill the fragrance oil. Patented atomizer design could works directly with pure essential oil and no need to dilute. This model is suitable to be put on desktop or wall-mounted. Though it looks like small, the atomization performance is not weaker than other machines that look larger in appearance.