Tuning Fork Switch vibrates to tuning fork by piezoelectric crystal and inherent frequency of tuning fork. It can monitor continuously to the change of frequency. When the tuning fork switch is used for low alarm (or low position control), the medium in the vessel is discharged downward passing through the tuning fork. When it is lower than the specific location and change the inherent frequency, this change is detected by electronic component, the output state is shifted; when it is used for high alarm (or high position control), the medium in the vessel go upward and contact with tuning fork which can shift the output state. Choose the inherent frequency of tuning fork (about 1300Hz) to prevent disturbance of equipments’ vibration, which could bring wrong switch operation.