A droplet deposited onto an orifice in the flat smooth plate can remain in a bistable state. Such system -a droplet and a flat surface- behaves as a bistable capillary switch, which can be used for threshold detection of some system's properties, e.g., the difference in the pressure on both sides of the plate. The droplet morphology changes abruptly at a certain value of the pressure difference and shows hysteresis. The specific behavior of the system is a result of the geometrical constraints defining the curvature of the liquid droplet at both sides of the surface. These constraints can be represented either by pinning angles at both sides of the plate or by the pinning angle at one side and the contact angle at the other. The dependence of details of the droplet morphology and energy on the difference in pressure at both sides of the plate is calculated by means of the semi-analytical model and Surface Evolver simulations.