These little tactile switches are commonly used as input buttons or as a reset switch in electronic projects. They are momentary switches - the pins are normally disconnected (open), connected (closed) when the button is pressed and disconnected again once the button is released.
Each switch comes with a removable round cap. This switch measures 12mm square and 12mm tall with the cap on. It works on best on PCBs (through-hole-mounting) and but also can be used on solderless breadboards.
