The Schmitt trigger will detect input signal between 4V and 8V, and it will be used to control a relay via a transistor which operate as a switch. The transistor will on the relay from off state when the input signal decreases to 4 V and will off the relay from on state when the input signal increases to 8V. For this project, the relay can be substituted by a light emitting diode (LED), that is, when the relay is on, the LED will light up.