Automatic fastening and alarming circuit for a vehicle seat belt

An automatic mounting and alarming circuit for a seat belt of a vehicle capable of automatically fastening the seat belt to a rider of the vehicle upon sitting on the seat, as well as warning the rider of a state where the seat belt is not or can not be fastened. The automatic fastening and alarming circuit includes a seat switch for detecting the sitting of the rider on the seat, a door switch for detecting the opening and closure of a door, an automatic fastening control circuit for supplying a releasing signal to an electromagnetic means to unlatch the retained seat belt upon judging the sitting state of the rider by detection signals delivered from both of the switches, a fastening detection switch for detecting the suitably fastened state of the seat belt to the rider and an alarming circuit for generating alarm information to the rider upon detection of the not-fastened state of the seat belt by detection signals delivered from the seat switch and the fastening detection switch. The automatic fastening and alarming circuit further includes a vehicle speed detection circuit having a hysterisis characteristic which generates alarm information to the rider when the vehicle speed exceeds a predetermined value with the seat belt not being fastened.