Timing circuit

Claim 1

A timing circuit comprising: a threshold device having an input point, and having first and second states, the threshold device being in its first state when the potential at the input point lies above a given voltage threshold and in its second state when the potential at said input point is below said given threshold, the threshold device being normally in its first state, a current source including a timing resistor connected to a source of charging voltage, a timing capacitor having a first side connected to said timing resistor and a second side connected to said input point, first and second switch devices connected to said first and second sides, respectively, of the timing capacitor and to a source of reference potential, said reference potential being above said threshold, and means for closing said first and second switch devices alternately to connect said first and second sides of the timing capacitor alternately to said source of reference potential thereby causing said timing capacitor to be charged cumulatively during successive closures of said second switch device, the potential at said input point of the threshold device changing progressively with each closure of said first switch device and falling below said given threshold only after a number of alternate closures of said first and second switch devices to cause the transition of the threshold device to a second state, while in the event of failure of the timing capacitor, the potential at said inpt point remains above said given threshold and the threshold device remains in its first state.