Digital timer have been used extensively with all types of system and device ranging from household appliance to large size computing system. Digital timers typically operate from a clock means driven directly by the line voltage, or can beoperated from crystal controlled oscillators that are widely used in devices such as microprocessor controlled equipment.This is particularly true of burner control equipment for programming the operation of burners that supply heat to boilers or furnaces. The failure of the timer in this type of equipment can lead to a catastrophic type offailure or explosion that has the potential of not only monetary damage, but the possible loss of life.
Most of the equipment used for timing in burner control equipment in the past has relied on a motor driven mechanism utilizing a small clock type timer that has proven to be reliable through many years of use. The clock type timer normallydrives a series of cams which operate switches at appropriate time intervals so that the program contained within the burner equipment will function as programmed. This type of equipment is being rapidly replaced by digital equipment, and particularlydigital equipment that includes a microprocessor for executing the control program. The operation of this new type of equipment relies on a digital timer and the failure of the digital timer clearly would create a potential failure of an unsafe nature.
The present invention is directed to a fail safe type of digital timer that relies on a digital clock. The digital clock can be driven by any type of cyclic source, such as the line potential or a crystal oscillator. The invention relies onsensing two outputs from the digital clock and inverting one of the outputs. The two outputs are fed to two similar counters and the outputs from the two counters are compared. Thecomparator has an output that is either constantly high or constantly low and this information is used to safely control a load means. The combination of the comparator output and a counter output is capable of being used to drive control equipment orother types of equipment where a fail safe type of timer is necessary.