Image Number 2 for United States Patent #4299556.
A timer circuit arrangement for use with a digital combustion control system functions to check failure of a safety timer which stops the combustion operation in the event that ignition is not completed within a predetermined time. The arrangement has a simple circuit which checks the safety timer in advance of initiation thereof. A pre-purge timer, which determines a predetermined time for feeding only combustion air into the combustion chamber for purge thereof in advance of the combustion, and the safety timer are built with a unitary construction. A series connection of flip-flop circuits is provided for carrying out frequency division of clock pulses to obtain predetermined times, and the flip-flop circuits for the safety timer are constituted by at least a part of the flip-flop circuits for the pre-purge timer so that the pre-purge may continue infinitely when the safety timer become out of order, thereby preventing the operation for combustion and ensuring safety of operation.