Circuitry for detecting malfunction of ink jet printhead

A device for detecting nozzle malfunction in an ink jet printhead having a predetermined number of nozzles, comprising:

• means for controlling said printhead whereby each of said nozzles is activated once in a manner which is required to print on a print medium a line having a number of ink dots equal to said predetermined number;
• means for detecting that one of said nozzles has failed to print an ink dot in response to its activation;
• means for photodetecting whether each location on said print medium which opposes one of said nozzles at the time of activation of said one of said nozzles has an ink dot printed thereat, each of said locations being photodetected in time sequence by scanning, said photodetecting means outputting a signal having a first level in response to each photodetected location which has no ink dot thereat and a signal having a second level different than said first level in response to each photodetected location which has as ink dot thereat;
• and
• means for processing the output of said photodetecting means, said processing means generating a signal which blocks printing by said printhead in response to a transition from said signal of said second level to said signal of said first level in the output of said photodetecting means during a predetermined window of time corresponding to the time required to scan a line having said predetermined number of ink dots;
• said processing means having a reference voltage source, a comparator having inputs respectively coupled to said reference voltage source and to said photodetecting means, means for measuring the time interval from a transition from said signal of said first level to said signal of said second level to said transition from said signal of said second level to said signal of said first level in the output of said photodetecting means, and means for outputting a shutdown signal in response to said time interval being less than said window of time.