US Patent 6888454 Fault diagnosis circuit for LED indicating light

A fault diagnosis circuit of the present invention includes: LED indicating units 3a, 3b, and so forth; current detecting circuits R1 and Q1 for detecting a flowing current by flowing a short-time pulse of a current through the LED indicating units 3a, 3b, and so forth based on pulses generated by a pulse generating circuit 7; comparing circuits 8 and 9 for comparing a current value detected by the current detecting circuits R1 and Q1 with their respective threshold values to judge a disconnection and a short-circuit; and retaining circuits 10 and 11 for generating a signal and retaining the signal when the current value is judged as being smaller and larger than the respective threshold values as a result of comparisons by the corresponding comparing circuits 8 and 9. Thus, it is possible to indicate a fault in an LED indicating light to the outside while making the turning ON of an LED unperceivable.