its operation the auto repair signal s1 is converted to a signal s2 by passage through the first inverter 2 and the inverted signal s2 is converted to a signal s3 by passage through the second inverter 3 the inverted signal s3 is then passed to one input terminal of an and gate 6 the other input terminal of the and gate 6 receives clock pulses s5 at preselected intervals from a reference time oscillator 5 in the case of the illustrated example the reference time oscillator 5 includes a pair of flip flops which divide the commercial line frequency ac3 v 50 60 hz as an alternative however a crystal may be used for generation of the clock pulses s5 the output signal s6 of the and gate 6 is converted to a signal s7 by passage through an inverter 7 and the signal s7 is applied to a time counter 8 the time counter 8 counts the signals s7 sequentially and passes each count value d to a comparator 9 since production time is used in the present example as the production index a time selector 10 is connected to the comparator 9 in the case of the illustrated example production time the preset production index is selectable stepwise by switch operation in a modification linear selection of production time is employable.