Selective threshold ignition circuit

A selective threshold ignition circuit adaptable for use in an electronic ignition system of an engine, said circuit comprising:

• sensor means for producing an AC electrical signal about a predetermined DC electrical reference level, said AC signal being created in response to and having a frequency related to the rotational movement produced by an engine;
• switching circuit means, having a predetermined DC threshold level, coupled to said sensor means for receiving said AC signal varying about said DC reference level and producing a corresponding spark timing signal by effectively comparing said received AC signal varying about said DC reference level with said DC threshold level and switching when said received AC signal varying about said DC reference level varies, with respect to said threshold level, from above to below said threshold level and from below to above said threshold level;
• selection means coupled to at least one of said sensor means and said circuit means for selecting at least first and second modes of operation for the combination of said sensor means and said switching circuit means in response to corresponding first and second control signals being received by said selection means, in said first mode of operation said DC reference level being substantially equal to said DC threshold value, in said second mode of operation said DC reference level being effectively different from said DC threshold level;
• and
• means coupled to said selection means for producing said first and second control signals and supplying them to said selection means;
• whereby said spark timing signal is adaptable for use in developing ignition pulses for the engine and the noise immunity of said ignition circuit is increased for one of said first and second control signals without substantially affecting the accuracy of the spark timing signal while a more accurate spark timing signal is obtained for the other one of said first and second control signals.