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Zero crossing detector
This circuit uses an LM139 (LM339) to form a zero crossing detector. This circuit is designed for a single power supply (V+). Voltage divider R4 and R5 establishes a reference voltage, V1, at the positive input. By making the series resistance R1+R2 equal to R5, the switching condition, V1=V2, will be satisfied when Vin=0. The positive feedback resistor, R6, is made very large with respect to R5. The resultant hysteresis established by this network is very small but is sufficient to insure rapid output voltage transitions. Diode D1 is used to insure that the inverting input terminal of the comparator never goes below approximately -100mv.