Shown to use LM324 voltage comparator composed of a test circuit. It features easy detection threshold level adjustment, can be tested DTL, TTL, CMOS and other logic levels. By a voltage comparator principle we can see: When inverting input (positive terminal) voltage is higher than the inverting input (negative terminal) voltage, the comparator output high; the other hand, the output low. RP to compare the voltage adjustment potentiometer, when UIN higher than the set voltage, the output of 7 feet high, shows 1, and the decimal point dp light; UIN below the set voltage, 1 foot high output shows 0, but the decimal point is not light; when it detects high and low clock pulse changes, if the frequency is very low, showing that 0,1 alternate display. At higher frequencies, 0,1 changing very fast, so I saw displayed as 0, while the decimal point dp light, this "dotted zero" that can detect the clock pulse.