Logic level tester circuit one

The logic level tester can detect the level of DTL, TTL, CMOS and other digital logic levels and the presence or absence of pulse signals . The circuit is shown in Figure 13-14. When the input signal is at a positive level , the red LED VLi lights up and VLz does not light up; when the input signal is at a negative level, the green LED VLz lights up and VLi does not light up.