Here is a simple logic state indicator to test whether the output of digital IC is in logic 1 or 0. The Bicolor LED lights Green when the logic state is 1 and lights Red when the logic state is 0.
The circuit uses only a few components and is based on the switching action of bipolar transistor. Normally the base of T1 is floating so that it will be off. This will allow T2 to conducts since its base gets bias through R2. The Red half of the bicolor LED lights indicating 0 logic. When the red probe connected to the base of T1 makes contact with a high signal output, T1 conducts and Green half of bicolor LED lights indicating logic 1. At the same time, T2 turns off and Red half of bicolor LED turns off. If the output is pulsating with alternate cycles, Red and Green LEDs lights alternately and if the transition is very fast both Red and Green LEDs lights giving yellow colour. The circuit can also be used as a continuity tester or Fuse tester.