LOGIC_PROBE_YIELDS_THREE_DISCRETE_STATES

Published:2009/6/25 4:02:00 Author:Jessie | From:SeekIC

The circuit uses a dual LED. When power is applied to the probe through the power leads, and the input is touched to a low level or ground, Q1 is cut off. This will cause Q2 to conduct since the base is positive with respect to the emitter. With Q1 cut off and Q2 con-ducting, the greendiode ofthe dual LED will be forward biased, yielding a green output.Touching the probe tip to a high level will cause Q1 and Q2 to complement, and the red diode will be forward biased, yielding a red output from the LED. An alternating signal will cause alternating conduction of the red and green diodes and willyield an indication approxi-mately amber. In this manner, both static and dynamic signals can be traced with the logic probe.



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