Design of an Audio Logic Tester

The design of a simple logic tester producing audio outputs is described. These audio outputs are distinct frequencies, approximately an octave apart, and correspond to the logic states '0,' '1,' and 'fault,' as defined for transistor-transistor logic (TTL) integrated circuits. The tester can be easily modified to detect pulses or logic levels of other types of circuits. It is expected that such a testing circuit will provide a useful aid in undergraduate digital laboratories.