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.