Posted on Jul 7, 2012

This tester provides an audible indication of the logic level of the signal presented to its input. A logic high is indicated by a high tone, a logic low is indicated by a low tone, and oscillation is indicated by an alternating tone. The input is high impedance, so it will not load down the circuit under test. The tester can be used to troubleshoot TTL or CMOS logic. The input consists of two sections of an LM339 quad comparator. ICla increases when the input voltage exceeds 67% of the supply voltage. The other comparator increases when the input drops below 33% of the supply.

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The tone generators consist of two gated astable multivibrators. The generator built around IC2a and IC2b produces the high tone. The one built around IC2c and IC2d produces the low tone. Two diodes, Dl and D2, isolate the tone-generator outputs. Transistor Ql is used to drive a low-impedance speaker.

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