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Electronic tone ringer
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Patent number 4,491,691
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Issue date Jan 1, 1985
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AT&T Bell Laboratories
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David M. Embree
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H04 - ELECTRIC COMMUNICATION TECHNIQUE
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Electronic tone ringer
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Patent number 4,276,448
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Issue date Jun 30, 1981
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Bell Telephone Laboratories, Incorporated
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Milton L. Embree
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H04 - ELECTRIC COMMUNICATION TECHNIQUE
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Relay driver circuit
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Patent number 4,224,654
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Issue date Sep 23, 1980
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Bell Telephone Laboratories, Incorporated
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David C. Goldthorp
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H01 - BASIC ELECTRIC ELEMENTS
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Transistor driver circuit
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Patent number 3,973,141
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Issue date Aug 3, 1976
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Bell Telephone Laboratories, Incorporated
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David Clayton Goldthorp
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H03 - BASIC ELECTRONIC CIRCUITRY
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