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Circuit Archives
Telecommunications/ Telephone
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Music on Hold Box
Reprinted with permission
from Rudolf Graf.
For this and over 1000 other electronic circuits, review and order Rudolf
Graf's Encyclopedia of Electronic Circuits on CD-ROM Vol-1
McGraw-Hill ISBN0078528119
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Description:
U1, an LS3404 melody
chip is activated when "hold" S1 is pressed, which causes SCR1 to conduct and
hold the telephone line via T1, R1, and LED1. The voltage across R1 and LED1 is used to
activate the melody chip. Q1 and Q2 form a restart circuit to keep the melody chip going
during hold. (Reprinted with permission from Popular Electronics, 12/91, p. 53. Copyright
Gernsback Publications, Inc. 1991 and figure FE100-10 from Encyclopedia of
Electronic Circuits Vol. 1.)
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