Published:2009-6-22 22:28:00 Author:May | From:SeekIC
U1, an LS3404 melody chip is activated when hold S1 is pressed, which causes SCRI to con-cluct 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.
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