Homemade telephone splitter
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Working principle Suppose there are two phones connected to the line, which are connected to L and L '. (We know that when the phone is in standby, depending on the switch on the local line L, there will be a DC voltage of 60V or 48V. When ringing, there will be 95V to 75V AC voltage. When the phone is off-hook, the line voltage is due to the internal resistance of the phone The intervention will be reduced to about 10V, this machine is designed based on this principle.) Both machines are on standby, if there is a phone call at this time, the AC current will ring through the above circuit. Suppose the extension on the L line is picked up first. The DC voltage on the line instantly breaks through D2. When R2 applies a trigger voltage to the control electrode of the BG1 thyristor through R2, BG1 turns on quickly to form a DC path, and the phone can talk normally. Since the line voltage drops at this time, the phone connected to L 'will not hear any sound. This is if you want to go to the standby phone, the off-hook phone can be dialed like a standby call (note: the phone must be in pulse dialing state), after the standby hears the ringing, go off-hook. You can talk after the online phone is on-hook, the principle is the same. The additional unit circuit can be expanded to several extensions, but at most no more than 10 extensions. 
Component selection BG1, BG2 is MCR100-6 and other small unidirectional thyristors, in principle, the reverse breakdown voltage is greater than 100V. D1, D2 are 18V regulator diodes. The entire circuit can be installed in the telephone junction box.
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