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Sensitive gate SCR telephone hold button

2015-01-16 23:19  
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Although a hold feature is standard on most new phones, a lot of us still use the origional bell phones. Those of us that require a hold feature will find this circuit very useful. It is easy to build, and is compact enough to be installed inside the phone with no real problem. It is also powered by the phone line itself, eliminating the need for batteries.A sensitive-gate SCR  provides a line-holding current of 20 to 40 mA, depending on loop resistance. It also lights an LED to give the user a positive indication that the telephone line is on hold. The 20 to 40 mA should prove sufficient to hold the majority of lines, but it might require increasing-by decreasing the size of Rl-in individual instances. When any receiver in the same loop is lifted, the low impedance of the offhook telephone set shunts holding current away from the SCR, thereby releasing the line and extinguishing the LED.Zener diode D2 ensures that the line-holding current drops below the SCR`s minimum conduction current. If the calling party tires of waiting on hold and hangs up, the release of the central-office relays from the calling side also releases the line from the hold mode.A telephone hold circuit for permitting either an incoming or an outgoing call to be held indefinitely, or as long as the party on the other end does not hang up. The circuit employs a silicon controlled switch for latching the circuit in the "hold" condition. The circuit also employs a light emitting diode to provide visual indication when the circuit is in the "hold" condition.