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Hold Phone Circuit for 2 Telephones Connnected in Parallel
This Hold Phone Circuit will enable you to hang up one phone without losing the connection and move to another room or hall and pick up a different phone which is connected in parallel and continue your conversation. This feature is useful especially when you are staying in a house with different levels and having 2 telephones connected in parallel. Circuit Description
When the phone is in on-hook condition, the voltage across the tip and ring is in the range of 48V DC to 50V DC. When it is in off-hook condition (the receiver is taken off its hook), the voltage drops to the range of 6V DC to 15V DC. As shown in the circuit above, a silicon controlled recifier or SCR is connected in series with a light emitting diode (LED) and a potentiometer vR1 which is used to limit the current flowing through the circuit. The SCR or also known as a thyristors is a switching device based on a PNPN structure. It can be viewed of as a 2 transistor structure - a PNP and a NPN transistor with their Base and Collector connected together. It is a reliable device and can be used to deliver billions of operations before failure. In this circuit, a recommended SCR of 1A and minimum peak inverse voltage of 100 V DC is to be used. The part BT169G is a T092 package with 1A continuous current and a PIV of 600V DC. Equivalent SCR can also be used in this phone circuit.
When the normally open switch SW is pressed, it caused the SCR to conduct. As a result, the LED turns ON and a load is placed on the phone line that consumes approximately the same amount of current as the standard phone. Pressing the normally open switch while hanging up the receiver places the phone on hold condition and the LED turns ON. Even though the switch SW is released after the pressing, the LED should remain ON as the SCR is still in the ON state. When the parallel second phone receiver is taken off-hook, the current through the SCR dips below the holding on state and caused it to be turn OFF and the phone will be used in a normal manner again. Ensure that you adjust the potentiometer VR1 until LED remains on. With the proper setting of the potentiometer, picking up the receiver of any phone will reset the hold circuit and turn off the LED. VR1 is set in such a way that the extra load on the line pulls the current through the SCR lower than its minimum holding current.
Phone Circuit Parts List
Source : Extracted from Popular Electronics Feb 1993, By Charles D. Rakes
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