Add-on-telephone-hold-button

Description: 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 off-hook 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.

The circuit utilizes a sensitive-gate silicon-controlled rectifier (SCR) as the primary component for holding the telephone line. The SCR is designed to maintain a holding current between 20 to 40 mA, which is contingent upon the loop resistance in the system. This current level is typically adequate for most telephone lines; however, adjustments may be necessary in specific cases by modifying the resistor Rl to optimize performance.

An LED indicator is integrated into the circuit to visually signal the user when the line is in a hold state. The LED illuminates when the SCR is conducting, providing a clear indication that the line is active and not in use. Upon lifting any telephone receiver connected within the same loop, the low impedance introduced by the off-hook condition effectively diverts the holding current away from the SCR. This action causes the SCR to turn off, resulting in the extinguishing of the LED and releasing the line from the hold condition.

Furthermore, a Zener diode, designated as D2, plays a critical role in protecting the SCR by ensuring that the line-holding current remains below the SCR's minimum conduction threshold. This safeguard prevents unintentional triggering of the SCR under normal operating conditions. Additionally, if the calling party decides to hang up while on hold, the disconnection of the line from the central office will deactivate the SCR, thereby releasing the hold state and allowing normal operation to resume. This circuit design effectively maintains line control while providing user feedback through the LED indicator.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.