Browse Prior Art Database

Loop and Ring Current Detector for a Telephone Line

IP.com Disclosure Number: IPCOM000079919D
Original Publication Date: 1973-Sep-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 2 page(s) / 46K

Publishing Venue

IBM

Related People

Lebrun, M: AUTHOR [+2]

Abstract

Loop DC current and ring AC current are detected by two optical isolators, as shown in Fig. 1A, and the discrimination between the DC current and the AC current is performed by logic circuitry shown in Fig. 1B.

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Loop and Ring Current Detector for a Telephone Line

Loop DC current and ring AC current are detected by two optical isolators, as shown in Fig. 1A, and the discrimination between the DC current and the AC current is performed by logic circuitry shown in Fig. 1B.

Two optical isolators 1 and 2 comprising light-emitting diodes LED1 and LED2 and phototransistors T1 and T2, respectively, are arranged on the telephone line and provide a signal on their output terminals 01 and 02, when the corresponding light-emitting diodes are forward biased.

Resistors R1 and R2, diodes D1 to D3 and D5 to D7 limit the current flowing in diodes LED1 and LED2, and diodes D4 and D8 protect these diodes against reverse voltages. Resistors R3 and R4 are used to adjust the sensitivity of the device.

Surge protectors P1 and P2 ground the lightning voltages which could be induced on the line.

When a ring signal is present on the line, both optical isolators are activated on each alternance and provide signals to the inputs of AND circuit A1 through integrators 11 and I2. Circuit A1 provides a signal on the RING DETECTION line.

When the handset is taken off, a DC current flows in the line and only one optical isolator is activated. The OR circuit provides a signal to an input of AND circuit A2, which receives a signal on its second input through inverter N. No signal is provided by circuit A1 in this condition. This a signal is provided on the DC DETECTION line.

Circuit A2 and inverter N preve...