Telephone call charge counting circuit - uses photocoupler bridge network receiver circuit to enable counting of low and high frequency common mode charge signals while rejecting no

摘要

A change counting circuit, for example for a telephone user, has a light emitting diode bridge network (51-54) whose inputs are connected across the two wires (a, b) of the telephone line and whose outputs are earthed. A photosensitive transistor bridge network is associated with the diodes with the transistor bridge outputs connected to a first NAND gate (6) whose output connects with a second NAND gate (7). A second input of the second NAND gate (7) connects through an inverter circuit (3) and a high pass fitter (2), 12-15 kHz, with both wires (a, b) of the line. The second NAND gate (7) gives a counter (10) through a monostable circuit (9) and transient filter (8). Receipt of either or both a low frequency (50 Hz) or high frequency (12-16KHz). common mode charging signal on the line generates a logic 1 at the second NAND gate (7) output to trip the counter (10). The circuit may be used where a telephone call charge signal is a common mode low a high frequency on both and is insensitive to normal call signals.