JPS6284251U - - Google Patents

Description

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of the wiretapping prevention circuit of the present invention, Figure 2
The figure shows the first telephone equipped with the wiretapping prevention circuit of the present invention.
FIG. 3 is a block diagram of a second telephone; Explanation of main drawing numbers, 9 ...Switch circuit, 13 ...Telephone/receiver circuit, 14 ...Voltage detection circuit, 20...Pulse width identification circuit, 21...Detection circuit, 22...Latch circuit,
23...Latch release circuit.

Claims (1)

[Scope of utility model registration request] A voltage detection circuit that detects the voltage of a telephone line in communal telephones connected in parallel to the same line, and a pulse width identification circuit that identifies a comparison signal of the voltage detection circuit for a certain period of time and outputs an identification signal corresponding to the detection signal. and when an identification signal outputted when a comparison signal indicating an off-hook is inputted to the pulse width identification circuit for a certain period of time, and a signal outputted from the communication circuit during an off-hook according to the footswitch are inputted,
Identification of a detection circuit that outputs a detection signal, a latch circuit that operates based on the detection signal of the detection circuit and outputs a latch signal, a switch circuit that controls a communication circuit based on the latch signal of the latch circuit, and the pulse width identification circuit. A wiretapping prevention circuit comprising a latch release circuit that is controlled by a signal to release the operation of a latch circuit, and prevents wiretapping by disabling the communication circuits of communal telephones other than the telephone in use.