SYSTEM FOR DETECTING DOOR-PHONE INCOMING

• Main Authors: MIYAZAWA MIKIO, TAKIZAWA HIDEAKI, UEKI MUTSUO, TAKAI TETSUYA
• Format: Patent
• Language: English
• Published: 22.05.1989
• Subjects: ELECTRIC COMMUNICATION TECHNIQUE; ELECTRICITY; SELECTING; TELEPHONIC COMMUNICATION

PURPOSE:To eliminate a control circuit, which controls the voltage sending of a direct current superimposing circuit, by superimposing a direct current voltage to a door-phone trunk line, executing comparison between the voltage of the door-phone trunk line which voltage-falls when a door-phone calling button is pushed down and threshold which is set in advance and obtaining an incoming when the voltage of the door-phone trunk line is lower than the threshold. CONSTITUTION:While the direct current voltage is superimposed from a power source POW of a direct current superimposed circuit 1 to a door-phone trunk line D, the voltage falling of a voltage V3 of the door-phone trunk line D which is generated when the incoming is matched is utilized. Then, by comparing the voltage V3 and a threshold VTH set in advance by a comparator 10a of an incoming detecting circuit 10, the incoming can be detected. Thus, it is not necessary to provide the control circuit to control the output voltage of the direct current superimposing circuit 10. Accordingly, the number of parts can be reduced and a low price can be obtained.