电话是人们现代生活不可缺少的通信工具。由于有相当长的电话线路暴露在户外,时常会出现电话线路被人盗用的事件。本文介绍一种简单易做的电话线路保护器。 电话线路保护器电路如图所示。端口COM和LINE分别按正、负极性接入电话线路,COM和LET接电话机。如线路安装正确,拿起话机LED指示灯亮;否则调换线路的极性。电话线路接通后,约50V电压经D5驱动三极管V1导通,使V3截止,IC4060不工作。拿起电话机后,50V电压经电话机(内阻约200Ω)、二极管D2驱动三极管V1导通,IC4060也不工作。 若电话线路被人盗用,COM至LINE端的电压下降到12V左右,使三极管V1截止,V3导通,IC4060起振,对电话线路发出450Hz的噪声,同时,经电阻R9使线路电压 Telephone is an essential communication tool for people’s modern life. As there is a fairly long telephone line exposed to the outdoors, there is often an incident of telephone lines being stolen. This article describes a simple and easy to do telephone line protector. Phone line protector circuit as shown. Port COM and LINE respectively positive and negative access to telephone lines, COM and LET to connect the phone. If the line is installed correctly, pick up the phone LED indicator light; otherwise, change the polarity of the line. After the telephone line is connected, the voltage of about 50V is turned on by the D5 driving transistor V1 to make the V3 turn off, and the IC4060 does not work. Pick up the phone, 50V voltage by the phone (internal resistance of about 200Ω), diode D2 drive transistor V1 conduction, IC4060 does not work. If the telephone line is stolen, the voltage from COM to LINE drops to about 12V, so that the transistor V1 ends and V3 turns on. The IC4060 starts to vibrate, causing 450Hz noise to the telephone line. At the same time, the line voltage