Cut Phone Line Detector
Electronic Schematics > Telephone related > Cut Phone Line Detector

A while ago I got an email asking for the schematic of a circuit to detect cut phone lines. It didn't take me long to find this circuit in Electronics Now. When the circuit detects that a phone line has been cut, it activates a MOSFET which can be used to drive a relay, motor, etc. It can also be connected to a security system.


This is the schematic of the cut phone line detector


Part Total Qty. Description Substitutions
R1, R2, R3 3 22 Meg 1/4 W Resistor  
R4 1 2.2 Meg 1/4 W Resistor  
C1 1 0.47uF 250V Mylar Capicitor  
Q1 1 2N3904 Transistor 2N2222
Q2 1 2N3906 Transistor  
Q3 1 IRF510 Power MOSFET  
D1 1 1N914 Diode  
Load 1 See "Notes"  
MISC 1 Wire, Phone Connectors, Circiut Board  


1. The "Load" can be a relay, lamp, motor, etc. The circuit can also be connected to a security system to sound an alarm in case the phone line is cut.

2. If the circuit is connected to a security system or other circuit, both circuits must be electrically isolated from each other using an opto-isolator, relay, etc. This also means that the Cut Phone Line Detector must be powered by a seperate 9V supply.

Title: Cut Phone Line Detector
electronic circuit
Published on: 2005-02-10
Reads: 634
Print version: Print version

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Electronic circuits > Telephone related > Cut Phone Line Detector