Cut Phone Line Detector

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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


Total Qty.
R1, R2, R3322 Meg 1/4 W Resistor
R412.2 Meg 1/4 W Resistor
C110.47uF 250V Mylar Capicitor
Q112N3904 Transistor2N2222
Q212N3906 Transistor
Q31IRF510 Power MOSFET
D111N914 Diode
Load1See "Notes"
MISC1Wire, 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.

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Ringing Phone Light Flasher, Phone Busy Indicator, FM Telephone Bug, Cut Phone Line Detector, Simple Phone Tap, Telephone Hold Button, Telephone Recorder, Remote Telephone Ringer


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Cut Phone Line Detector
Thursday, May 09, 2013 3:27:34 AM
hi. thanx for the circuit. is there a replacement for the MOSFET because i have to get it on the Ugandan market.
Cut Phone Line Detector
Sunday, September 13, 2009 7:32:15 PM
Someone complained the resistor values being excessively high. The reason for that must be to extend the battery life. The circuit is not a good one as it draws significant battery current even when phone line is intact. Since the input impedances seen by the transistors are highly elevated, the circuit wil also be very sensitive to noise.
Gm Karutz
Cut Phone Line Detector
Friday, August 07, 2009 8:30:24 AM
is this device can be use in grounded telephone lines? if doesnt, please give me some schematic to detect grounded telephone line...thnx
priyesh jain
Cut Phone Line Detector
Tuesday, March 24, 2009 1:03:49 AM
This circuit is so easy for the students those work on the minor project and it is very useful to detect the phone tapper.
niraj kumar
Cut Phone Line Detector
Sunday, March 08, 2009 10:35:34 AM
this is vary good is vary use full in our day to day life.please if posible send benchmark analaysis to my email .
Cut Phone Line Detector
Friday, July 11, 2008 1:40:24 AM
is this applicable in wider range of detecting the cut of a wire?
Mike Tarr
Cut Phone Line Detector
Wednesday, May 28, 2008 11:54:54 AM
Can I find a circuit already built?
Cut Phone Line Detector
Tuesday, April 01, 2008 11:26:15 AM
its a good circuit but i want to find like that circuits when talking will start and circuits cut the line which are predefine by us for exaple after 5 mins,10 mins and etc. if u have a like this circuits please send it my email address on urgent basis email address is
Cut Phone Line Detector
Tuesday, February 12, 2008 11:28:34 AM
if i used the wirless phone it could be work and tell me how i connected with it and if you give me the more and more knowledge plz help me any one knows it thanks
Cut Phone Line Detector
Monday, January 07, 2008 3:51:05 AM
thank you, very good but sir could you please provides more information about this.
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