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.
Cut Phone Line Detector
Sunday, January 06, 2008 4:55:11 PM
Oops! I missed the the diode on the input. I guess it does have ring protection after all. Sorry!
Cut Phone Line Detector
Sunday, January 06, 2008 4:51:13 PM
As another poster said "Remove the reference to GND"!! It's a very bad thing to do, and will most certainly set off alarms it Ma Bell! I also agree that most of the resistor values are excessively high. & the input has no protection from the AC Ring voltage!
Cut Phone Line Detector
Saturday, December 22, 2007 4:28:17 PM
soooo good circuit but we need more information about it like data sheet of transistors
Cut line detector
Sunday, November 18, 2007 10:00:26 AM
I just looked at the cut phone line detector schematic. I need a circuit that will detect a break in in continuity of a single wire that I thread physically through a display of Christmas wire forms that if cut will in turn activate a 110VAC circuit to a siren/whaler, lights etc. PS. I have a basic knowledge of electricity/electronics and would prefer to have someone build me such a circuit. It looks like a fairly basic relay circuit, but is probably more in depth. Thanks
Cut Phone Line Detector
Wednesday, November 07, 2007 2:45:30 PM
Dear all My question is how could we use this circuit, i.e. what is the function of this circuit. another question is I am looking for circuit its function to find lost telephone wire
Cut Phone Line Detector
Saturday, October 27, 2007 10:06:10 AM
can we use same circuit for extentions of an EPABX because normal line(phone )voltage is 48V , extention voltages are around 24V to 48V depending on manufacturer. what happens if ringer voltage of 75V rms comes .because It is ac voltage.
Pesewu Samuel
Cut Phone Line Detector
Friday, October 12, 2007 9:00:56 AM
I need a help on how to build a cut ac[220-500volt] power line detector as my project work.
Cut Phone Line Detector
Tuesday, September 25, 2007 7:10:41 AM
could you please tell me what is the working principle of this device and what is it used for?
Cut Phone Line Detector
Wednesday, August 15, 2007 3:18:34 PM
I suggest that you eliminate the "GND" notation in the schematic and draw-in a 9V battery. Otherwise, users may connect the red wire to earth ground, shorting the telephone line.
Cut Phone Line Detector
Wednesday, July 25, 2007 6:08:38 AM
got any idea how to build the security system if this circuit were to be connected to the a simple security system monitored by a computer?
Cut Phone Line Detector
Tuesday, July 03, 2007 8:16:38 PM
This circuit will work & there is no need to build it to check However R1 + R2 = 44 Meg seems way too high and some 2N3904 transistors may not have enough gain and the minimal base current will not turn it on. I would make R1+R2 = 22Meg. and even less This value (high) will NOT draw much current from the phone line to detect off hook condition. Cheer
Cut Phone Line Detector
Thursday, June 21, 2007 11:23:12 AM
sir could you please provide the circuit that detects the tapping of phone lines by the wrong persons
ode namakam
Cut Phone Line Detector
Monday, May 07, 2007 8:00:47 AM
The design Schematic is sweet!
Encrypted Phone Conversation
Friday, March 30, 2007 1:27:33 PM
hey, i was searching for a circuit or whatever ideas to have a secret phone conversation so that if anyone picks up the phone in from another room connected to the same line (or try to spy), they couldn't understand what voice is being transmitted but the receiver would still get to have the private conversation
Cut Phone Line Detector
Friday, March 16, 2007 7:40:55 PM
i need more explanation in this circuit, if it found please send me in my email . i need circuit wireless if it found thank you very much
Cut Phone Line Detector
Thursday, March 15, 2007 3:39:42 PM
The circuit is fine since the GND is just a local circuit ground and not Earth ground. However if you don't know the correct polarity of the phone line you should add a bridge rectifier between the line and the circuit. Otherwise make sure your GND is connected to the most negative wire on the phone line.
Cut Phone Line Detector
Tuesday, March 13, 2007 4:55:10 PM
Red wire from the phone co has -48v. This will short out your dial tone. The green wire has ground from the co. This sucker will not work. Been there 39 years now. Telephone man
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