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.

Schematic

Parts

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

Notes

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.

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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Comments

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anonymous 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.

anonymous 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 device.it is vary use full in our day to day life.please if posible send benchmark analaysis to my email .

ivy 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?

anonymous 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

kamran 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

revannath 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.

anonymous 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!

none 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!

hazim 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

joe 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

Jehad 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

anonymousmurali 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.

vaatsal 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?

anonymous 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.

lindy 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?

Angelo 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

ramesh 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!

anonymous 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

mohd 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

anonymous 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.

anonymous 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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anonymous	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.
anonymous	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 device.it is vary use full in our day to day life.please if posible send benchmark analaysis to my email .
ivy	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?
anonymous	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
kamran	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
revannath	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.
anonymous	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!
none	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!
hazim	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
joe	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
Jehad	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
anonymousmurali	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.
vaatsal	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?
anonymous	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.
lindy	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?
Angelo	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
ramesh	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!
anonymous	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
mohd	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
anonymous	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.
anonymous	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