Cut Telephone Detector Construction Schematic

[Navid Naghdi]

CUT PHONE LINE DETECTOR SCHEMATIC

Since nearly many electronics enthusiasts have been asking for
a schematic of a cut Telephone line detector, I decided that I
should contribute something that is of interest.

HOW IT WORKS:

When a cut phone line has been detected, a MOSFET is activated
which drives a relay, motor, etc. It also too can be connected
to a security system.

CONSTRUCTION SCHEMATIC:
_________o_______ +9V
| |
| __|___
R4 | Load |
| ------
| __|
| | Q3
o--------|<--.
b |/ e |___|
----| Q2 |
Q1 | |\ c |
|/ c | |
Green ----R1--------o----| | |
| | b |\ e | |
R2 | | | |
_|_ --- | | |
C1 === /_\ D1 | | |
| | | | |
Red __________o____o_____o______o____________o__ GND


COMPONENTS LIST

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
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.

 

[Fred Bloggs]

CUT PHONE LINE DETECTOR SCHEMATIC

Since nearly many electronics enthusiasts have been asking for
a schematic of a cut Telephone line detector, I decided that I
should contribute something that is of interest.

HOW IT WORKS:

When a cut phone line has been detected, a MOSFET is activated
which drives a relay, motor, etc. It also too can be connected
to a security system.

CONSTRUCTION SCHEMATIC:
_________o_______ +9V
| |
| __|___
R4 | Load |
| ------
| __|
| | Q3
o--------|<--.
b |/ e |___|
----| Q2 |
Q1 | |\ c |
|/ c | |
Green ----R1--------o----| | |
| | b |\ e | |
R2 | | | |
_|_ --- | | |
C1 === /_\ D1 | | |
| | | | |
Red __________o____o_____o______o____________o__ GND


COMPONENTS LIST

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
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.

I am not sure that you want R2 in there unless you want the circuit to
trip during a RING- the 0.47u is -j17K at 20Hz so that the divider ratio
will be 0.017/22=0.0008 which x90VAC or so is 70mV- or no effect. But
with R2 it will be 48VDC + 1/2 x 90VAC and this should cut Q1 off every
half cycle. And I don't see R3 at all. On SED we demand absolute
accuracy on all posted schematics- hehe- j/k'ing- but you may want to
scrutinize that ckt more closely before you build it.

 

[Fred Bloggs]

I am not sure that you want R2 in there unless you want the circuit to
trip during a RING- the 0.47u is -j17K at 20Hz so that the divider ratio
will be 0.017/22=0.0008 which x90VAC or so is 70mV- or no effect. But
with R2 it will be 48VDC + 1/2 x 90VAC and this should cut Q1 off every
half cycle. And I don't see R3 at all. On SED we demand absolute
accuracy on all posted schematics- hehe- j/k'ing- but you may want to
scrutinize that ckt more closely before you build it.

You need to make C1= 4.7u if you want that thing to coast through a RING
without tripping-use film or Tantalum.