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Cut Phone Line Alarm --
Burglars, rapists and other criminals will often
cut a phone line before they break into a home. The cut phone line will prevent many
alarm systems from alerting the police. The battery powered circuit below will sound
an alarm when the unit no longer detects the voltage present on an active phone line. |
| A bridge rectifier front end
makes the circuit phone line polarity independent. It shows four 1N4004 diodes being used
but you can also use a standard low current bridge rectifier package with a 200v rating or
more. The bridge is wired so the output voltage goes negative relative to circuit ground.
The phone line will typically be somewhere around 30v when in standby mode and drop to
something less than 15v when a phone is connected to the line. In the circuit, this
translates a negative voltage fed to the gate of the 2N7000 FET, which kept the transistor
turned off. When the phone line is cut, the voltage fed to the gate of the FET will
swing to +9v, which will turn on the FET. When Q1 turns on, it also turns on Q2,
which routes power to a 555 timer circuit. The 555 timer is wired as a 1Hz square
wave oscillator, which drives the transistor Q3. That transistor drives an alarm
beeper. With the components shown, the circuit draws nearly zero current in standby
mode. Therefore a good quality 9v battery should last for many years. |
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Click on Drawing Below
to view PDF version of Schematic |
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Master Schematic
Category List -
David A. Johnson, P.E.
Alarm Circuits
Burglar Alarms
Telephone Circuits |
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