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Circuits designed by David Johnson,
P.E.
Last Updated on:
Saturday, December 23, 2017 03:21 PM
Master Category List - Dave's Circuits
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Alarm Circuits
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Cut Phone Line Alarm
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| 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. |
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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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