Electronic Circuits and electronic circuits, electronic schematics plus an extensive resource for hobbyists, inventors and engineers

DiscoverCircuits.com has links to 45,000+ electronic circuits, cross-referenced
into 500+ categories.    We have searched the web to help you find quick design ideas.

HOME Schematics Hobby Corner Dave's Circuits Electronic Resources Contact Info
Imagineering Ezine    Discover Solar Energy Dave Johnson & Associates Faraday Touch Switches
Welcome to DiscoverCircuits.com - online since 2002

See Media Kit for
Advertising Opportunities

Attention Designers:  Please alert us by eMail if we have linked to sites
where your original material has been copied without your permission.
We will change to the LINK to your site.  
Thank you.


Circuits designed by David Johnson, P.E.
Last Updated on: Wednesday, October 18, 2017 05:04 AM

Master Category List - Dave's Circuits

The contents & graphics of Discovercircuits.com are copyright protected.
LINKING to Dave's circuits is permitted but DO NOT COPY any files to your WEB SITE server

More  Alarm Circuits   Burglar Alarms   Telephone Circuits



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.

Click on Drawing Below to view PDF version of Schematic

Cut Line Phone Alarm circuit

More  Alarm Circuits   Burglar Alarms   Telephone Circuits

Master Category List - Dave's Circuits
eMail David A. Johnson, P.E. about this circuit


Imagineering Ezine    Discover Solar Energy Dave Johnson & Associates Faraday Touch Switches

 About Us   |  Advertise on DiscoverCircuits.com   |   Report Broken Links  |    Link to DiscoverCircuits.com  |   
Privacy Policy

Discovercircuits.com endeavors to credit original designers.   Please alert us by eMail so we can
delete links to material that have been copied without your permission
.   Thank you.

Copyright of all original material on this website is the property
of David A. Johnson, P.E. (Dave Johnson & Associates ) unless otherwise noted.  

Linking is ALLOWED but COPYING any content or graphics to your web site is EXPRESSLY PROHIBITED.
All material is provided "as is" without guarantees or warranty of any kind, either expressed or implied.