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Thursday, March 26, 2009

Cut Phone Line Detector

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.

  • 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 or 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

  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.

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