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

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.