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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 | 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.