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Telphone Line Use Monitor

The LED is off. When the line is in use, the voltage drops to about 8 volts AC. Thus Q1 is off and Q2 is allowed to turn on and the LED will light up.

This circuit detects the fact that ANY phone on a land line is in use. The phone company does not want any circuit to load the line down, introduce voltage, or have any path to ground. This circuit meets all of those requirements. Any phone on the line that is activated, will cause the LED to light up.

When the line is not in use, the voltage on it is about 53 volts AC. This is enough to turn the zeners on. (Use a 15 volt zener up to a 20 volt one. Here two 7.5 volt ones were used). This in turn will turn transistor Q1 on and thus the voltage on its collector is very low and Q2 cannot be turned on.

Regarding the LED, a Super RED LED RL5-R8030 from was used and it only required a 22K resistor for proper illumination.

If you use other LEDs, they might require that you reduce the value of R3. This of course will reduce battery life. The printed circuit board is 2 1/2 inches wide by 1 3/4 inches high. The 1/2 inch thick piece of Oak is 2 1/2 inches by 2 1/2 inches.

Caution: Incoming calls (ringing) can cause voltages of more than 90 volts on the line and as such, it is recommended that this circuit be constructed and then plugged into a phone outlet without touching the wires.

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