Telephone In-Use Indicator Schematic


Description
This circuit will illuminate a LED if one of your telephones is in use. It should work in all
countries (Including UK) that have a standing line voltage above 48 Volts DC.

Webmasters Note:
Please note that it is illegal to make a physical permanent connection to your telephone line
in some countries (this includes the UK and Ireland). If building this circuit it is advisable to
use a plugin cord so that the unit can be unplugged should a fault occur. If in doubt consult
either your telephone or cable operator.

In use indicator

Circuit Notes
If all extension phones are on-hook and the line voltage is around 48
V, Q1 will conduct thus effectively shorting the gate of Q2 to its
source, so it will be off and the LED will be disabled.

Lifting the handset of any phone on the line causes the line voltage to
drop to 5-15 V. The gate voltage of Q1, equal to some 6% of the line
voltage, will then be too low and Q1 will be turned off. So Q2’s gate
is now biased at approximately 1/2 of the line voltage, Q2 turns on and
the LED indicates that the line is in use. The circuit itself is
practically invisible to the other telephone devices using the same
line.

LED1 must be low-current and its current-limiting resistor must be 2k2
or more. The other components’ ideal values may vary slightly,
depending on the local telephone line parameters. The circuit is
powered off the telephone line.

If other types of MOSFETs are used, the 500k trimmer can be adjusted to
ensure that Q1 is biased fully on while the line is not in use (LED1
off), and vice versa. If Q2 is not a BS108 but some other 200 V MOSFET
with a higher G-S threshold voltage, it might be necessary to increase
the value of the lower (or decrease the value of the upper) one of the
two resistors connected to the gate of Q2.

Plain (bipolar junction) transistors can be used instead and the
circuit also works fine, but the resistor values are then much lower –
letting ten times more microamps of current pass through while the line
is not in use, and even this MOSFET design still could not meet formal
minimum on-hook DC resistance specifications.

Both prototypes’ PCBs were 4×1 cm. The current-limiting resistor for
LED1 is 2k2 in both cases. DO NOT ground any of the leads or conducting
surfaces in this circuit. A more reliable design would also include
some kind of over-voltage protection etc.

In their normal course of operation, telephone lines can deliver
life-threatening voltages! Do not attempt to build any of the
circuits/projects unless you have the expertise, skill and
concentration that will help you avoid an injury. Please see theDisclaimer on this site.

There are also legal aspects and consequences of connecting things to telephone lines, which vary from country to country. Keep away from telephone lines during a lightning storm!


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