A privacy circuit allowing one telephone exclusive access to the line.
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). It is therefore illegal to use this circuit
in the UK, although it is shown for Countries that do not have these restrictions.
As soon as the first user has lifted the handset of his phone, all the
other extension telephones – each one of them connected to the same
line through its own privacy circuit – are effectively cut-off. This
kind of circuit can also protect a modem user from Internet connection
interruptions caused by family members trying to use other telephones
in a household.
Many circuits have been proposed. After building and testing some ten
different (but essentially similar) designs based on SCRs or triacs,
with diacs, zener diodes etc, the conclusion was that all of them could
work, but there always was at least one foreseeable situation, a
combination of factors (usually telephone types) that could make a
specific circuit more or less unusable. Still, one simple (and not at
all original) design seems to be more universally applicable and is
shown in the picture.
When the first user lifts the handset, the line voltage of
around 48 V is high enough to make current flow through the zener
diode, triggering the SCR (in this user’s device) which starts to
conduct. A fraction of a second later, the central office ’sees’ that a
phone is off-hook and lowers the line voltage to 5-15 V. If the handset
of another extension phone is then lifted, the low line voltage will
not be able to pass through the zener diode and turn on the SCR in this
second phone’s privacy circuit, so this extension is disabled. The
first SCR will conduct as long as the line current is above 4-5 mA.
(During conversation, the current through a telephone line is around 25
mA.) The TIC47 can be replaced with some other 200 V sensitive-gate SCR
with a holding current of 5 mA or so.
One tiny flaw of this privacy circuit appeared while it was
tested on a line together with a vintage back-up telephone (inductive
mechanical bell ringer) without a privacy circuit of its own: when the
main phone’s handset has been taken off the hook, it sometimes takes an
additional click or two on the hook before the dial-tone comes through.
This is really a minor inconvenience, and only in a pretty unlikely
The diode bridge can be omitted, but then the line leads’ polarity must be carefully observed.
The ring (say 90V@25Hz) triggers the SCRs early enough in each voltage cycle, so all extensions ring.
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 the
Disclaimer on this site.
There are also legal aspects and consequences of connecting things to telephone lines,
which vary from country to country.