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British Version of the Telephone Black box - Non-BT approved !
==============================================================
The Circuit.
============
X-------|------S1------|-------Y
| |
|------D1------|
| |
|---D2----R1---|
| |
|------C1------|
S1 - SPST Toggle D1 - 50-200 volt diode (IN400x range)
D1 - Standard LED R1 - 6.1K Resistor (or combination)
C1 - 1uf Non polarised
Note: The two diodes are placed the opposite way to each other.
Modern telephones have four wires goinn to them. These are
normally coloured red, white, green and blue. The red and white
wires carry the actual line, the green prevents 'tinkling' and
the blue isn't used. Three of these wires (including the blue
and green) pass straight through the black box, untouched. The
remaining wire (either red or white - either will do) should
have the box circuit connected in series with it at points X & Y.
The Theory.
===========
When a phone is answered, approx 100 mA is drawn from the line
which tells the exchange that you have answered. However, only
about 80 mA is actually required to polarise the mike element,
so by limiting the current, you can talk to the caller whilst
their exchange thinks that the phone is still ringing (and
thus they are not charged).
The main current limiting is carried out by R1. 6.1K resistors
are hard to come by so that resistance may have to be made up
with a combination of one or more resistors.
All other components merely improve running: C1 ensures that
speech quality is unaffected and D2 is used to indicate that
the box is switched on.
The Practice.
=============
Wait for the incoming call to ring then switch the box on. The
LED (D2) should pulse with each ring. If it doesn't, reverse
the polarity of both D1 and D2. Lift the handset and the LED
should light continuosly - you may now talk to your caller
completely free of charge ! To commence charging halfway
through a conversation, switch the box off - you will hear a
click as the callers meter (in their exchange) starts turning.
Once you have started chaging, you cannot stop it even if you
turn the box back on.
The Snags.
==========
As well as being able to work from any normal telephone,
the box will work when the caller phones from a Cardphone or
old style payphone, however new style coin payphones do not
cut in the microphone until the called party answers.
On trunk or international calls, the call time is limited to
three minutes, as the exchange gives up after that time (as
it assumes no-one is in, and therfore needn't hold up the line).
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Written March 1988 by K.S.Reach (C)1988 The Hackers Academy.
This article may be downloaded and put on any bulletin board
but all credits m