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