File to view: blackbox.dox c 
British Version of the Telephone Black box - Non-BT approved!

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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 - 1"F Non polarised



Note: The two diodes are placed the opposite way to each other.



Modern telephones have four wires going 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 the 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 charging, 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
