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). --------------------------------------------------------------- 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