Black boxes were devices which, when attached to home phones, allowed all incoming calls to be received without charge to the caller. The black box (as distinguished from

blue box A blue box is an Electronics, electronic device that produces tones used to generate the in-band signaling tones formerly used within the North American long-distance telephone network to send line status and called number information over voi ...

es and red boxes) was a small electronic circuit, usually a

resistor A resistor is a passive two-terminal electronic component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active e ...

zener diode A Zener diode is a type of diode designed to exploit the Zener effect to affect electric current to flow against the normal direction from anode to cathode, when the voltage across its terminals exceeds a certain characteristic threshold, the ''Z ...

in series with the line. It relied on (now-obsolete) telephone exchanges controlled by mechanical relays. These exchanges used a relay to detect a drop in line voltage (usually to less than -10V off-hook, compared to -48V when on-hook) to begin billing for a call; a separate relay controlled ringing on the line. The black box placed a

in series with the line, so that the off-hook voltage was closer to -36V: just enough to stop the ringing, but not enough to trigger billing. A bypass

capacitor In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...

was often added to prevent the device from attenuating AC signals such as transmitted voice. A call originating from a telephone fitted with a black box would still be charged for by the telephone company unless some method to circumvent the call charging was deployed. Black boxes were commonly built by phone

phreak Phreaking is a slang term coined to describe the activity of a culture of people who study, experiment with, or explore telecommunication systems, such as equipment and systems connected to public telephone networks. The term ''phreak'' is a se ...

s during the 1960s to 1980s (and in some places like Eastern Europe, well into the 2000s) in order to provide callers with free telephone calls. Sometimes several friends would incorporate a black box into each of their telephones to enable them to hold long conversations with each other without having to pay for them. Another use of black boxes was in the incoming

modems The Democratic Movement (, ; MoDem ) is a centre to centre-right political party in France, whose main ideological trends are liberalism and Christian democracy, and that is characterised by a strong pro-Europeanist stance. MoDem was establish ...

of computers running

bulletin board systems A bulletin board system (BBS), also called a computer bulletin board service (CBBS), is a computer server running software that allows users to connect to the system using a terminal program. Once logged in, the user performs functions such as ...

that were popular back in the 1980s and early 90s.

Electronic switching system In telecommunications, an electronic switching system (ESS) is a telephone switch that uses solid-state electronics, such as digital electronics and computerized common control, to interconnect telephone circuits for the purpose of establishing te ...

s rendered black boxes obsolete, as no audio path was established until the call was answered. The infinity transmitter, an eavesdropping device which in its original design relied on an audio path to the target line remaining open before a call was answered or after it was hung up by the recipient, was similarly affected by the demise of mechanical switching.

References

{{Phreaking Boxes

Black Box In science, computing, and engineering, a black box is a system which can be viewed in terms of its inputs and outputs (or transfer characteristics), without any knowledge of its internal workings. Its implementation is "opaque" (black). The te ...

See also

References