The black box (as distinguished from blue boxes and red boxes), was a small electronic circuit added to a telephone which provided the caller with a free call. A call originating from a telephone fitted with a black box would still be charged for by the telephone company unless the called telephone was also fitted with a black box, or some other method to circumvent the call charging was deployed. Black boxes were commonly built by phone phreaks during the 1960s to 1980s 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 were in the incoming modems of computers running bulletin board systems that were popular back in the 1980s and early 90s.
Black boxes operated by taking advantage of a feature built into many telephone exchanges where calls are only metered when the called telephone is off-hook, and metering is suspended if the called telephone is placed on-hook. A black box makes an off-hook telephone appear electrically as if it were on-hook, therefore fooling the telephone exchange into thinking that the called telephone is on-hook.
Electromechanical telephone exchanges such as Strowger and crossbar used two relays to detect the line status of a called telephone. In British Strowger telephone exchanges these were the F relay and D relay in the final selector. The F relay was used to detect a ring trip, and when activated, switched the called telephone from the ringing supply to the audio path. The D relay was used to start and stop the metering of the telephone call. The trick was to activate the F relay but not the D relay. The ringing supply consists of an AC signal of approximately 75 V superimposed on a direct current (DC) signal of approximately 48 V. Telephone ringers are connected in series with a capacitor, blocking the DC but allowing the AC to pass through and sound the ringer when the telephone is in an on-hook condition. The coil of the F relay is designed to be insensitive to AC and will only activate with a direct current. When the telephone is taken off-hook, it appears to the exchange as a low-value resistor, completing the circuit for the DC signal which activates the F relay. The F relay remains activated even if the called telephone is subsequently placed in an on-hook condition, which means that only a momentary pulse of DC is required to activate it. A continuous direct current of sufficient value through the called telephone is required to activate the D relay. This means that the D relay only activates when the telephone is off-hook and deactivates when the telephone is on-hook. An on-hook telephone minus the ringer circuit appears to the exchange as an open circuit.
The simplest black box consists of a capacitor and resistor in parallel, connected in series with the telephone. Installing a black box into the telephones commonly used during the 1960s to early 1980s was usually simple because many were assembled using screw terminals meaning that no soldering was required. For example, a black box could be installed in a 706 telephone between terminals 18 and 19 in place of the link. The purpose of the resistor is to supply a small amount of DC to operate the carbon transmitter and trip the F relay. Its value had to be sufficiently high in order to reduce the DC to a level where the D relay would fail to activate with the telephone off-hook. A value of 10 kΩ was typical although it was very much open to experimentation as the minimum currents required to activate the F and D relays varied according to the length of the local loop and the type of telephone exchange equipment used. If its value was too high then the F relay would fail to activate and if its value was too low then the D relay would activate with the telephone off-hook. The capacitor provided a low-impedance path for the AC audio signal. Its value was typically in the region of 0.1 μF to 1 μF but wasn't critical. More refined black boxes placed a diode in parallel with the resistor to provide a larger DC pulse to activate the F relay than if only a resistor was used, or replaced the resistor with a 36 V Zener diode which regulated the voltage drop across the telephone to a level where the D relay would not activate, regardless of the length of the local loop.
A more complex design of black box used a nine-volt battery to power the telephone (thus avoiding the problem of trying to draw just enough current to power the telephone without drawing so much that metering was started). The battery fed power to the telephone via an inductor and the telephone line was connected (via a capacitor on one wire) to the terminals of the inductor. The inductor was of a high enough inductance to ensure that the AC audio signals to and from the telephone were allowed to pass from and to the telephone line, while allowing the current from the battery to pass through, powering the telephone. The ring-tripping diode was connected directly across the telephone line via a push-button that had to be operated manually before picking up the telephone handset.
British Strowger telephone exchanges reversed the line polarity of the calling telephone whenever the D relay was activated. This could be used to test whether a black box worked or not. Alternatively a black box could be tested using a payphone. If the black box works then the coin does not drop. The final selectors in British Strowger telephone exchanges were fitted with a warning light which illuminated whenever the selector was in use but the D relay was not activated. Larger telephone exchanges were staffed and exchange staff would often manually release final selectors with illuminated warning lights.
Many digital telephone exchanges are designed to render black boxes ineffective. Some exchanges no longer suspend metering whenever the called telephone is placed on-hook. Other exchanges automatically terminate a call whenever the called telephone is placed in an on-hook condition for a certain amount of time.
Black boxes were mentioned in an article in New Scientist during the early 1980s. The components were correctly described but no details were provided on how to build and install a black box.
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