A new and improved apparatus for preventing unwanted telephone calls comprising, in combination:
- a housing having a front face, a rear face, a top surface, a bottom surface, and a pair of side faces formed therebetween defining an interior space, the housing having an input jack situated on the rear face of the housing and selectively connected to a conventional telephone jack, the housing further having an output jack situated on the rear face thereof and adapted to be releasably coupled to a telephone output line connected to a conventional telephone;
- an encoder situated within the interior space of the housing and comprising a single input line, four output lines, and a clock, the encoder adapted to generate a four-digit binary representation of an analog signal indicative of a first single decimal digit upon the receipt thereof via the input line, the encoder further adapted to generate a clock pulse upon the transmission of the binary representation of the analog signal on to the four output lines;
- state memory situated within the interior space of the housing and comprising four input lines, four output lines, and a clock input line, the input lines connected to corresponding output lines of the encoder and the clock input line connected to the clock of the encoder, whereby the state memory is adapted to maintain the state of each of the output lines thereof identical to those of the encoder upon the receipt of a clock pulse;
- password circuitry including a light emitting diode digital display positioned on the top surface of the housing, selection means, and four output lines, the password circuitry allowing a user to select a second decimal digit with the selection means and depict the decimal digit via the display, the password circuitry is adapted to further maintain a binary representation of the second decimal digit on the four output lines thereof;
- digital comparator circuitry comprising four exclusive-NOR gates each having inputs coupled to corresponding output lines of both the state memory and password circuitry, the digital comparator circuitry adapted to produce an active state on the output of each gate upon the matching of the output lines of the state memory and the output lines of the password circuitry;
- switching means situated within the interior space of the housing and connected to the input jack, output jack, and the output lines of the comparator memory, the switching means adapted to allow communication between the input jack and output jack upon the activation thereof effected by all of the output lines of the comparator memory being of an active state;
- playback means comprising an integrated circuit adapted to allow the digital recording of a brief prompting message thereon and further allow the playback of such message upon the activation thereof;
- and
- control circuitry situated within the interior space of the housing and connected to the playback means, output jack, and the input line of the encoder, the control circuitry, in a first mode of operation, adapted to activate the playback means and allow the transmission of the message to another party via the output jack upon the receipt of a call, to allow the transmission to the encoder of an analog signal generated by the depression of a digit on a phone of the other party, and afford ringing of the telephone of the user upon the activation of the switching means, thereby allowing communication between the input jack and the output jack upon the receipt of a proper single digit password, the control circuitry adapted to allow conventional use of the telephone in a second mode of operation and further unconditionally preclude communication between the input jack and output jack of the housing in a third mode of operation.