Method and apparatus for off-hook detection

An apparatus, connected to the tip line and the ring line of a telephone, a first potential difference existing between the tip line and the ring line, the telephone having a handset, for detecting an off-hook condition on a telephone, comprising:

• a. a bridge circuit having a first input connected to the tip line and a second input connected to the ring line and having a first output and a second output, a second potential difference existing between the first output and the second output, the second potential difference equal to the absolute value of the first potential difference;
• b. a differential amplifier, operationally coupled to the first output and the second output, that generates a difference signal representative of the second potential difference;
• c. a first comparator, responsive to the difference signal, that generates a first indicator signal when the difference signal is above a first value corresponding to a maximum value of potential difference between the tip line and the ring line when the handset is off-hook;
• d. a second comparator, responsive to the difference signal, that generates a second indicator signal when the difference signal is below a second value corresponding to a minimum value of potential difference between the tip line and the ring line when the handset is off-hook;
• and
• e. a microprocessor, responsive to the first indicator signal and the second indicator signal, directed by a computer program to:
• i. generate an off-hook signal indicating that the handset is off-hook when the difference signal is between the first value and the second value;
• ii. continue to generate the off-hook signal if the difference signal drops below the second value and remains below the second value for less than a predetermined period;
• and
• iii. cease to indicate that the handset is off-hook if the difference signal drops below the second value and remains below the second value for at least the predetermined period.

A series of operational steps to be performed on a computer used to detect an off-hook condition of a telephone and to perform control functions of an auxiliary telecommunications device, the telephone and the auxiliary device being operationally coupled to a telephone line on which DTMF digits are dialed, the telephone having a handset, the computer responsive to a voltage difference signal indicative of a voltage value between the tip line and the ring line of the telephone, comprising the steps of:

• a. recording DTMF digits being dialed into a memory when the difference signal indicates that the voltage value is below the maximum value corresponding to the off-hook condition;
• b. starting a timer when the voltage signal indicates that the voltage value has fallen below a predetermined value;
• and
• c. interrupting the recording of DTMF digits and resetting the memory when the timer indicates that a preselected period has passed while the voltage value is below the predetermined value.

An apparatus connected to the tip line and the ring line of a telephone line, comprising:

• a. a telephone connected to the tip line and the ring line;
• b. an auxiliary device connected to the tip line and the ring line;
• c. means for detecting an off-hook condition in the telephone, comprising
• i. a bridge circuit having a first input connected to the tip line and a second input connected to the ring line and having a first output and a second output, a second potential difference existing between the first output and the second output, the second potential difference equal to the absolute value of a first potential difference existing between the first output and the second output;
• ii. a differential amplifier, operationally coupled to the first output and the second output, that generates a difference signal representative of the second potential difference;
• iii. a first comparator, responsive to the difference signal, that generates a first indicator signal when the difference signal is above a first value corresponding to a maximum value of potential difference between the tip line and the ring line when the handset is off-hook;
• iv. a second comparator, responsive to the difference signal, that generates a second indicator signal when the difference signal is below a second value corresponding to a minimum value of potential difference between the tip line and the ring line when the handset is off-hook;
• and
• v. a microprocessor, responsive to the first indicator signal and the second indicator signal, programmed to determine when the handset is off-hook and to generate an operative control signal to a facsimile machine when the handset is off-hook, the microprocessor being directed by a computer program to:
• A. record DTMF digits being dialed into a memory when the values of the first indicator signal and the second indicator signal indicate that the value of the difference signal is within a range corresponding to an off-hook condition;
• B. start a timer when the second indicator signal indicates that the difference signal has fallen below the second value;
• and
• C. interrupt the recording of DTMF digits and reset the memory when the timer indicates a preselected period has passed while the value of the difference signal is below the second value;
• and
• d. means, responsive to the detecting means, for inhibiting the operation of the auxiliary device when the detecting means detects an off-hook condition.