October
2004, Issue 171
Telephone
Message Watchdog
An
Intelligent Call-Forwarding System
by
Jingxi Zhang, Yang Zhang, & Huifang
Ni
PHONE
LINE INTERFACE
The
telephone message watchdog system contains the H8S/2398
evaluation board from the kit and analog telephone interface
circuitry. The MCU in the evaluation board handles touch-tone
signal processing.
Figure
1 is a simplified diagram of the device interface to
a telephone line. In compliance with part 68 of the
FCC rules, the MCU circuitry is isolated from the telephone
line by an isolation transformer and optocouplers. An
AC optocoupler (IC2), which is connected between the
telephone input line and the answering machine, serves
as a line loop-current detector. When the answering
machine engages the line, a 20-mA loop current flows
through the optocoupler LED emitter, causing the optically
coupled photosensitive transistor to conduct. The transistor
collector voltage VCE drops and triggers the MCU to
enter the touch-tone detection state.
|
(Click
here to enlarge)
|
Figure
1—IC2 is a bidirectional optocoupler for the answering
machine’s loop current sensor. IC3 is for line-engage
control. IC4 is for delivering line voltage condition
information to the MCU. |
When
the switch is set to the normal position, the system
is in working condition. The touch-tone audio signal
is coupled to the system through an isolation transformer.
In order to prevent the system from drawing a DC current,
a capacitor is used in series with the isolation transformer.
An op-amp conditions the incoming audio to optimal voltage
for the MCU’s A/D converter.
After
the caller hangs up the telephone receiver, the loop
current drops and the IC2 photosensitive transistor
collector voltage rises. The rising voltage triggers
the MCU to stop the touch-tone detection state and start
the message forwarding. First, the line condition detection
circuitry checks the line condition. When the line is
free, there is approximately –48 V across the tip and
ring lines. If anyone is using the line, the voltage
is dropped to a lower level and the line condition circuitry
turns the photocoupler IC4 photosensitive transistor
on to signal a “line busy” condition to the MCU. If
the phone line is free, the MCU drives another photocoupler
(IC3) to close a resistor path across the tip and ring
lines. This action draws a 20-mA DC loop current and
engages the telephone line. When the remote telephone
switching office senses the loop current, it acknowledges
by sending a dial tone and preparing to receive touch-tones.
The MCU, in turn, starts the touch-tone generator and
dials your pager number. The generated touch-tone is
amplified by an op-amp and coupled to the telephone
line by the isolation transformer.
If
you turn S1 to the setting position, it allows a telephone
to connect to the device. You can then use a regular
telephone’s touchpad to preset the pager number and
home phone number.
