October
2004, Issue 171
Telephone
Message Watchdog
An
Intelligent Call-Forwarding System
by
Jingxi Zhang, Yang Zhang, & Huifang
Ni
CIRCUIT
CONSTRUCTION
As
we mentioned, we used Basic Micro’s H8S/2398 starter
kit board. The telephone line interface circuit was
constructed on a separate board. The starter kit board
piggybacks on top of the interface board. A 14-conductor
ribbon cable electronically connects both boards (see
Photo 1).
|
(Click
here to enlarge)
|
Photo
1—The bottom is the telephone interface board, which
connects to the top evaluation board by a 14-conductor
ribbon cable. |
Figure
4 is a schematic of the telephone line interface circuit.
T1 is a 600-W isolation transformer. T1 and C1 compose
the audio coupling circuit. C1’s voltage rating should
be higher than 100 V. We selected a 1-µF, 250-V metal
polyester capacitor. D1 and D2 provide surge protection.
|
(Click
here to enlarge)
|
Figure
4—JP1 and JP2 are the connectors of the starter
kit board. The power supply block is connected to
the evaluation board power supply switch 5-V regulated
source and 9-V unregulated source. Note the point
in-between T1 and R2 is a connection point to the
optional audio monitor circuit. |
IC1
is a Texas Instruments dual single-supply, rail-to-rail
(I/O) op-amp, OPA2340. One channel of the OPA2340 (IC1a)
is for a touch-tone detection signal-conditioning amplifier.
It provides a 5-V output range, which is optimal for
the ADC. OPA2340 also accepts rail-to-rail input. That
makes the DTMF tone generator design easier. The DAC
can directly drive the 5-V ranged DTMF signal to the
op-amp. Two channels of generated basic DTMF tone are
mixed by IC1b, the other half of the OPA2340. The mixed
signal is then fed into the isolation transformer T1.
IC2
is an AC optocoupler for the loop current sensor. Because
the loop current can be as high as 70 mA in a short
circuit situation, we selected the Fairchild H11AA3
optocoupler, which has a good IF rating (IF = 100 mA).
IC3 is Toshiba’s TLP222G optocoupler, which contains
an infrared-emitting diode coupled with a bidirectional
photo-MOSFET circuit with a 350-V/120-mA rating. Therefore,
it’s safe to use as a relay to connect a 600-W resistor
(R7) across the tip and ring lines when the MCU engages
the phone line.
Bridge
rectifier D4, Zener diode D5, high-voltage transistors
(VCE = 400 V) Q1 and Q2, and optocoupler IC4 comprise
a phone line condition detector. When the phone is on
the hook, there is a –48-V voltage across the tip and
ring lines. D4 routes the positive voltage polarity
to the 16-V Zener diode D5. Because the on-hook line
voltage is higher than the Zener diode’s breakdown voltage,
the Zener diode conducts and the small current flowing
through D5 causes Q1 to conduct. As a result, Q2 is
shut off and no current flows through the IC4 emitter
LED. The IC4 photosensitive transistor is therefore
turned off, and the line input to the MCU stays high.
If the line is in use, the line voltage drops to a lower
level in the range of 5 to 10 V. In this case, D5 is
not conducting, and therefore causes Q1 to shut off.
As a result, Q2 turns on and the IC4 emitter LED lights
up. This causes the line input to the MCU to drop to
logic 0, indicating that the phone line is busy.
S1
is a 4PDT mode switch. In Normal mode, the telephone
line is connected to the DTMF tone detector. When it
is switched to Setting mode, a specific phone port is
connected to the DTMF tone detector. This allows a regular
telephone (no power) connected to this port to dial
the owner’s preset pager number and home phone number.
To allow the regular telephone to be used, the port
is also fed a power supply through resistor R2. D1 is
a protection diode to prevent damage from mistakenly
connecting a 48-V phone line to this port. In that case,
D1 becomes nonconductive.
