QUASAR CODES 3140 - DTMF Telephone Relay Switcher
Page
2
This device connects to the telephone line and can be
used to remotely control up to 4 relay outputs using a
DTMF (tone dialing) telephone. A number of user
settings are available to improve the useability and
security of the device. The
unit
comes complete
with
a
small plastic case with silk-screened front and
rear
panels.
NB This unit is not PABX compatible!
The unit is controlled by an Atmel 89C2051
microcontroller. Source code is
not
available.
**
WARNING! The maximum connection voltage is
48Vac unless the screw terminal block connectors are
insulated to prevent exposure to hazardous voltages.
FEATURES
Ouputs Relays
x
4
Output rating 5 Amps @ 240Vac
**
/ 24Vdc max.
Operating voltage 12
-15
V
dc
centre positive
(
regulated
)
Operating current
30mA with no relays operated
120mA with all relays operated
Connections
1. 2.5mm DC power jack
2. RJ-11 for telephone line
3. 4 x 3-way pluggable screw
terminals for relay contacts
Case size
13cm (W) x 10cm (D) x 3cm (H)
(6.1” x 4” x 1.3”)
User Settings
(see text)
1. Password
2. Tamper
3. Rings
to
answer
4. Auto
hangup
time
5. Lockout
CIRCUIT DESCRIPTION
We will not review the operation of how telephone
lines work or the detailed operation of the various ICs
used. See the ‘References’ section for Web links where
you can get this information.
The brains of the switcher is the Atmel AT89C2051
microcontroller, IC4. Incoming ring is detected via C4,
R7 and the opto-coupler IC2 and connected to pin 6 of
the microcontroller. This pin also has the ‘Password
Reset’ switch connected to it (more on this later).
The incoming call is answered by connecting the
circuit based around Q1 and Q2 (an electronic holding
coil) to the line via IC1, a bi-directional opto-isolator.
This circuit has a low DC resistance but a high AC
impedance which is required by telephone circuits
when the line is looped. The RC network consisting of
R4, R5 and C3 is used to provide impedance matching
to the telephone line. The metal oxide varistor
protects the switcher from telephone line transients.
The configuration and values used here are optimi
s
ed
for use with most phone systems thoughout the world
without any need for customisation. They can be
changed by the end-user to meet specific needs if desired
e.g. to meet local approval standards.
DTMF detection and decoding is provided by IC3.
This chip, an 8870, is a complete DTMF receiver
which is able to detect and decode all 16 DTMF tone
pairs into a 4-bit code. When a valid DTMF digit is
detected the 4-bit code is placed on pins 11-14 and a
‘data avaliable’ output, pin 15, is set to a logic high.
The output relays, RL1-4, are controlled via the relay
driver IC6. One output from the relay driver (pin 14) is
used to output a 800Hz software generated tone into the
telephone line via the impedance matching network. This
tone is used to signal the user when commands have
been completed or of any command errors.
User settings are stored in IC5, a 24C01 EEPROM. The
microcontroller ‘talks’ to this device using a 2-wire I2C
bus via pins 18 and 19.
IC7 provides two functions – a regulated 5V power
supply output and detecting when the phone line is
‘looped’ (in use). This chip is a Maxim MAX666 low
power programmable voltage regulator with on-chip
low-battery detection, mainly intended for use in battery
powered equipment. The regulator output voltage is set
via the ‘VSET’ input (pin 6). By tying this pin to ground
the output is fixed at 5 volts. We have used the low-
battery input to monitor the state of the phone line.
An idle (not in use) phone line normally has about
50VDC across its pair of wires. This voltage drops to
less than 20V when the line is in use. The MAX666 Low
Battery Input (LBI, pin 3) is one side of a simple voltage
comparator which compares this input to an internal
1.30V reference voltage. When the voltage at the LBI
input falls below the reference voltage then the Low
Battery Output (LBO, pin 7) will go
LOW
.
The phone line is connected to the comparator input via
the diode bridge B2 and resistors R15 and R16. The
diode bridge makes sure that the voltage polarity
connected to LBI is the same regardless of which way
around the phone wires are connected. The resistors form
a voltage divider which is used to set the ‘low battery’
threshold at LBI. For the values used the LBO output
will go
LOW
when the phone line voltage drops below 25
volts, indicating that the line is in use (looped).
PCB ASSEMBLY (AS3140 users
can
skip to page 2)
First check the components supplied in the kit against the
component listing. Identify all the components. It is
generally best to solder the lowest height components
first: the resistors, diodes & IC sockets. Leave the
connectors and relays until last.
Note that two components, resistor R14 and capacitor
C11, are mounted inside the IC sockets for IC3 and IC4
respectively. This is no problem – just fit these
components before the IC sockets.
Make sure to get the diodes and electrolytic capacitors
the correct way around. Match up the bar on the diodes
with the bar on the overlay. The negative lead of
electrolytic capacitors is marked by a ‘stripe’ whereas
the PCB overlay indicates the hole for the positive lead.