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baby does not overload the input to IC2.B.
This setting is best performed by viewing the
output of IC2.B with an oscilloscope while
adjusting P2. Lastly, the monitor trigger level
is adjusted by preset P1. This adjustment
ensures that the unit does not respond to
background noises in the nursery. The baby
monitor will not dial-out each time this com-
parator is triggered, the frequency of the
sound must be above a threshold, and the
sound must persist for a time greater than a
minimum period. Both of these parameters
are stored in EEPROM and can be altered at
will.

When the microcontroller recognises the

sound as a baby cry it will switch relay RE1
to connect the monitor to the telephone line
(equivalent to lifting the receiver of a phone),
the red LED will switch on continuously and
then it will call the stored telephone number
by using tone dialling. After a preset ring
time (two to three rings) the monitor will
hang up. The monitor will now remain in a
‘standby’ state for a pre-programmed period to
prevent it from being continually triggered.
The red LED will flash during this period,
finally the unit will switch back to its opera-
tional mode.

Dial out

Pressing pushbutton S2 will immediately call
the dial-out phone number stored in memory.
External equipment (e.g. house alarm or tem-
perature monitor) with a normally open type
relay or open collector output can wire this
output in parallel to S2 so that the baby mon-
itor will dial out when the alarm condition is
detected.

Enter the number

The method of entering the dial-out phone
number is a little long winded but once it is
stored in EEPROM the chances are that you
will not need to change it too often. A long
press of pushbutton S1 (longer than two sec-
onds) will switch the monitor unit into pro-
gramming mode (the red LED will come on

and the green will go out). Now
enter the first digit of the dial-out
phone number: press pushbutton S1
corresponding to the digit (if the
number is zero do not press S1 at all)
after tapping in the digit press S2 to
enter it and move on to the next digit
of the number. Carry on until the
complete number is entered and
then a long press of S1 will finish the
process. If you need to program a
time delay in the dialling process
(The number may involve a connec-
tion through an exchange) then just
press S1 10 times where you want
the delay in the phone number. If
you make a mistake when entering a
digit then two long presses of S1 will
return you to the start of the number
and you can make another attempt
at re-entering it.

Checking the number

The stored dial-out phone number
can be checked by pressing and
holding pushbutton S2. The red LED
will now light continuously and the
green LED flashes to indicate the

value of the first programmed digit,
repeating after a short delay (the
LED will remain off if the number is
zero). Pressing S2 then moves on to
the next digit. After the last digit the
red LED will go out and the green
LED will light continuously to indi-
cate that the unit is ready for use.

Programming

All the baby monitor software is
written in ‘C’. Constants used in the
program (the noise detector mini-
mum frequency, various timer values
and the dial-out phone number) may
be changed without resorting to
Compiler software but by using a
simple programming device to
directly overwrite the values stored
in EEPROM:

Pressing S1 and S2 together at
power-up will load the default values
to EEPROM.

(012016-1)

GENERAL

INTEREST

Elektor Electronics

11/2002

Address Function

Default

$00-$1F

Dial-out phone No. (ASCII coded). $00 is the terminating
character.

$20

Low frequency cut-off. Sounds above this frequency are classified
as a baby cry. (f