Elektor EPROM, Руководство пользователя

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GENERAL INTEREST
46 Elektor Electronics 11/2002
There are several different types of baby mon-
itor on the market. The most common pro-
vides a short-range radio link (typically
100 m) between the transmitter unit in the
nursery and the receiver unit carried by a par-
ent. Another type uses audio to modulate a
carrier frequency sent along mains wiring to the parents receiver plugged into a
mains outlet. The RF model has a
limited range and the mains modu-
lation method requires that the par-
ent cannot wander too far from a
mains socket. Both systems are sus-
ceptible to interference.
The device described here goes
about it in a different way: The unit
is plugged into the house telephone
socket and positioned in the nursery
so that its in-built microphone can
pick up sounds that the baby makes.
The baby monitor microcontroller
will analyse all sounds and deter-
mine if it hears a cry. When a cry is
detected and persists for a given
time, it will dial-out a pre-pro-
grammed number (typically the par-
ents’ mobile). There is no need to
answer the call, a glimpse at the
mobile’s display will indicate that
the house phone called you and
junior is demanding attention!
Noise discrimination
The baby monitor circuit diagram is
shown in
Figure 1 . The electret
microphone MIC1 detects sounds in
the nursery. Resistor R1 provides a
bias from the supply rail while R7
and C4 are used to decouple any
noise on the microphones supply.
The AF (audio frequency) output sig-
nal is fed to the inverting input of
IC2.A via capacitor C1. IC2 is a so-
called rail-to-rail dual op-amp from
ST (available from Farnell). This type
of IC is necessary in this application
because it allows us to set a high
Telephone Baby
Monitor
cut through the static
Design by T. Finke
The usefulness of baby monitor alarms using (FM) radio has always been
limited by poor range and its susceptibility to interference. This novel
design uses a microcontroller to take a fresh look at the problem.