GENERAL
INTEREST
47
11/2002
Elektor Electronics
vide a reference voltage of half V
A
for the
non-inverting input of IC2.B. Capacitor C9
smoothes any noise on this reference voltage.
Both amplifiers have a bandwidth of 10 kHz,
which is more than enough for this applica-
tion.
Brown-Out protection
Brown-outs occur when the mains supply
undergoes a brief interruption, just enough to
cause your houselights to dim momentarily.
These interruptions cause havoc in micro-
controller systems that do not have brown-
out protection. The microcontroller supply
voltage starts to fall, producing unreliable
operation but then recovers again before a
reset can be generated. The processor can
mis-read data values and instructions, end-
ing up in an undefined state and systems
using EEPROMs can have stored values cor-
rupted. The brown-out protection circuit used
level of amplification. The supply
voltage to this IC is 12 V.
The audio signal is output from
this stage on the wiper of preset P2.
The signal quality is not especially
hi-fi but it is not necessary for this
application. Any high frequency
ringing or oscillations are damped by
capacitor C2.
The potential divider formed by
resistors R2, R3, P1 and R4 produces
a fixed voltage at approximately
half-rail (2.5 V) for the non-inverting
input of IC2.A capacitor C3
smoothes out this voltage.
The signal is now fed to the com-
parator input of IC1.
Figure 2
shows
the internal features of this chip. One
input of the comparator (AIN1) is
connected to an adjustable reference
voltage (REF). The other input
(AIN0) is connected to the AF signal
(CMP) and when this signal rises
above the reference voltage the com-
parator output will switch and gen-
erate an interrupt in the microcon-
troller. If the microcontroller estab-
lishes that the sound is a baby
crying it will change the AIN0 pin to
an output and then output a zero to
effectively short out the AF signal
(R8 limits the short circuit current).
Next the microcontroller sends out
the DTMF signals on the PWM pin to
dial the number. A more detailed
description of this process is
described under the heading ‘DTMF
signals’.
Amplifier IC2.B has a dual pur-
pose to amplify audio or the PWM
signal. The amplification factor
required here is around two for
DTMF signals and ten for audio sig-
nals. Resistor chain R13 and R14 pro-
2
3
1
IC2.A
S2
S1
C13
22p
C12
22p
X1
10MHz
K2
+5V
ISP
P2
22k
R6
220k
R5
100
R2
8k2
R1
10k
C3
47
µ
K1
T2
BC557B
R21
47k
R23
180k
R22
33k
R20
4k7
R17
220
R18
220
D1
D2
C10
1
µ
R19
4k7
T1
BC547B
RE1
1 : 1
TR1
D3
1N4148
R16
560
+5V
7805
IC3
C16
100n
C17
10
µ
63V
C19
470
µ
25V
+5V
VCC
MISO
MOSI
RESET
GND
SCK
a
b
a'
b'
012016 - 11
R4
10k
AT90S2313-10PC
(MISO)PB6
(MOSI)PB5
(AIN1)PB1
(AIN0)PB0
PD2(INT0)
PD3(INT1)
(SCK)PB7
(OCI)PB3
PD0(RxD)
PD1(TxD)
PD6(ICP)
PD4(T0)
PD5(T1)
XTAL1
XTAL2
RESET
IC1
PB4
PB2
20
10
19
18
17
16
15
14
13
12
11
5
4
1
2
3
6
7
8
9
R12
33k
R11
15k
R10
1k5
R9
100k
C6
1
µ
R15
1k
+5V
REF
PWM
C14
100n
C18
100n
D5
1N4002
MIC1
MCE2000
C1
680n
C2
68p
10k
P1
R3
2k2
C4
220
µ
10V
R7
47
IC2
8
4
R24
47
D4
12V
1W3
C15
100n
63V
6
5
7
IC2.B
63V
C11
470n
C7
100n
C5
1
µ
63V
R8
1k5
R14
100k
R13
100k
25V
C8
680p
C9
10
µ
63V
LM-NP-1001
9...12V
+V
A
+V
A
CMP
IC2 = TS922IN
Figure 1. The circuit contains both analogue and digital circuitry.
Содержание EPROM
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