Build this 2-tone
alarin Inodule
You've just finished designing an alarm
project and you need an ear-catching alarm
to go with it. If that's the case, then this
alarm sounder is the go. It produces a
melodic 2-tone sound, packs quite a punch
for its size and uses only a few parts.
By DARREN YATES
This circuit was basically designed
for all those times we needed a simple
circuit that made an "easy on the ears"
sound. It's very similar to the alarm
siren in the Egg Timer project published in the November 1990 issue of
SILICON CHIP. It's perfect for any alarm
type application such as a water level
monitor, automotive gauge monitor,
or in a kid's game to indicate a winner.
The module uses "easy to get" parts,
can be built in a couple of hours and
shouldn't cost any more than about
$10 to make.
It has only one input point and is
simple to operate. Just connect the
control input to the positive supply
rail to make the alarm sound, and
ground it to turn the sound off again.
Circuit diagram
Let's take a look at the circuit diagram in Fig.1 and see how the circuit
works. The module uses a single 4093
Schmitt trigger NAND gate IC, a few
transistors, a couple of diodes and
little else.
Gate IC1a is connected up as the
control oscillator. This determines
how fast the circuit toggles between
the two tones. If pin 1 is connected to
the positive supply rail, the gate is
enabled and the circuit begins to oscillate.
If you look at the truth table in
Table 1, this will show you why. If
one of the input pins of the gate is
low, then it doesn't matter what you
do with the other, the output will
always remain high. This corresponds
to lines 1 and 2 of the truth table as
well as lines 1 and 3 since it doesn 't
matter which input is taken low.
However, once you take one of the
input pins high , the output then becomes the opposite of the value on
the other input pin, matching lines 3
and 4 as well as lines 2 and 4 in the
truth table.
The NAND gate here is a special
type known as a Schmitt trigger gate.
Normal gates have a very small threshold area or "hysteresis" level where
an input voltage causes a change in
the output level. For a normal 4011
NAND gate, the threshold level is
about half the supply voltage plus or
minus a few millivolts.
The Schmitt trigger gate, however,
The parts for the 2-tone chime are all
installed on a small PC hoard. The
project can drive a small loudspeaker
to good volume & is ideal for a wide
range of applications requiring an
audible alarm (eg, door chimes, games
& automotive monitors).
48
SILICON CHIP
----------------------+3V-15V
16VWI
10
+
01
.,,
1 PCB,codeSC08111901, 73x
46mm.
5 PC pins
1 8Q 57mm loudspeaker
47k
1M
120k
Semiconductors
1 4093 quad 2-input NANO
Schmitt trigger (IC1)
1 BC558 PNP transistor (01)
1 BC338 NPN transistor (02)
1 BC328 PNP transistor (03)
3 1 N914 signal diodes (D 1-D3)
-:-
150k
B
ELJc
VIEWED FROM
BELOW
TWO-TONE CHIME MODULE
Fig.1: the circuit is.based on three Schmitt trigger NAND gates: ICla, IClc &
ICld. ICla is the control oscillator & sets the rate at which the circuit toggles
between the two tones generated by IClc & ICld. The outputs from the tone
oscillators are then mixed & fed to an audio amplifier (Ql, Q2 & Q3).
has a fairly large area, usually about
1/3rd of the supply voltage. This
means that the input voltage must go
below 113rd of the supply voltage before the gate recognises it as a low
and above 2/3rds of the supply rail
for it to be seen as a high. So, if we
have a 9 volt supply rail then the
upper threshold is about 6 volts and
the lower threshold is about 3 volts,
giving us a hysteresis level of about 3
volts.
By adding just two components, we
can turn this gate into a simple squarewave oscillator. Looking back at the
circuit diagram , notice that there is a
lMQ resistor connected between the
output and one of the input pins (pin
2), and a 0.lµF capacitor from that
input pin to ground. Ifwe assume the
other input is held high, then this is
how the oscillator works.
Initially, the capacitor has no voltage across it and the gate recognises
this as a low. If you look back at the
truth table in Table 1, you can see
that one high input and one low input gives us a high output. The capacitor now begins to charge up via
the current flowing from the gate output and through the lMQ resistor.
The output remains high until the
voltage across the capacitor reaches
2/3rds of the supply voltage. When
this happens, the gate realises it now
has a high on both inputs, and so
pulls the output low. (Check this again
PARTS LIST
on the truth table shown below).
The capacitor now begins to discharge through the lMQ resistor, out
through the low output. Once the
voltage falls below 113rd of the supply voltage, the gate sees a low on one
of its inputs and so sends the output
high again. The capacitor again begins to charge up through the lMQ
resistor until it reaches the upper
threshold level and so the cycle c.ontinues.
There are three of these oscillators
in the circuit, formed by gates ICla, c
& d. ICla forms the control oscillator
while IClc and ICld are the two tone
oscillators. The frequency of ICla is
much lower than the other two so
that the two tones can be easily distinguished. The frequency of this type
of oscillator can be easily varied by
changing either the resistor or capacitor value. Reducing the value of either component increases the frequency and increasing the value decreases the frequency.
TABLE 1
INPUT A
INPUT B
OUTPUT
(pin 1)
(pin 2)
(pin 3)
Low
Low
High
High
Low
High
Low
High
High
High
High
Low
Capacitors
1 100µF 16VW electrolytic
1 10µF 16VW electrolytic
1 0.1 µF metallised polyester
(greencap)
2 .018µF metallised polyester
1 .0039µF metallised polyester
Resistors (0.25W, 5%)
1 1MQ
2 10kQ
2 150kQ
1 1kQ
1 120kQ
1 68Q
1 47kQ
Miscellaneous
Hookup wire, solder, etc
The outputs from these two oscillators are then coupled together by
two lOkQ resistors to a common
.0039µF capacitor. These components
form a very simple audio mixer. The
signal is then fed to the base of transistor Ql, which is biased on by the
4 7kQ resistor.
Transistors Ql-Q3 together form a
low-power audio amplifier. Ql provides the voltage gain and biasing for
the push-pull output stage based
around transistors Q2 and Q3. The
68Q resistor and diode D3 provide a
voltage drop of about 1 volt between
the bases of transistors QZ and Q3 to
reduce crossover distortion, to provide a clean output signal. They also
set the quiescent current to about
5mA. (You could even use this amplifier circuit for your own projects!)
The amplified audio signal appears
at the junction of the emitters of Q2
and Q3 and is coupled to the loudspeaker via a lO0µF capacitor.
Construction
If you make or buy the PC board,
check that there are no shorts or
breaks in any of the tracks. If there are
JANUA RY 1991
49
Fig.2: be sure to orient all polarised parts correctly
when installing them on the PC board. These parts
include the IC, transistors, diodes & electrolytic
capacitors.
any, correct them now before you do
any soldering.
Once you're happy that everything
is OK, solder in the PC pins and the
wire links. Next install the resistors
and the greencaps. Use the wiring
overlay diagram to make sure you
have them in the right place on the
board.
Now install the electrolytic capacitors. Check that you have them correctly installed before you solder them
in. Once you've done that, solder in
the diodes, transistor and the IC.
Testing
If you're happy that everything is
OK, you can now test the circuit operation. First, connnect a wire link
between the control pin and the posi-
CAPACITOR CODES
0
0
0
0
Value
IEC Code
100n
0.1µF
.018µF 18n
.0039µF 3.9n
EIA Code
104
183
392
Fig.3: use this full-size artwork to etch your own PC
board & to check the board for etching defects.
tive supply pin. If you now connect
up a voltage supply of between 3 and
15 volts, you should get an instant
chime from the speaker.
Disconnect the power and remove
the link between the positive supply
and control pins and replace it with a
link from the control pin to the ground
pin. If you again switch on the power,
you should hear nothing.
If this doesn't work, disconnect the
power and check the board for any
accidental solder splashes between
the tracks. You should also check that
you have all the components correctly
installed.
Experimenting
Since we are using only dirt-cheap
and common components, you may
like to experiment a bit with the circuit in the following way:
To increase the frequency at which
the circuit cycles between the two
tones, decrease the lMQ resistor connected to ICla. You will notice some
strange effects, the lower in resistor
value you go, but don't go below about
lOkQ otherwise the gate begins to
dissipate too much power. You can
decrease the frequency also by increasing the resistor value.
If you want to change the frequency
of the actual tones themselves, you
can increase or decrease the value of
the 150kQ and lZ0kQ resistors connected to IClc and ICld respectively.
As before, increasing the resistor value
decreases the frequency and decreasing the resistor value increases the
frequency. Again, don't go below
l0kQ.
The volume of the chime can be
increased by replacing the .0039µF
capacitor with a larger value - say
.0082µF. Be careful though, as this
increases the current consumption of
the amplifier from about 30mA to
90mA. Don't go above .0lµF or you
may end up damaging the output transistors.
Most importantly, do not take the
control input higher than the circuit
supply voltage otherwise you will kill
the IC. What you can do is to run the
chime module from an existing power
supply and generate the control signal from that supply voltage.
SC
RESISTOR CODES
0
0
0
0
0
0
0
0
50
No
Value
4-Band Code (5%)
5-Band Code (1%)
1
2
1MQ
150kQ
120kQ
47kQ
10kQ
1kQ
68Q
brown black green gold
brown green yellow gold
brown red yellow gold
yellow violet orange gold
brown black orange gold
brown black red gold
blue grey black gold
brown black black yellow brown
brown green black orange brown
brown red black orange brown
yellow violet black red brown
brown black black red brown
brc,wn black black brown brown
blue grey black gold brown
1
2
SILICON CHIP
