This is only a preview of the May 1999 issue of Silicon Chip. You can view 33 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
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CARBON
MONOXIDE
ALARM
By JOHN CLARKE
Exposure to carbon monoxide
(CO) gas produces an
insidious form of poisoning
which at best can give the
victim a headache and at
worst can result in death.
This CO Gas Monitor
warns you of rising
CO levels and emits a
loud tone when the
concentration
reaches a
preset
threshold.
Features
•
•
•
•
•
•
•
•
•
Sensitive detection of CO gas (<200ppm)
Uses rugged and reliable semiconductor sensor
Sensitivity adjustment
Precautionary CO level visual alarm
Main higher CO level visual and auditory alarm
Main alarm reset on continuous tone alarm
Automatic purging of sensor
7-minute CO sensing time every ten minutes
2-minute sensor heat purging time
MAY 1999 61
D
ON’T BE TOO COMPLACENT here is that people often associate diz- on the dashboard of your vehicle or
mounted towards the rear of a van or
about the risks of CO poison- ziness and nausea with “car sickness”
ing in your car, particularly if or “motion sickness”. However, it’s station-wagon.
you drive an old “bomb”. Admittedly, quite possible that what is described
While presented as a standalone
there’s not much risk in a modern car as “car sickness” is really a good dose
unit and perfectly practical as deof carbon monoxide.
or if the vehicle is well maintained.
scribed, even this small case might
On older cars, however, there’s a
Table 1 shows the effects of various
look out of place in a today’s modern
very real danger if the seal around the levels of CO concentration. As you cars – that’s if you can find a suitable
rear door or bootlid has deteriorated can see, even small concentrations mounting position at all.
sufficiently to allow exhaust gases to
spell real danger. That’s because
A more practical arrangement
seep into the cabin.
carbon monoxide has over 200 times might be to mount the PC board
If the bootlid or rear door no longer more affinity with the haemoglobin
without the case under the dashboard,
in your blood than oxygen. It literally
seal properly, exhaust gases can be
with the CO sensor, front panel LEDs
stops the blood supply from carrying and switches mounted somewhere
sucked into the rear of the vehicle
oxygen and if enough haemoglobin is more suitable.
as you drive along. Rust holes are
affected, the brain suffers from oxygen
another problem.
Another idea would be to mount the
starvation.
Opening a window doesn’t ease the
project, complete in its case in, say,
In severe cases, a blood transfusion the boot, again with the CO sensor,
situation; in fact it can often turn a bad
situation deadly by altering
LEDs and switches brought
airflow within the vehicle.
to a small plate mounted
Table 1: Effects of CO Gas Poisoning and Symptoms
You could be driving along
on or under the dashboard.
seemingly unaffected while
Regardless of where it’s
CO Gas Symptoms
your rear seat passengers cop
installed, this unit is very
Concentration
a bad dose of carbon monsensitive to the presence of
50ppm
Exposure for a few hours normally
oxide (CO) poisoning with
CO gas. Just driving along
results in no symptoms
possible fatal consequences.
in heavy bumper-to-bumpThe other big danger is
er traffic with the window
100ppm
Exposure for a few hours results in
from a poorly maintained exopen or the airconditioner
a slight headache in the forehead
haust system. If there’s a hole
set to “fresh-air” is enough
500ppm
Exposure for one hour results in a
anywhere in the system, it’s
to trigger the unit, for exheadache with increasing severerity
possible that exhaust gases
ample.
over time
could seep past any defecSo far, we’ve only talked
tive seals and into the cabin.
about using this alarm in
1000ppm
Exposure for 20-30 minutes results
The answer here is obvious
a motor vehicle. With a
in a headache, dizziness and nausea;
– make sure that the exhaust
suitable 12V supply, the
possible death within 2 hours
system is regularly inspected
unit could also find use in
4000ppm
Exposure results in possible death
and correctly maintained.
a service workshop or anywithin 30 minutes
Driving in bumpwhere else where exposure
er-to-bumper traffic can also
to CO gas is a risk.
is the only way to save the victim
expose you to excessive concentraThe unit is very easy to use and has
from death.
tions of CO gas. Because there are
just two switches and four indicator
By the way, it’s estimated that a LEDs on the front panel. The functions
so many cars so close together, it’s
heavy cigarette smoker will have of the Power and Reset switches are
inevitable that there will be some
about 5% of his/her available hae- self-explanatory, as is the function
exposure to exhaust gases.
moglobin tied up by CO at any given of the Reset switch. The other three
This applies particularly if you
drive with the window open or with time. The symptoms for CO poisoning
LEDs are designated “Heat”, “Alarm”
begin to occur when this percentage and “Warning”.
your interior air fan or airconditionreaches about 10%, while the onset
ing set to “fresh air”. Setting the fan
The Heat LED indicates a heat
or airconditioner to “recirculate” is of death occurs at about 20%.
purging cycle for the gas sensor, while
These figures suggest that a smoker the Alarm LED lights (and an internal
the answer here, especially if you are
is far more susceptible to CO poi- piezo alarm sounds) when a critical
stopped at traffic lights.
soning than a non-smoker, simply CO level is reached. Finally, the WarnHow do you know when you’re
because they are starting out from a ing LED flashes to give a preliminary
being exposed to excessive CO levels?
Well, you might not know until it’s 5% higher base.
warning that the CO concentration is
too late. That’s because CO is utterly
on the rise.
CO monitor
colourless and odorless but nonetheThe CO sensor itself protrudes from
Because CO gas is impossible for the rear panel of the case. This is a
less quite deadly.
First symptoms, from quite small the individual to detect, we set out to
low-cost semiconductor unit made
design an effective yet easy-to-build
concentrations of CO gas, are headby Nemoto. It contains a heating
aches, nausea and dizziness, while CO monitor. The end result is the
element and a semiconductor senexposure to higher levels quickly self-contained unit described here.
sor surrounded by a catalytic layer.
It is housed in a small plastic in- These parts are contained in a 19mm
causes unconsciousness and death.
er cylindrical case with six
An interesting point to consider strument case which can be placed diamet
62 Silicon Chip
Self-contained in a plastic case and plugging into a car cigarette
lighter socket, the CO Alarm can be moved from vehicle to vehicle.
Alternatively, it could be “built in”, with or without the case.
°
pins protruding from the base and
with a double gauze wire mesh over
the element.
The double layer of wire mesh is
there to prevent an explosion if the
sensor is exposed to dangerous concentrations of inflammable gas.
In operation, the sensor is heated to
a temperature of 130-170°C and when
CO gas becomes trapped on the catalytic layer, electrons are transferred
to the semiconductor element. This
markedly reduces the effective resistance of the semiconductor element to
reveal the presence of the gas.
When the gas dissi
pates, the resistance of the semiconductor layer
returns to normal. Over time, other
gases such as hydrogen, petrol vapours and alcohol vapours are ab-
sorbed onto the catalytic surface and
cause contamination.
To prevent false readings, these are
periodically burnt off by raising the
temperature of the heating element to
around 450°C.
By the way, in case you’re wondering, this unit is really only suitable for
detecting carbon monoxide. It is fairly
insensitive to hydrocarbon vapours
(although it can detect very high concentrations), which means that it is
unsuitable for detecting petrol fumes.
Block diagram
Fig.1 shows the block diagram of
the CO Alarm. The circuit is powered from a 12V supply (eg, via the
cigarette lighter socket) and this is
regulated to give a +5.5V rail using
REG1. This rail then supplies the
heater and the semiconductor element
in the CO sensor, along with the rest
of the circuit.
The sensor heater element must
be driven correctly to suc
cessfully
purge any contaminating gases on the
semiconductor element.
The specifications state that the
CO sensor be heated to 450°C for 1-3
minutes, while the CO sensing time
should be 6-10 minutes. These times
are set by the Timer circuit, with LED2
switching on during the heat purging
period.
In this case, the Timer circuit heats
the sensor element for two minutes
when power is first applied and repeats this 2-minute heating (purge)
cycle every 10 minutes after that.
Fig. 1: operation of the CO Alarm
is easily understood when you
break it down into circuit
elements, as shown in this block
diagram.
MAY 1999 63
Fig.2: the circuit might seem a little complicated at first glance but it’s quite simple. It uses just three ICs, six
transistors, a 3-terminal regulator and a handful of other parts plus, of course, the carbon monoxide sensor.
During each 2-minute heating period
and for one minute afterwards, the
signal output from the CO sensor is
grounded via Q3 in the Output Control section of the circuit. This is done
to prevent false readings.
At the end of each 3-minute period,
Q3 turns off and so the signal from
the CO sensor is fed to the following
comparator stages. There are two
comparator stages here – a “latching
com
parator” based on IC1a and a
“warning comparator” based on IC1c.
Basically, the warning comparator
monitors the CO sensor output during
the 7-minute sensing period. If the CO
64 Silicon Chip
level reaches a moderate level during
this time, it enables a Flasher circuit
(IC1d). This in turn drives LED4,
which flashes on and off to provide a
preliminary warning.
If the CO level subsequently rises
past a critical point, the latching
comparator (IC1a) lights LED3 via
transistor Q4. It also activates a tone
generator circuit based on IC1b and
this then drives the piezo alarm via
Q5 and Q6.
Note that the piezo driver is modulated by the flasher so that the sound
occurs in short bursts rather than
continuously.
The latching comparator now remains in this state until it is reset.
This takes place automatically at the
end of the first minute of the sensing
period. If the CO level is still high after
the reset, the comparator immediately
returns to the latched-on state.
Conversely, if the output from the
CO sensor is below the comparator
threshold at the time of reset (ie, the
CO level has dropped), the comparator output switches low and turns off
the alarm. Alternatively, the circuit
can be reset manually at any time,
so that the CO level can be retested.
If CO is present, the output from
the CO sensor will normally
only go low when heat purging
starts again at the end of the
10-minute cycle. Provided it
had already been triggered
during the latter part of the
sensing period, the piezo
alarm will continue to sound
into the purging period but the
tone will change from pulsed
to continuous.
This continuous tone indicates that the manual reset can
be used to silence the alarm.
The circuit
OK, so much for the basic
theory of operation. To find
out how it all works in practice, take a look now at the
full circuit diagram (Fig.2). It
might seem a little complicated at first glance but it’s really
quite simple. It uses just three
ICs, six transistors, a 3-terminal regulator and a handful of
other parts – plus, of course,
the CO sensor.
The +12V rail from the car’s
battery comes in via switch S1
and is applied directly to the
input of REG1, an LM317T
adjustable regulator. Zener diode ZD1 protects the regulator
from voltage transients, while
the 100µF capacitor provides
supply decoupling.
In operation, REG1 produces 1.25V between the adjust
(ADJ) and output (OUT)
terminals. The 120Ω resistor
between these terminals sets
the current between them to
10.4mA and this current flows
Fig.3: use this component layout in conjunction
with the photo overleaf to help with construction.
to ground through trimpot
VR1. Setting VR1 to 408Ω
gives 4.25V between ADJ and
ground, which means that the
applied, its output at pin 3 is high and period is 0.693 x 150kΩ x 220µF. This
output of REG1 will be at 4.25 + 1.25V
the 220µF capacitor charges towards gives figures of 38.12s and 22.87s
= 5.5V. In practice, VR1 is simply ad- the positive supply rail (Vcc) via the respectively, for a total period of just
justed for the correct output voltage.
100kΩ and 150kΩ resistors. When the over one minute (60.99s).
A second 100µF capacitor decou- voltage at pin 6 subsequently reaches
In turn, pin 3 of IC2 clocks IC3, a
ples the regulator output, while LED1 2/3Vcc, pin 7 switches low, as does 4017 decade (divide-by-10) counter.
provides power indication. The 470Ω pin 3, and the capacitor discharges This counter has 10 independent
resistor in series with LED1 limits the via the 150kΩ resistor until it reaches
outputs which sequentially go high
1/ Vcc. At this point, pin 7 goes open
current through it to about 7mA.
on receipt of a clock signal from IC1.
3
IC2, a 555 timer wired in astable circuit again, pin 3 goes high and the
When power is first applied, IC3 is
capacitor charges once more to 2/3Vcc.
mode, forms the heart of the clock
reset via the 10µF capacitor on pin
circuit. Its timing components are This cycle repeats indefinitely while 15 (this capacitor briefly pulls pin
ever power is applied.
connected to pins 2 & 6 and consist
15 high) and so its “0” output at pin
The charging period for the 220µF 3 is high. As a result, transistor Q1 is
of a 220µF capacitor and the 150kΩ
capacitor is simply 0.693 x (100kΩ + turned on via D6 and the associated
and 100kΩ resistors. It operates as
150kΩ) x 220µF, while the discharge 4.7kΩ base resistor. Q1, in turn, drives
follows: initially, when power is first
MAY 1999 65
This photograph of the
completed project,
looking from front to
back, gives you a good
idea of how large the
project is. It will also help
with component
placement during
assembly.
the base of Q2 which also turns on
and connects pin 6 of the CO sensor
to ground to apply the full 5.5V rail
across the heating coil element.
Q2 also turns on LED2 to indicate
that the heater is operating.
At the same time, transistor Q3
turns on via diodes D6 and D8. This
transistor shunts the output of the CO
sensor to ground via a 10kΩ resistor,
to prevent the following comparator
stages from detecting any false signals.
When IC2 subsequently clocks the
“1” output (pin 2) of IC3 high (after
one minute), transistors Q1-Q3 all
remain on due to the forward bias now
provided via diode D7. At the end of
the second minute, the “2” output (pin
4) of IC3 switches high and forward
bias to Q3 is supplied via D9. Conversely, D8 is reverse biased and so
Q1 & Q2 switch off to end the heating
(purge) cycle after two minutes.
Note, however, that a residual current still flows through the heater coil
to ground via the parallel 180Ω and
3.9kΩ resistors on pin 6 of the sensor.
The effective voltage across the heating coil is now only 0.8V and so the
temperature quickly drops towards
the desired 130-170°C operating range
for CO sensing.
66 Silicon Chip
Q3 remains on during this time,
to short the sensor output to prevent
false readings while the temperature
stabilises. At the end of the third minute, IC3’s “2” output goes low, transis
tor Q3 turns off and the signal from
the sensor is now fed to comparators
IC1a & IC1c via a 10kΩ resistor. Diode
D1 isolates the comparator inputs.
Warning comparator
IC1c, part of an LM324 quad op
amp, is the warning comparator. Its
pin 13 inverting input is biased to
1.28V by a voltage divider consisting
of 33kΩ and 10kΩ resistors and this
sets the comparator threshold. The
output from the CO sensor appears
at pins 5 & 7, while trimpot VR2 sets
the sensitivity.
Normally, when CO concentrations
are low, the output from the sensor
is less than the comparator theshold
voltage (1.28V). As a result, pin 14 of
IC1c is low and D5 pulls pin 9 of IC1d
low to prevent this flasher oscillator
from operating.
Conversely, if the CO sensor output
rises above 1.28V (ie, if excessive CO
is detected), the voltage on pin 12 of
IC1c will be greater than the voltage
on pin 13.
When this happens,
pin 14 of IC1c switches
high and reverse biases
D5, thereby allowing the
flasher oscillator based on
IC1d to operate.
IC1d is also part of
the LM324 quad op amp
package and is wired as a
0.5Hz oscillator. Its period
of oscillation is set by the
100kΩ feedback resistor
between pins 8 & 9 and
by the asso
ciated 10µF
timing capacitor. The
two 10kΩ resistors on
pin 10 nominally bias the
non-inverting input to half
supply (1/2 Vcc), while the
10kΩ feedback resistor
between pin 8 and pin 10
provides hysteresis. This feedback
resistor provides upper and lower
threshold voltages of +3.67V and
+1.83V respectively.
The circuit works as follows. When
no CO gas is present, pin 9 of IC1d is
held low and so the output at pin 8
is high and PNP transistor Q7 is off.
However, if CO gas is detected, D5
becomes reverse biased as described
previously and so the 10µF capacitor
on pin 9 of IC1d charges via the 100kΩ
feedback resistor until it reaches the
upper threshold voltage (ie, 3.67V).
At this point, pin 8 switches low and
so Q7 turns on and lights LED 4 via a
470Ω resistor.
The 10µF capacitor now discharges via the 100kΩ feedback resistor
into pin 8 until it reaches the lower
threshold voltage (1.83V). When it
reaches this point, pin 8 goes high
again, Q7 turns off and the 10µF capacitor again starts charging towards
the upper threshold voltage. This cycle
continues indefinitely and so LED4
flashes at a 0.5Hz rate while ever CO
gas is present.
Latching comparator
IC1a is the latching comparator. Its
pin 1 output switches high when the
sensor output reaches half supply (ie,
2.25V), as set by the two 10kΩ bias
resistors on pin 2. This high output
in turn pulls pin 3 high via D2 and a
series 10kΩ resistor and so the comparator output is latched high, even if
the sensor output immediately drops
below 2.25V. This turns on Q4 which
in turn lights LED3 (alarm).
Note that when pin 3 of IC1a is
latched high, D1 is reverse biased.
This ensures that the high on pin 3
has no affect on the sensor output.
As soon as pin 1 of IC1a switches
high, D3 is also reverse biased and
so IC1b starts oscillating. This “tone
generator” stage works in exactly the
same way as the oscillator based on
IC1d, except that the timing components on its pin 6 input are much
smaller in value. As a result, IC1b
oscillates at about 3kHz.
IC1b drives Q5 & Q6 which together
function as a push-pull output stage.
In turn, these drive the piezo alarm
to provide the audible alarm. Note,
however, that the 3kHz alarm tone is
not continuous but is modulated by
IC1d. That’s because each time the
output of IC1d switches low, it also
pulls pin 6 of IC1b low via D4 and a
series 4.7kΩ resistor and thus disables
the tone generator. IC1d oscillates at
a 0.5Hz rate, which means that the
tone generator stage (IC1b) operates
in 1-second bursts.
The latching comparator can be
reset at any time by pressing the Reset
switch S2. This momentarily pulls
pin 3 of IC1a low via a 1µF capacitor. If the voltage from the CO sensor
is below the latching comparator
threshold, then the comparator output
stays low and the circuit reverts to
the monitoring status. If not, it will
go high again immediately after the
reset and retrigger the alarm.
Alternatively, if the Reset button
isn’t pressed, the latching comparator is automatically reset when the
“4” output (pin 10) of IC3 goes high
and switches on transistor Q8. This
occurs one minute into the sensing
period (ie, at the end of the fourth
minute).
As for a manual reset, the alarm is
immediately retriggered if the sensor
output is still above IC1a’s threshold
voltage; otherwise it resumes its monitoring role.
If the alarm immediately retriggers
after the automatic reset, it will con-
tinue to sound until IC3’s “4” output
switches high again 10 minutes later.
This means that the alarm will even
continue to sound during the next
heat purging period (unless, of course,
the reset button is pressed). When the
heat purging process starts, however,
Q3 turns on and so pin 14 of IC14 goes
low. As a result, oscillator IC1d is disabled which means that it no longer
drives LED 4 (via Q7) or modulates
the audible alarm.
Therefore, the audible alarm
switches from pulsed to continuous
tone when the heat purging cycle
begins. Pressing the Reset switch
will now turn the alarm off, since the
sensor output is effectively grounded
by Q3 and can no longer retrigger the
latching comparator.
Note that the heat purging process
does not start until six minutes after
the automatic reset has taken place.
That’s because IC3 is a decade counter
and it takes a further six minutes for
outputs “5-9” (not shown on the circuit) and then “0” to go high in turn.
Construction
Building the CO Alarm is easy since
virtually all the parts are mounted on
a single PC board coded 05303991
(117 x 102mm). Fig.3 shows the assembly details.
Before installing any of the parts,
carefully check your PC board for
etching defects by comparing it with
the published pattern. In particular,
check for shorted or broken tracks and
undrilled holes.
Begin the assembly by installing
the three wire links, then install PC
stakes at the external wiring points.
You will need 10 PC stakes in all –
four for the CO sensor leads, two for
the power supply connections, two
for switch S2 and two for the piezo
alarm.
Once the PC stakes are in, you can
install all the resistors. Table 2 shows
the resistor colour codes but it’s also
a good idea to check them using a
digital multimeter, just to make sure.
The three ICs can then be installed,
followed by the diodes and the zener
diode. Make sure that these semiconductor parts are correctly oriented.
Now for the transistors. Be careful
here, because there are three different types used and they all look the
same. In particular, be careful not to
confuse the BC327 and BC337 types
(one is a PNP transistor, the other an
Parts List
1 Nemoto NAP-11A
semiconductor type CO gas
detector
1 PC board, code 05303991,
117 x 102mm
1 front panel label, 133 x 27mm
1 small instrument case,
110 x 140 x 35mm (see text)
1 automotive lighter plug
1 piezo transducer
1 SPDT toggle switch (S1)
1 momentary contact switch (S2)
1 1m length red/black figure-8 wire
1 60mm length 0.8mm tinned
copper wire
1 80mm length yellow hookup wire
1 80mm length blue hookup wire
1 160mm length red hookup wire
1 cordgrip grommet
10 PC stakes
4 5mm LED bezels
Table
3: Capacitor
2 3mm
screws
and nuts Codes
4
small
self-tapping
screws to
[sb]Value IEC EIA
secure
PC
board
[sb]0.1uF 104 100n
[sb].015
153 15n
Semiconductors
1 LM324 quad op amp (IC1)
1 555 timer (IC2)
1 4017 divide-by-ten decoder
(IC3)
1 LM317T adjustable regulator
(REG1)
2 BC547 NPN transistors (Q1,Q8)
4 BC337 NPN transistors (Q2-Q5)
2 BC327 PNP transistors (Q6,Q7)
1 16V 1W zener diode (ZD1)
9 1N914, 1N4148 switching
diodes (D1-D9)
4 5mm red LEDs (LED1-LED4)
Capacitors
1 220µF 16VW PC electrolytic
2 100µF 16VW PC electrolytic
4 10µF 16VW PC electrolytic
1 1µF 16VW PC electrolytic
2 0.1µF MKT polyester
1 .015µF MKT polyester
Resistors (0.25W, 1%)
1 330kΩ
1 220kΩ 1 150kΩ
3 100kΩ
1 33kΩ 19 10kΩ
2 4.7kΩ
1 3.9kΩ 1 2.2kΩ
2 1kΩ
5 470Ω 1 330Ω
1 180Ω
1 120Ω 1 10Ω
1 500Ω horizontal trimpot (VR1)
1 5kΩ horizontal trimpot (VR2)
Miscellaneous
Solder, etc
MAY 1999 67
Table 2: Resistor Colour Codes
No.
1
1
1
3
1
18
2
1
1
2
5
1
1
1
1
Value
330kΩ
220kΩ
150kΩ
100kΩ
33kΩ
10kΩ
4.7kΩ
3.9kΩ
2.2kΩ
1kΩ
470Ω
330Ω
180Ω
120Ω
10Ω
4-Band Code (1%)
orange orange yellow brown
red red yellow brown
brown green yellow brown
brown black yellow brown
orange orange orange brown
brown black orange brown
yellow violet red brown
orange white red brown
red red red brown
brown black red brown
yellow violet brown brown
orange orange brown brown
brown grey brown brown
brown red brown brown
brown black black brown
5-Band Code (1%)
orange orange black orange brown
red red black orange brown
brown green black orange brown
brown black black orange brown
orange orange black red brown
brown black black red brown
yellow violet black brown brown
orange white black brown brown
red red black brown brown
brown black black brown brown
yellow violet black black brown
orange orange black black brown
brown grey black black brown
brown red black black brown
brown black black gold brown
NPN type). Note that Q8 needs to be
bent over flat on the PC board to allow
room for the Reset switch.
Next, install the capacitors and
note that the electrolytic types must
be oriented correctly. Table 3 shows
the value codes for the MKT polyester
types.
Regulator REG1 has its leads bent at
right angles so that it can be mounted
with its metal face flat against the PC
board. Bend the leads as shown in
the photo, so that they mate with the
board mounting holes, then secure the
regulator to the board using a screw
and nut before soldering the leads. A
separate heatsink isn’t required for
REG1 – its metal tab allows sufficient
cooling.
The board assembly can now be
completed by installing VR1, VR2
and the four LEDs. Mount the LEDs
with a 20mm lead length so that they
can later be bent over and pushed
through the bezels fitted to the front
panel. Make sure that the LEDs are
correctly oriented – the anode lead
is the longer of the two. In particular,
note that LED4 is oriented differently
to LEDs 1-3.
Case
And here is the final assembly, looking from the rear. The round grey object on
the bottom left is the CO sensor, which could be mounted external to the case.
68 Silicon Chip
As previously discussed, the prototype CO Alarm was housed in a
low-profile instrument case measuring 110 x 140 x 35mm.
Whether or not you use this case is
up to you and your particular method of mounting. If you do, you will
Table 3: Capacitor Codes
Value
0.1µF
.015µF
IEC
104
153
EIA
100n
15n
have to drill four holes in the front
panel to take the switches, plus four
more to accept LED mounting bezels.
Another two holes are drilled in the
rear panel for the cordgrip grommet
and CO sensor.
First, the front panel. The best way
to go about this job is to attach the
label and then use this as a guide for
drilling the holes. Alternatively, you
can use the full-size artwork published with this article as a drilling
template.
Take care with the holes in the
rear panel – both the sensor and the
cordgrip grommet (for the 12V supply
leads) should be a tight fit. The best
way to make the sensor hole is to
first drill a small pilot hole and then
carefully enlarge it to size using a tapered reamer. The other hole should
be carefully profiled to suit the shape
of the cordgrip grommet.
The PC board can now be installed
in the case and secured using four
self-tapping screws. These go into the
integral standoffs in the base of the
case. This done, mount the switches
on the front panel, then slide the panel into its slot at the front of the case
and push the indicator LEDs through
their matching bevels.
All that remains now is to complete the wiring as shown in Fig.3.
Use automotive cable for the supply
leads and make sure these are firmly
secured to the rear panel using the
cordgrip grommet.
The CO sensor can be wired using
light-duty hookup wire, while switch
S2’s contacts solder directly to the PC
stakes adjacent to Q8.
We mounted the piezo transducer
on the lid of the case using hook and
Fig. 4: use this same-size PC board pattern to make your own board or to check
a commercial board for etching/drilling defects before commencing assembly.
loop fasteners but a dab of super
glue would also work. Finally, attach
a cigarette lighter plug to the 12V
supply lead.
Testing
You’re now ready for the smoke
test. Rotate VR1 fully anticlockwise,
apply power to the circuit and measure the voltage on the output (centre)
lead of REG1.
Adjust VR1 for a reading of 5.5VDC
and check that both LED1 and LED2
are now alight, indicating that power
is present and that the heating cycle
has begun.
If LED2 fails to light, try switching
the power off and then on again, to
activate the power-on reset for IC3. If
that fails, check the 5.5V rail on pin 4
of IC1, pin 8 of IC2 and pin 16 of IC3.
Assuming that all is well, wait for
two minutes and check that the Heat
LED (LED2) extinguishes. If you want
to check operation of the sensor, place
it near the exhaust pipe outlet of a
running engine. Both the CO warning
LED and the main alarm should be
activated after a short time.
Switch the power off and on again
if you want to initiate the heat purging sequence immediately. This will
also stop the main alarm if it has
latched on.
Installation
The CO alarm is installed inside
the vehicle and can be placed on
the dashboard. Note that if you are
already using the lighter socket for
some other purpose, you can obtain a
double lighter socket from automotive
retailers or from Jaycar.
VR2, the sensitivity control, should
initially be set to mid-position and this should suit
most applications.
If you want greater sensitivity, adjust VR2 anticlockwise. Conversely, to
decrease the sensitivity (eg,
if the unit generates lots of
nuisance alarms), adjust
This same-size front panel artwork can be copied and used directly and/or used as a
VR2 clockwise from its
drilling template for the front panel. Artworks for panels and PC boards are also
mid-position.
SC
available on the SILICON CHIP website, www.siliconchip.com.au
MAY 1999 69
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