FIGURE 3. This “air quality” sensor detects propane, LPG, and
other toxic and flammable gasses. It needs only a rudimentary
interface circuit, and connects directly to an analog input.
Photo courtesy Parallax, Inc.
is, it triggered and sounded its alarm when using the Test
button. I have no idea if the unit’s smoke detection abilities
are diminished at the reduced voltage, but I suspect this
may be the case. Anyway, running the board at five volts
saves you from having to power it from a separate battery,
so it’s worth looking into.
On the other hand, at the reduced voltage there wasn’t
enough rise in signal to trigger the Arduino; the voltage at
pin D2 wasn’t enough to register as a HIGH. This occurred
whether or not the zener diode was used. An option in this
case is to connect a voltage comparator to the hacked
output of the smoke detector, and adjust its reference
voltage to trigger at the reduced signal level (in my tests, it
was a little under two volts).
the “Test” button on the smoke alarm. The software should
branch off to its “I smell smoke” subroutine. For a final test,
light a match, and then blow it out. Wave the smoldering
match near the smoke detector chamber. Again, when the
detector samples the smoke particles in the air, the output
changes and the software runs the “I smell smoke”
Limitations of Robots
Running the Smoke Alarm
on Five Volts
I found the SA300 unit operated under five volts — that
You should be aware of certain limitations inherent in
robot fire detectors. In the early stages of a fire, smoke
tends to cling to the ceilings. That’s why manufacturers
recommend that you place smoke detectors on the ceiling
rather than on the wall. Only when the fire gets going and
smoke builds up, does it start to fill up the rest of the room.
FIGURE 4. Basic connection
diagram for a gas sensor that uses
an internal heating element. Adjust
the potentiometer for best
sensitivity. Attach the sensor as
shown to the Arduino 5V and Gnd
connections, and analog pin A0.
Smoke alarms detect the smoke
from fires but not noxious fumes.
Some fires emit very little smoke but
plenty of toxic gasses, and these are
left undetected by the traditional
smoke alarm. Moreover, potentially
deadly fumes can be produced in the
absence of a fire. For example, a
malfunctioning gas heater can
generate poisonous carbon monoxide
gas. This colorless, odorless gas can
cause dizziness, headaches,
sleepiness, and — if the concentration
is high enough — eventually death.
52 SERVO 07.2011