by Ron Newton
The Electronic Sniffer
An Electronic Nose That Knows ...
My wife and I were taking our morning constitutional walk up the hill when I noticed the smell.
The wind was from the south.
“Do you smell that?”
“What?” she said, “I don’t smell anything.”
“Our neighbor must be gluing some PVC pipe or
fiberglassing something.” I didn’t think much more about
it, but the smell continued off and on for the next couple
A month later, the wind was from the north and we
had taken a different route.
“Wow!” I said, “Is that ever strong!”
“I don’t smell a thing” she replied. Then, it hit me
like a ton of bricks. I am a retired chemist and I don’t
know why it hadn’t clicked before: acetone or ether
or a combination thereof. There was a drug lab in the
I went home and scrounged through my junk box and
I found a Figaro TGS 822 gas sensor. I breadboarded it up
to a 12 volt battery and hooked up a voltmeter. It still
functioned. Human noses quickly become desensitized to
odors. The Figaro gas sensors do not. It didn’t take me
long walking around the neighborhood to find the house
the smell was coming from. I called drug enforcement
and explained my suspicions. Three months later, there
was a drug bust.
Everyone does not need one of these things to find a
neighborhood drug lab, but it has many other uses. By
changing the sensor, you can detect the following:
propane, methane, hydrogen, carbon monoxide, ammo-
nia, hydrogen sulfide, organic solvents, CFCs, and carbon
dioxide, to name a few.
Methane is found in mines and is odorless. Miners
used to carry a canary to detect it. So, if you are exploring mines, it might have a use. Carbon monoxide is also
odorless and is a deadly killer and can be found in tunnels
— or around your gas appliances. Ever wonder what the
carbon monoxide level is in the Holland Tunnel? The carbon monoxide unit can be used for checking out bad gas
heaters or has use with fire departments. The TGS 822 is
good for organic solvents.
The project is simple. It uses a round board that fits
into the head of a four D cell Mag-lite that is available at
any hardware store. You can find them for less than
$20.00 if you shop around. The good thing about using
this flashlight as a power source is that there are no
modifications required and you are able to still use it as
a flashlight. Many of the sensors, like the TGS 822, have
a heater and draw a hefty current 660 milliwatts or 132
milliamps. The D cells will power the unit for 24 hours.
The TGS 822 cost is $14.50, as of the January 2004
price list. Other sensors vary in price. All are obtainable
directly from Figaro.
Parts placement on the PCB.
NUTS & VOLTS
The heater requires a 5 volt regulated power supply. I
used a low drop out National LP 3875, which will allow
operation down to 5.1 volts, but doesn’t exceed 7 volts
input. A Wheatstone bridge is used to provide the most
sensitivity and correct for the ambient temperature. As
the sensor changes its resistance when it encounters a
gas, the voltage changes across the Wheatstone bridge.
The brain of the unit is a PIC12C671 A/D microprocessor.
The voltage from the Wheatstone bridge is fed into the
microprocessor and, with some simple math algorithms,
is converted into sound. A bi-color LED is used to make
sure you are on the right side of the crossover point of the
You will need a programmer to program the PIC chip
or it is available pre-programmed (see Parts List). The
code is listed on the Nuts & Volts website: www.nuts
All of the components — with the exceptions of the