How a breathalyzer works (alcohol sensor)----make money online

hey everyone someone recently gave me

this little handheld breathalyzer and so

I thought we'd take it apart today and

see what makes it work and then later

I'll build my own breathalyzer so it's

got one button here and a display and a

port that you breathe into and I'm going

to press the button and it beeps and it

has this little countdown routine and

we'll talk about that in a minute but

it's actually doing is warming up the

sensor physically warming it up and when

the countdown gets down to zero it beeps

to tell you to start blowing and I'm

gonna blow into the tube

and it eats in it says zero since I

haven't had anything to drink if we

press the button and have it start its

countdown routine and then don't blow

into it the machine knows that you

aren't actually blowing into it and will

say something like FL for fail on the

display okay let's take it apart

okay here it is with the cover taken off

here's the port where you blow in and

this little piece of plastic has an exit

here and so when you breathe into it

your breath travels here to the sensor

here and then you can see there's a

couple events that go to the outside so

it doesn't actually force your breath

through the sensor it really just passes

it across the sensor here and most of it

escapes the chamber through that vent in

the back so what it looks like on the

back there if we look at the front of

the board of course it's got the display

there it's got a pot over here for

presumably calibrating it or setting

something at the factory it's got a

pretty good sized diode and inductor

here and it's got a five lead SMD

package here which probably makes a

boost converter this thing runs off of a

single triple-a battery and when we flip

the board over you'll see there's some

components that definitely require more

voltage than one and a half volts so

you've got a little microcontroller

they're a bunch of passives and some

transistors the microcontroller is a

whole tech HT 46 are Oh 65 and I looked

that up and it's your basic 8-bit MCU

with integrated ad and it also has an

integrated display controller for LCDs I

don't know if they sort of modified that

to work with the LEDs or if it could

power both or what but there's that and

then this is just the piezo beeper the

wires going to the beeper on the back so

a pretty simple circuit it's kind of

amazing how a lot of modern electronics

get boiled down to the same battery

power supply microcontroller and user

output type stuff I searched around a

little bit for alcohol sensors and found

that the market is dominated by a

specific line of sensors made by the

same company this particular sensor is

most likely in mq3 zero 3a and we can

see that it's a three terminal device

and two of the terminals are for the

heater and the third terminal is

signal and basically it's just a

variable resistor that changes

resistance based on the concentration of

alcohol vapor blowing across it the

datasheet indicates that the sensor

requires point nine volts for the heater

and that it takes several minutes to

come to equilibrium so that you can

actually start measuring but in a

product like this it wouldn't really

work to wait a few minutes for this to

warm up and I think the manufacturer

realized that so the datasheet says that

you can also give it two volts for about

ten to fifteen seconds and then back

down to 0.9 volts and that will ensure

that the thing is hot enough and that's

and that's what they're doing here and

that's why it has that 15 second time

down when you start the device I did a

little bit more searching about these

alcohol sensors and they're apparently

made with tin dioxide so the heater

heats up this ceramic pellet that's

coated in tin dioxide and then the sense

resistor basically goes through the tin

dioxide to the heater the heater is

relatively low resistance the thing

burns about 100 milliwatts when it's

heating and the you know so the

resistance from that third lead through

the tin dioxide to the heater is

relatively high so it doesn't really

matter which pin you use as the other

side of the sense circuit here's a

slightly larger sensor that I bought

from Sparkfun awhile ago

and this one is called the M q3

presumably made by the same company and

I'll hold it up to the light so you can

see the sensor element inside and the

way it's built it has this ceramic tube

that's covered in the tin dioxide so has

the stuff actually work it's a little

tough to find decent explanation but I

think it works like this when an

incoming ethanol molecule lands on the

surface of the tin dioxide it reacts

with oxygen that's also just hanging

around in the area and since it's hot in

there you know it's this heater heating

the whole thing up the ethanol will burn

you know react with the oxygen and leave

water and co2 there so the trick is that

when you pull oxygen that's out of the

tin dioxide its conductance will change

and thus the resistance of that whole

thing will change and this isn't

actually pulling off

out of the latus it's not like it's

converting it from tin dioxide back to

tin I think it's just pulling out oxygen

atoms that happen to be hanging around

adsorbed oxygen and so the more ethanol

that comes in more of that oxygen gets

pulled away so that may make you think

that the oxygen concentration of the gas

coming in is important and it certainly

is the data sheet shows that the oxygen

concentration is very critical into the

performance of this thing and they've

been calibrated for atmospheric air you

know about about 20 percent oxygen if

you blow higher concentrations of oxygen

at it it will not read correctly in

addition to lower concentrations it's

also you know temperature and humidity

sensitive and those variables have been

tried to been a you know calibrated for

breath so relatively high humidity and

temperature of twenty something or

thirty see here's a chart that shows

your estimated blood-alcohol

concentration as a function of time and

so we're assuming the chart says one two

three or four drinks and so these are

all consumed at once at time zero and it

does take some time for the body to

absorb the alcohol and then it's

metabolized at a relatively constant

rate which is why these descending lines

are fairly linear this table shows the

correlation between blood alcohol

concentration as a percent volume and

ppm of alcohol in the exhaled breath and

so I think this has to be determined

empirically basically this depends on

how permeable the membranes are in the

lungs and how likely it is that the

ethanol molecules will transfer back

into the air the alcohol sensor in the

little handheld breathalyzer is

specially set up to measure alcohol

concentrations in this range it was

basically made specifically to be a

breathalyzer sensor this chart shows the

output of the sensor in terms of

resistance divided by its nominal

resistance as a function of

concentration of alcohol and ppm you can

see they also give curves for butane and

hydrogen here too so the sensor will

sense other gases it just happens to be

that you don't exhale much butane or

hydrogen

if you have been drinking pretty much

the only thing coming out of you is

water and ethanol and co2 you can see

the concentration here in sort of the

mid scale range is from ten to a

thousand which covers the ranges that

you'll likely encounter in exhaled

breath so to test this out I drank one

and a half ounces of vodka exactly 20

minutes ago and so my blood alcohol

content should be at that peak that we

saw on the graph so let's try it out it

shows at 0.01 percent blood-alcohol

concentration just for reference I weigh

like about 150 something pounds here's a

lash-up of that other sensor this one

has a much higher power consumption it's

almost seven or eight hundred milliwatts

just about it's a 5 volt heater and I've

got it set up with the heater running

continuously and the meter monitoring

the sense resistance and so I'm going to

blow into this to begin and direct some

of my breath into the sensor here

you can see a pretty market decrease in

the sense resistance and then it starts

heading back up when I stop breathing on

it one interesting thing is that that

little handheld breathalyzer knows when

you are breathing into it so it must be

using either the change in resistance of

the heater itself or the change in

resistance of the sense resistor to

figure out if there's actually breath

moving across the sensor I suspect some

of those transistors in there are a

constant current source for the heater

and it knows that if it has to start

dumping more current or if the voltage

has to go up then the power consumption

and the heater must be going up to

maintain that current I'm not really

sure about that but I think there is a

there must be a sense in there to

determine if you're blowing enough air

across it okay check the description for

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