Transcript Building A Photometer

How to Build a Photometer
Building A Photometer
• At the heart of any
of these devices is a
PHOTORESISTOR.
• It’s a resistor which
changes because of
the amount of light
striking it.
• A photoresistor is a resistor
whose resistance decreases
with increasing incident light
intensity.
• A photoresistor is made of a
high resistance
semiconductor.
• If light falling on the device is
of high enough frequency,
photons absorbed by the
semiconductor give bound
electrons enough energy to
jump into the conduction
band.
• The resulting free electron
(and its hole partner)
conduct electricity, thereby
lowering resistance.
How does a
photoresistor
work?
So, to measure LIGHT, you
can measure RESISTANCE
• A common way to
measure resistance is
with a multimeter.
• Notice it’s set to Ω.
That’s ohms, the unit of
resistance.
• See the probes? We’ll
connect the leads of the
photoresistor to them.
Here it is connected to the
multimeter
• We used alligator
clips, but you could
use ordinary wire, or
even connect the
leads of the
photoresistor directly
to the multimeter
probes.
Covering the
photoresistor will
change the
multimeter
reading
• This is a fancy
autoranging multimeter,
so although the
numbers displayed look
about the same, it’s
actually gone from 1026
ohms to 16340 ohms.
Our sample will be in a test
tube, so a test tube rack might
be a good holder.
Here’s a way to get the
photoresistor in position
• We’ve used tape
to attach it to an
empty test tube
next to the space
where we’ll put
our test tube with
liquid sample.
Let’s make a light source
• 9 volt battery
• LED (light
emitting diode)
• 100 to 300 ohm
resistor (this limits
current flow to the
LED)
Here’s the LED
connected and
working
• Battery > resistor > LED > back to Battery
• We used alligator clips, but we could have just twisted
wires together.
• We used a battery clip to attach to the battery, but we
could have just taped wires to the battery terminals.
• The LED is polarized (current only goes one way). So if
it doesn’t light up, reverse the connections.
Here’s the LED mounted to
our test tube rack
• Potential issues to experiment
with:
• Is the light pointing at the
photoresistor?
• Is the light going to go through
(vs above) the liquid sample?
• For the color of the sample
you’re using, is there a best
choice for the LED color?
• How much does ambient room
light affect your measurements?
And we insert a test tube for
measurement
• In it goes, between the photoresistor and
the LED
We could build a holder from a
small cardboard box
• Good: the box
can block out
ambient room
light.
• Bad: we have to
be sure we’re
measuring
through the
sample, rather
than around it,
A hole for the
photoresistor
(taped in
place),
another for
the LED,
and
another for
the sample
tube
If you’ve got probes, you could
of course use them
• We’ve done this
same work with
Vernier light
sensors instead
of photoresistors.
• Of course, the
probe is really
just a
photoresistor
inside !
You could also use a
spectrometer probe
• These have their own
light source, and can
measure light intensity
across the entire
spectrum of visible light.
You have to make a
decision then, about
what wavelength of light
you want to focus on.
• This Vernier
spectrometer accepts
cuvettes and works very
nicely.
You could also use a
photometer designed and built
at ISB.
• Whatever you use, you
have to work with it to
make sure it
consistently says
• “different” when 2
samples are different
• “same” when 2
samples are the same
Whatever you use, you should
be able to make a “calibration
curve”
• Put in known
amounts of milk
• Measure the output
• Create a graph
showing the
relationship
(don’t expect it to be
linear, necessarily).
• This graph can be
used to determine
the amount of milk in
unknown samples.