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.

How does a photoresistor

work?

• 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.

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, since we can’t see it directly.

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.