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.resistorresistance A photoresistor is made of a high resistance semiconductor. 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.frequency photons electronsconduction band The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.holeresistance

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 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.