How to Build a Photometer 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, since we can’t see

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.