Build a Spectrophotometer

This low cost spectrophotometer uses an infrared LED (light emitting diode) and phototransistor to measure the amount of light that passes through a liquid. A multimeter provides a numerical reading (in milliamps) that corresponds to the amount of light detected by the phototransistor after it is passed through a sample. More light passes through clear liquids than cloudy ones, producing a higher milliamp reading on the multimeter. Less light passes through a more concentrated sample, producing a lower reading on the multimeter.

Spectrophotometry is often used in the laboratory to calculate bacterial growth curves, perform starch assays, and measure protein concentration in a solution.

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Step-by-Step Building Instructions

Setting up and Using the Spectrophotometer

The design for this spectrophotometer was adapted from The Bug that Eats Toxic Waste, by Dan Dillon, Bob Schumacher, and Dr. Brent Peyton. Their document details a multi–disciplinary, multi–day project that uses a spectrophotometer to chart bacterial growth curves.

Blowing a Fuse on the Multimeter

If a multimeter is turned on and off while the circuit is hooked up, you run the risk of blowing the fuse, especially when you are using the mA setting. When a fuse is blown, you will typically see 0.00 as the display for mA, even though the meter seems to be reading VDC correctly. This can be confusing if you are trying to troubleshoot problems with your spectrophotometer.

Most multimeters come with a replacement fuse. If you are getting 0.00 mA readings and your wires are all tightly connected, try replacing the fuse. If you need more fuses, refer to the user manual for your multimeter to make sure you get the right kind.

Measuring in Milliliters

Medicine syringe dispensers with milliliter markings can be found in many drug stores, vet clinics and pet stores. These offer a great alternative to pricier lab equipment and can actually be more convenient for dispensing liquid into a test tube.

Viewing Infrared Light

The human eye can't see infrared (IR) light, but many digital cameras and cell phone cameras can. This can come in handy when you want to test whether the IR LED in your spectrophotometer is on. To see if your camera detects IR light, point a TV remote control at the camera and push one of the remote buttons. You should see a purple light in your camera's viewfinder.