USB Spectrophotometer

What is a spectrophotometer you may ask? Basically it's a device that uses different wavelengths of visible light (and sometimes even infrared or ultraviolet) to collect data. The light is passed through the substance, which is usually in liquid form or is in solution, and the intensity of the light reaching through the sample is measured. Different substances of different concentration will interact with the light differently, so that the composition or concentration of the substance can be calculated.

Materials needed:

LEDs of different colours - as many as you want

Resistors to match the LEDs - depends on amount of LEDs

Photoresistor - used to measure light intensity

USB cable - used to provide power to the circuit

Case - helps to keep out unwanted light and keeps things organized

Multimeter - to measure the resistance of the photoresistor, and to help build the circuit

Optional:

Switches

Connectors

Sorry it looks bad :P

The first thing I did was attach all of the negative leads of the LEDs to a single wire and gave each positive lead its own wire. After that, I attached the resistors to their respective LEDs, and used electrical tape to secure it. ( soldering is recommended although i couldn't find my soldering iron )

Low intensity LEDs are recommended as the light will sometimes be focused on the photoresistor when a sample is being tested.

Now, cut the USB cable in two. Keep the male end (the part that plugs into your computer) and strip it. There should be four wires and some mesh, but we only need the red and black wires. Strip the ends of the black and red wires and attach a switch to the black wire (negative) attach the other end of the switch to the negative wire from the LED array.

UGGGGG. This is terrible. I got the bright idea to use a connector from a computer power supply. DON'T USE THEM! Use ANYTHING but them. It didn't work well but it was all I had around. Each positive should go to its own connector through the resistors that we attached earlier. Use the appropriate number of connectors for the number of LEDs you have.

The purpose of this is to set which LED turns on. Depending on which female connector you attach it to, the corresponding LED will light up. This connector is simply attached to the positive end of the USB cable.

Attach two leads to the end of the photoresistor. The multimeter will be used to to read the resistance of the photoresistor to determine the intensity of the light after it has passed through the sample.

Place everything in a container of some sort. Make sure no light can get in to the casing. I did mine in a closet with the lights off so it was not a problem for me. When setting up the case be sure to do the following things...

Set the LEDs so they face the photoresistor. Leave a gap between them so you can place the sample in between them.

Have the leads of the photoresistor outside the box so you can attach them to the multimeter.

Have the connectors for the LEDs as well as the positive probe outside the box so you can actually USE them.

Place a sample between the photoresistor and the LEDs. Connect the male connector to the LED you want to turn on, and plug the USB cable into a port. Connect the multimeter to the two leads off of the photoresistor and set it to measure resistance. Measure the resistance for each LED and record this data.

NOTE: the resistance will get lower the more light passes through the sample.

also the resistance will be higher when there is no sample because the light is focused on the resistor when a clear sample in placed in.