Laboratory Grade Colorimeter (inspired by a Classic, the Klett Summerson Colorimeter)

The venerable Klett-Summerson colorimeter was first produced in the 1950s and is still produced today. I decided to make a smaller version of it which turned out to be a neat design.

The DIY lab grade colorimeter has 11 different LEDs as a source of light. Red, Orange, Yellow, Green, Cyan, Blue, Purple, UV (395nm), warm white, cool white and Pink. The color can be selected by a mechanical wheel that is rotated so that the selected color shines into the cuvette.

I can take standard lab cuvettes as well as sample vials.

It has a retro look with an analog meter (a recycled VU meter).

And it is small and cute!

The body of the colorimeter will hold a rotating wheel that contains 3 mm LEDs arranged radially.

I built the LED wheel first by drawing the 3-layers of the wheel from 3mm thick plywood. The rim of the central wheel from which LED light will escape is not cut through to keep the wheel parts together. The rim of the slightly larger central wheel will be cut through after gluing the 3 discs together.

I glued the bottom and central wheel together and then placed the LEDs by passing one wire through the hole in the bottom wheel.

I had 16 slots but only had 11 different LEDs. So left the remaining slots empty.

I then slid the top disc over the LEDs by passing the second wire of each LED through that corresponding hole.

The positive wires from the LEDs on the bottom wheel were soldered together.

The negative wires from the LEDs were wrapped from the one hole to the other in the wheel forming a radial pattern of 11 wires.

The edge of the central wheel was ground away with a Dremel so that the LED light could escape.

A zinc coated steel washer was soldered to the positive leads on the bottom wheel.

The LEDs were tested to make sure they lit ok.

I made the body of the colorimeter from 5mm plywood that was cut on a laser cutter. The layers were glued on top of each other as shown till light layer was reached.

I put two photo resistors into the designated slots. One photodetector was to detect absorbed light, the detector that is in line with the LED. The other, perpendicular to the light path was to detect scattered light. In the end I did not use the scattered light feature of this unit.

I glued on the last layer to seal in the photoresistors. The top plate would be screwed on later forming a lid on the holder.

By adding bronze springy strips to the top and bottom of the LED wheel I essentially created contacts which would pass current through one of the 11 LEDs at a time.

As I rotated the LED wheel, the bronze contacts attached to the bottom and top plates would make contact with only one LED.

The bottom strip on the bottom plate would continuously touch the steel washer and the small bronze strip on the top plate would only touch one LED wire that radiated out from the holes in the top wheel closing teh circuit.

As the wheel was rotated it should stop when the LED was aligned with the cuvette holder. To achieve that I cut out a small slot in the stack of plates on the opposite side of the cuvette holder into which I added an open brass ring. Open = cut the ring open.

The ring would slot into the depressions cut into the central disc of the LED wheel and hold it in place till pressure was applied to the wheel to rotate it to a new position.

I designed and laser cut the sides and top on the laser cutter.

Then glued the rear of the colorimeter to the cuvette and LED wheel holder assembly.

The electronics is simple. There are two circuits, both fed by 5V from a USB A socket.

One of the circuit is a transistor based constant current source to feed the LEDs at 20 mA constant current.

The second circuit is nothing but an analog ohmmeter that measures the resistance of the photo resistor.

My VU meter has a non-linear scale which turned out to be a positive as the absorbance values (aka Ohms) were more spread out.

I will show how I calibrated the scale in a separate instructable using CuSO4 solutions.

I built the electronics dead-bug style by gluing the components on the set zero potentiometer and then soldering them together.

The electronic cube was connected to the photoresistor, to the LED wheel switch, and to the VU meter.

The panels were painted and then screwed onto the sides. The top was attached with a strip of bent aluminum.

The cuvette holder lid was made out of a strip of aluminum that was screwed onto the sides (not shown here, but shown in the final assembly and in the video).

I tested the LED wheel by placing a white plastic strip into the cuvette well and then switching the wheel to different colors. Except for the green LED all the others shone as expected.

I most likely will make a version 2 of this colorimeter - can simplify it a bit more and make it a bit more compact. Let me know if you have suggestions.

This instructable got lengthier than I had expected so will make another one on how to calibrate the absorbance with CuSO4 solutions and how to use it for some interesting assays.