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Test Tube Photometer
Flow-through Photometer

Low-cost hardware setup for measuring light extinction and optical densities


You can find a description on how to build the OLD VERSION of the photometer here.


This photometer can be used for science. We published a paper about it in Current Microbiology. (Yes, the journal has a paywall. It maybe was not the right decision to publish it there but I'm trying to make up for it by providing all info here in the repository. If you want the paper I can send it to you. Just drop me an email.)

What's a photometer?

Photometers are used to measure light extinction in liquids to analyse their content. They basically shine light through a certain amount of the liquid from one side and measure the light that's left on the other side. Among other things, photometers are used to measure the densities of bacterial cultures on a regular basis. You can determine them by comparing photometer measurements of culture media with (sample value) and without bacteria (blank value). Using physical laws you can then calculate the optical density of your sample.

Why a test tube photometer?

Well, I'm a lazy person. In order to measure the optical density of my bacterial cultures I always had to take a sample from them. This usually meant I had to work sterile to avoid contamination of my cultures which is a bit time consuming. That's why I asked myself if there is a way to measure the density directly in the test tube without having to open it. There are already some test tube photometer out there but they are expensive and often require the use of specific test tubes. That's why I started to develop a photometer which I could use with the standard test tubes.

And why a flow-through photometer?

With a flow-through photometer can be used to monitor light extinction in any liquid independent of the container. The sample is constantly pumped through the photometer and OD is measured over time. I started to work on a flow-through version of the test tube photometer as part of a Biomaker project. We were developing an low-cost microbial bioreactor and I was looking into ways to measure the culture density in the device over time. But you can also use it for example to measure culture densities in a beaker or monitor the turbidity of river water.

Assembly instructions

The photometer is designed to be modular. You can add as many measurement cells as you like (well ok. you are limited by the amounts of analog inputs your microcontroller has).

There are two was of building the photometer:

1. Using a breadboard and cables

Advantage: You don't need many special parts apart from the sensor (opt101). Some cables, resistors and a breadboard are required as well.

Disadvantage: The connections are not very stable which could produce reading errors. Wiring the opt101 just with cable is a bit confusing (at least it was for me). Apart from that, the legs of the sensor break easily if you bend them to often and too much.

2. Using PCBs (Arduino Nano HAT and the sensor PCB)

Advantage: All the electronics are easy to put together. The connections are stable and secure. You can quickly add connect more measurement cells.

Disadvantage: Well.. you need the PCBs. But you can easily order them from a PCB manufacturer with the gerber files in this repository. You'll find more info about this in /hardware/pcb/

Both ways of building the photometer are described in the instructions folder.


This is a side project of mine and it is not actively developed. I work on it sometimes and try to improve the design and fix errors. I try to do my best to document it as good as possible so anybody can copy this and build their own photometer. You are welcome to contribute to the project by adding more designs for the housing and improving the building instructions.

Report issues

Notice a mistake or errors? Please file any bugs, requests, or questions in the issue tracker!

If you are new to GitHub issue tracker take a look here.

Who is behind all this?

Please contact me if you have any questions.

Creative Commons License
This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.


Everything you need to build a test tube or flow-through photometer







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