Edge Collective

UV Spectrophotometry

Designing a DIY UV Spectrophotometer

Quick links

Background

Kearns et al have been working to develop lower-cost methods of testing that leverage a particular feature of biochar (and similar filters): some contaminants adbsorb much more easily than others. It turns out that the presence of one class of more weakly-adsorbed contaminants -- dissolved organic matter -- is relatively easy to assess with a lower-cost laboratory method: UV spectrophotometry.

The details of this approach are laid out in a 2020 article by Kearns et al:

And specific instructions for performing the UV absorbance test are laid out in the supplementary section, here:

Further: while commercial UV spectrophotometers used in such tests are usually over $2000, there are several designs available for DIY forms of the instrument, costing under $100 in parts. We found one design to be particularly simple and well-characterized:

Our aim is to build a working prototype of this UV spectrophotometer, compare its performance to similar commercial instruments used in a laboratory, and then see if such an instrument might be easy to build and use in a community workshop setting.

Misc notes

Sun Mar 3 08:35:31 PM EST 2024

Building the circuit

Precision voltage to current converter

Adafruit 4-pin jumper cable here

Adafruit 4-pin jst ph stemma here

Adafruit JST 4-pin jst ph stemma SMD here

Mon Mar 11 05:20:16 PM EDT 2024

LED polarity

uv detector IN-C35PPCTGU0

datasheet here

custom footprint:

emitter v_0.1 ordered 3/11/24

detector v_0.1 ordered 3/12/24

Fri Mar 15 11:00:43 AM EDT 2024

instructrable on 555 as switch mode supply

Sat Mar 23 03:52:20 PM EDT 2024

Solder mask goof-up on UV emitter part!

Reference for UV emitter IN-C35PPCTGU0

Also: two pins on Q1 on emitter are 'flipped' -- need to fix pins -- was able to solder 'upside-down' anyway to fix

Mon Mar 25 08:17:39 PM EDT 2024

Made a quick 3d enclosure using openscad:

openscad and stl files are here

Fri Mar 29 08:57:08 PM EDT 2024

Water filtering, 1 micron

Selection at McMaster here

Nice pairing on Amazon:

Most recent prototype

Fri Apr 5 08:31:52 PM EDT 2024

Working on detector v_0.2 ...

Sub-schematic that will be moved to a 'control board':

Version 0.1 footprint assignments:

Version 0.1 pinout:

(need to add 2.5V ref)

Fri Apr 5 11:01:04 PM EDT 2024

Detector schematic:

Emitter schematic:


Revision 0.2 of emitter and detector

Sat Apr 6 10:25:06 PM EDT 2024

Edited footprint for LED (remove solder mask from center pad)

Revising mosfet pin assignment on emitter ...

2N7000 datasheet

2N7000 pinout:


emitter ver 0.2

Sat Apr 6 10:22:42 PM EDT 2024

board layout:

schematic:

)

board files:


detector ver 0.2

Sat Apr 6 10:22:55 PM EDT 2024

board layout:

schematic:

)

board files:

Mon Apr 15 08:18:11 PM EDT 2024

Collecting water samples and filtering through 1 um filter ...

3/4" NPT male + 3/4" barb fitting on Amazon

electric water transfer pump on Amazon

calibration of total organic carbon / uv254, EPA document pdf

uva254 go

'request a quote' for uva254 go here

might want to search for 'total organic carbon' (TOC)

An easy spectrophotometric acid-base titration protocol for dissolved organic matter

correlating uv254 to TOC, BOD, and COD video

Great paper on UV254 methods and applications. Note: correlations between uv254 and disinfection byproducts! references here

Mon Apr 29 09:29:59 PM EDT 2024

UV-Vis spec of Organic Compounds

UV-Vis spec, student resources

Nice chapter on uv absorbance

Nice general reference on uv-vis spec

Some UV-Vis chemistry examples

Sun May 5 02:54:23 PM EDT 2024

Notes on testing v0.2

idea: place all non-emitter/detector components on 'bottom' of board to allow easy access during testing

use voltage doubler on emitter board, as per this part here

replace the 5K R15 on detector board with R15 = 619 ohms

add current sense breakout above R2 on emitter board

add 555 boost and 555 emitter input on external feather board

add usb socket to motherboard for power

I was getting a measly 1.0V at the "2V5" reference pin - some buffoon (that is, me) seems to have chosen poorly the value of R15. I have replaced the 5K with a 619 Ohm resistor - and much better! I get 2.5Vreference.

And the signal at "leak" seems to act as I expected, in room light anyway. I still have to set up the LED and test a blank and wet cuvette ...

so, that T-network has a nice high gain!

And the input signal is 2.491 to 2.517 at 1kHz

Implies the equivalent feedback R is about 53.8Meg

There about 26nApp into the the stage with this test

So , 26mVpp into 1Meg for test current, and about 1.4Vpp output

Now to find what I did wrong at the LED stage ...

With 130us rise time that's about 2.7kHz tia bandwidth

Don't run the LED at 10kHz ..

OK I have a solid 0.5Vpp signal at "leak" node when I ping the LED board with 0V to 3V square wave at 500 Hz. On that emitter pcb I had to swap out R1 (was 49.9K) to 49.9 Ohms, and R2 (was 24.9K) to 24.9 Ohms. So, no cuvette, in the opto-mechanical system Craig designed, I see 0.5Vpp signal. Not bad.

less nice, I see some noise - I think that high gain TIA is also picking up 60Hz, and some other crap. more scope-ing to figure this out .... maybe we want a metal box ?

Fri May 10 03:39:30 PM EDT 2024

Fri May 17 08:25:56 PM EDT 2024

Version 0.2 of hardware does indeed work!

Input: 1 kHz square wave, 50% duty cycle, 3.3V pp

Video of initial test with cuvette filled with distilled water. Interesting: voltage at DETECT slightly higher with liquid-filled cuvette than without any cuvette ... lens effect?
Raw battery sag is about 0.2V
Test setup
LEAK blocked
LEAK unblocked
RECT blocked
RECT unblocked
DETECT blocked
DETECT unblocked

Initial 'linearity test' attempt using tannin powder

Video of initial tannin test. Initial solution: 25 mg tannin powder in 5 mL of distilled water (this dropped DETECT signal to 'zero'); then diluted 5 mL of that solution into 25 mL of distilled water.

Next steps:

Sun May 19 06:02:51 PM EDT 2024

Similar platform: https://iorodeo.com/products/uv-open-colorimeter

What would be required to make their system work for UV254?

Sun May 26 01:51:00 PM EDT 2024

555-based boost circuit design

Reference is circuit 1 from this tutorial

Power mosfet IRF2907Z datasheet here

op-amp is LT1716

Mon May 27 04:44:00 PM EDT 2024