In my previous post hacking around with the DU530 I showed a spectra from a CFL which matched published spectra. However I was still having a number of issues, particularly with the stepper driver. In particular I was having issues with the stepper skipping steps.
I’ve been using an EasyDriver (clone) and found that if I disabled microstepping everything was working correctly, but when microstepping was enabled steps would be skipped.
I installed a shunt resistor and took a look at the current output on a scope (over the shunt). I used a couple of 0.22Ohm resistors as this is what I had to hand…
The current trace looked like this:
So for some reason it was hitting it’s maximum current output and then just sitting there. I don’t fully understand why, but reducing the current (via the current adjust pot on the board) and increasing the supply voltage (I pushed it to 24, which seems like it’s more than should be required, but still drawing only ~300mA) resolved the issue. I don’t fully understand why the current adjust should have this effect and should probably investigate further. But the traces look fine and I don’t see any missed steps:
With these changes I was again able to resolve the CFL spectra but now I have more freedom with speed/acquisition time…
Acquiring on the scope at a lower speed seems to result in more noise (likely from vibration), but at least a still visible spectra:
This means I can probably move on to acquiring the photodiode output on a microcontroller… a picture of the EasyDriver/shunt is below for reference:
I recently wrote about various UV-Vis spectrophotometers on the substack. As part of this I discussed the Nanodrop and how its architecture differs from other instruments and the advantages this gives.
The CCD is quite possibly a Sony ILX511 this sensor was a popular CCD in a number of spectrometers, I’ve previously designed a small interface for this sensor.
The layout seems pretty simple, the light from the fiber comes in, hits a mirror then is reflected off a diffraction grating and heads to the CCD:
The auction contains a number of other photos, which includes a fiber cable. I assume this fiber (and all the other components) must be UV compatible which will increase the cost. But overall, its a very simple optical system.
Thermo documentation suggests that older Nanodrops use Xenon Flash lamps. These give a relatively broad output. It doesn’t seem like the Nanodrop has any kind of sensor on the output of the lamp (unlike more traditional UV-Vis instruments) this will make taking a blank measurement all the more important. The blank will be compensating for both emission differences across wavelength in the lamp and the absorbance of the buffer.
Newer Nanodrops appear to use LEDs, I assume this must be a combination of UV and visible LEDs to cover the entire spectrum.
Other references show the Nanodrop setup, and suggest that fused silica (quartz) is used:
The Nanodrop patents are pretty informative, and make it reasonably clear that the fiber optical “pedestals” are just the ends of a couple of SMA connectors “typically the end of an industry standard SMA fiber optic connector 10, FIG. 3a (found as connectors on the ends of optical patch cords like p/n P-400-2-UV-VIS from Ocean Optics inc. of Dunedin, Fla.)… For most SMA connectors the approximate 2 mm end diameter can be effectively covered with 2 microliters of water or a water-based solutions.”
A few other pictures from the auction are below, while the auction is listed as Nanodrop monochromator. I suspect this may however be mislabeled, and am continuing to search for Nanodrop teardown images…
I’ve continued to do some work with the DU530 (see previously blog posts).
I decided to see if I could drive the grating motor and register the spectra from a CFL light through the grating.
As shown above I used an EasyDriver and Arduino to drive the stepper. This showed some slipping (using a 200ms square wave generated from the Arduino to drive the step input). The CFL bulb was just jammed into the lamp box. You can fairly easily see the spectra coming out the grating:
I then registered the output of the far side photodiode using a 10M gain transimpedance amplifier. I continuously ran the stepper and monitored the output on a scope:
Spectra recorded on my scope
This matches the spectra published online extremely well, though the resolution is limited in the scope recording:
Obvious next step would be to record the output with an ADC/microcontroller, this would let me do slower scans. I’m also concerned that the motor is occasionally slipping, which suggests I might want to review the stepper driver…
I wanted to do some basic experiments with the DU 530 UV-Vis Photospectrometer (see previous post) photodiode amplifier. In particular I wanted to confirm that they were running the diodes unbiased. I could have reassembled the unit and checked voltages, but I decided to test the photodiode board in isolation instead.
The board contains two seemingly identical (aside from the feedback resistor, the other uses a 180K instead of 1.5M) transimpedance amplifier circuits:
I was able to power up the opamps with +/- 10V. V- on pin 13 of the header, V+ on pin 9. I connected ground directly to the ground plain on the top of the board. By inputting a test signal (function generator output through a 100M resistor) I was able to measure low frequency current with the amplifier. The bandwidth seem very limited (few 100Hz?) but I didn’t accuracy characterize the board, and in any case things were quite noisy as I wasn’t using any shielding.
But the output was clean enough to get a rough sense of what was going on (and was as expected, inverted):
12Vpp input going through 100M resistor and out though ICL7650 in TIA configuration (~1.5M feedback resistor)
There are a few differences between the layout here and the basic transimpedance layout. R1 seems to be providing the bulk of the gain, C3 should limit peaking. Don’t fully understand why R3 and R5 are required. The output from the amplifier heads out to an analog mux. The board has 3 opamps (one for each photodiode, and IIRC there’s a header which connects to a temperature sensor). So this mux no doubt selects the opamp output to go to the header. From looking at the main PCB, it seemed like pin 14 went to the ADC on the main board. But I have no plans to further investigate this side of the circuit at this point.
My name is Nava Whiteford. I’ve worked for a few sequencing companies. I have equity in a few sequencing companies based on my previous employment (I try to be unbiased in my posts). You can contact me at: [email protected]
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