One of the attractions of buying at the bottom end of the electronics market by mail order from China is that you never quite know what will come your way. Sometimes it’s a diamond in the rough, while with others it’s a mess. Occasionally along comes something which should work but doesn’t, and that’s the moment when you wonder if you could fix it. [Nyanpasu64] had just such a device, an HDMI to VGA converter with audio that didn’t work. What could be wrong?

The HDMI to VGA chip has an onboard audio digital-to-analog converter (DAC), and it’s a delta-sigma design. This type of DAC is frequently used in audio applications because it works by shifting its switching frequency many times higher than the input sample rate, thus reducing considerably the distortion. This one wasn’t performing as advertised though, and the problem turned out to be that switching frequency being all over the output. Clearly the filter wasn’t working, which led to the design of a new filter. The write-up is therefore an extensive dive into filter design, and in part also a discovery of the effect of impedance on them.

For a super-cheap module to cause so much work, one might ask why not simply spend a few more dollars and get a better one. But had they done that we wouldn’t have seen this write-up, so we’re sticking with team cheap.

We’ve looked at audio DACs, in the past.

When light reflects off a surface, not all of it reflects off at the same wavelength; some photons impart a portion of their energy to raising the vibrational energy of the surface’s molecules, and are thus scattered away at a lower energy and longer wavelength. This is called Raman scattering, and the precise wavelength shifts are characteristic of the particular molecule being illuminated. It can therefore be used in Raman spectroscopy to identify molecules; these spectrometers are normally elaborate, expensive instruments, but [Allegedly Science] was able to build a simple system with surprising sensitivity.

The system is named the CubeRaman, after the cube-shaped body containing the main optical path. It uses a cheap 532-nm laser module as a monochromatic light source, with a bandpass filter to eliminate stray infrared light. The beam then reflects off a 45-degree dichroic mirror and passes through a microscope objective onto the sample. Raman-shifted light then scatters back through the objective, passes through the dichroic mirror and a long-pass optical filter, and is focused by an achromatic lens onto the slit of a spectrometer. The entire housing is 3D-printed, as are most parts of the kinematic mounts; the kinematic mounts use adjustment screws running through inserts in the mount, with the tips of the screws held in place by magnets.

[Allegedly Science]’s first test was with a raw diamond, which clearly showed the expected Raman shift. When trying to test a chemical inside a glass bottle, it mainly returned the signature of silica, making thin-walled cuvettes essential. Ethanol inside a plastic bottle was similarly interesting; varying the focal distance changed whether it detected the characteristic shift of ethanol or polypropylene. Nevertheless, [Allegedly Science] thinks there’s still room for improvement, particularly by eliminating stray light and using a narrower slit in the spectrometer.

Although we’ve seen an open-source Raman spectrometer before, this design is significantly more accessible. It does still require a separate spectrometer, though, so it might be worth considering some other spectrometer options.