To build my modular synthesizer, and to have the variety of module designs I want for a
complete system, I've decided to engage certain time-saving measures for construction. One is to label the PAiA front panels with Dymo label maker tape instead of having front panels manufactured or silkscreened. Another is to use semi-custom construction on stripboard PCBs, rather than use full-custom pad-per-hole protoboards, or develop full-custom PCBs. There are just two many module designs I want to do -- laying out some 18 PCB designs with correctness would be a really lengthy process.
But related to semicustom-construction is also carefully arranging circuit designs so as to use a minimal number of IC components. Because now they have to be stuffed and soldered together on stripboards, and having fewer ICs will take less assembly time. In fact, to set modern times in perspective, I examined the transistor count on the Moog System 55 modules. These were astonishingly compact circuit designs! Less is more.
But related to semicustom-construction is also carefully arranging circuit designs so as to use a minimal number of IC components. Because now they have to be stuffed and soldered together on stripboards, and having fewer ICs will take less assembly time. In fact, to set modern times in perspective, I examined the transistor count on the Moog System 55 modules. These were astonishingly compact circuit designs! Less is more.
To get more functionality with fewer ICs, the LM3900 comes to mind, because it is a very versatile device. I'm
presently developing an ADSR envelope generator circuit design, and wanted to
implement certain parts of it with the LM3900, because this one device could
take on multiple functions -- hence reduce IC part count on a stripboard. In starting this ADSR circuit design, I initially had some trouble figuring out how to design a LM3900 positive Schmitt Trigger. So, resolving that required this little mini-investigation. It was rewarding though, reminding me a lot of the greater utility of the LM3900 device.
Engineering notebook and scope fotos follow, below.
Positive Schmitt Trigger engineering notes. Developing first from a theory, which is hinted in AN-72, but not illustrated explicitly. I could not get anywhere with that approach, even mirroring what I had learned developing a negative Schmitt Trigger. Then, a flash of insight was to instead start with a practical circuit, and work backwards after measuring it, in order to derive a more general theory of design. That approach was more successful!
Original insight: just build a regular comparator, around which hysterisis could be added.
Then experiment with adding a suitable positive feedback resistor, to get a moderate desired ΔV hysterisis to shift the trigger points away from one place. Here, 3.3MΩ was added as positive feedback,shifting the one trigger point to two. Measured LTP=2.218V, UTP=3.562V, leading to a ΔV hysterisis of 1.343V. The precise values in the ADSR application are non-critical, I just wanted something < 5V, so that a variety of waveforms would be acceptable for gating the ADSR.
This scope foto shows a measurement of the very wide signal swing of the LM3900 operating at +15V.
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