Getting things sequenced



So far my analog drum machine project has concentrated heavily on the sound generation. While this is admittedly the most important part of the machine, the generators will not generate a single sound without trigger pulses. That’s why each drum machine contains some kind of sequencer. And this is what we’ll concentrate on next.

I used Arduino and a modified version of Jan Östman’s O2_minipops software as a sequencer during the sound generator testing. But this is not the way I’m going to go with the actual project. I don’t normally use Arduino in my projects. It is good for quick tests and hacks, but for a “real” project, preferably something else.

From the very beginning of this project I have had a vision that the sequencer user interface should be similar to TR-808: sixteen buttons in a row with lights, showing steps of one channel at a time; changing the step status by pressing the corresponding button; rotary switch to select the active channel; one button to start and stop the playback. This kind of basic step sequencer would be quite easy to implement with any microcontroller, but…

My main inspirations for this project have been Boss DR-55 and Korg KR-55, both from the late 70’s, and the sound atmosphere points heavily to 70’s. Andromeda Space Rockers fit well into this atmosphere. So it would be nice to implement also the sequencer in the 70’s style.

In the 70’s a microprocessor or a microcontroller was an expensive beast. That’s why neither Boss nor Korg have it, they use discrete logic chips in their sequencers. Maybe discrete logic would be good choice in this project also. That kind of devices are always fun to build, and they are so 70’s. The more I think it the more I like the idea.

Although the functionality of the sequencer is quite simple, implementing it from scratch with discrete 7400/4000-series logic is not that straightforward anyway. There are quite many details to consider. Some kind of starting point would be really useful. But where to find it? Boss and Korg are not much of a help here. Korg is not programmable at all, its rhythms are in ROM. Boss is programmable, but its user interface logic is completely different than what I’m after. TR-808 has the right kind of user interface but it uses a microcontroller, so not suitable for me.

Searching the internet again. There are all kinds of schematics that are more or less but not quite. But then I hit the jackpot: Matt Sarnoff has built an awesome drum machine using only 7400- and 4000-series logic. It is built as a contest entry to Dangerous Prototypes 7400-contest, so also the sound generators use logic chips only. I’m not interested in the sound generators now, I have already too many of them, but the sequencer part is just what I need. This is the kind of schematics I was looking for!

Here is a presentation video of the machine. The sequencer functionality is just what I was thinking of. In the Github page, there is a good explanation of the inner workings of the sequencer.


But nothing is perfect, not even this sequencer. There are couple of features that do not fit to my purposes as such:

  • Sequencer has only 4 channels, I will need 7
  • Sequencer is made to trigger logic-based sound generators, the trigger pulses are very short (hundreds of nanoseconds), my generators need 10 ms pulses
  • The buttons which I’m planning to use as step buttons need higher voltage than 5V for driving the lights, they cannot be driven directly from TTL output
  • The programmed patterns are erased when the power is turned off; it might be nice to have a non-volatile memory
  • And since the memory is non-volatile, it would be good to be able to store more than one pattern

It seems that several changes are needed to the original schematic. But still it is a good starting point, there are many ideas that can be used as such. And many details that I would not have thought, it would have needed many hours of testing with the breadboard to find out all those, if starting from scratch.


In the next post, we will dig into those changes.



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