Drum Synth & Integrated 16 Step Sequencer

I’d been looking for a good sounding drum synthesizer schematic for a while with not much luck. Trawling through hours and hours of YouTube videos and websites with incomplete schematics, I found it hard to find one that fitted my needs. I wanted something a bit more interesting than the (in my probably controversial opinion) overused and quite bland 808 sound.

After going through eBay looking at vintage drum machines for inspiration, I found the Boss DR-55… Apparently reknowned for it’s hard-to-use built in sequencer. But that wasn’t an issue for me, seeing as I was planning to build an arduino powered sequencer (more on that in a bit). A few days of research later & it turns out a company called circuitbenders.co.uk make a replica PCB for the synthesizer part of this drum machine, they call it the CB-55 – you can get it here on eBay. This is the one I got! And I have to say, I love the sound it makes. There are a couple of mods you can do to play with decay/tuning, which I haven’t got round to doing yet… One day!

There’s a pretty good build guide for the PCB that Circuit Benders made so that makes it very simple to build – the only problem is that you’ll need to source the components yourself as it doesn’t come in kit form. I’m based in the UK so I got the parts from RS, Farnell & the rest from my stash of components!

Once you’ve built it, it’s a good idea to test it here before you go any further – all you’ll need is a 5v source to trigger the sound, and something to listen to the output on (you can hook the output up to a guitar amp or similar) just be careful to put the volume pretty low when you first power it up… Just in case there’s a fault somewhere on the board, it might send a DC signal (or another loud noise) to the amp.
Once you’re happy with the drum sounds all triggering nicely, it’s time to start on the sequencer. My choice to power the sequencer was an Arduino Nano… I just had to work out how to do it. I’m a very very novice Arduino programmer, so the code I make does work, although to someone more experience its probably horrific.

To begin with, I knew I’d need some I/O expander ICs as there just aren’t enough pins on the Arduino Nano, but the price for an Arduino with the right amount of pins is just ridiculous. So I decided to use the Arduino Nano along with a couple of MCP23017 chips, this will increase the number of IO pins by 32. More than enough for what you need to make an 8 step sequencer with 5 channels (using 1 channel for each drum sound, plus trigger LEDs, input buttons and sequencing LEDs). 

All the switches I’ve used are momentary switches. The 5 vertical LEDs are just standard 3mm green LEDs, but the 8 horizontal LEDs are 3mm bi-colour (red/blue) so that you can see both the step number and programmed sequence of the selected channel at a glance.

After settling on what interfacing I needed, this is the panel I made:

Rhythm Stick 55 Front Panel

Now for the more interesting part… How it all works. Below are the schematics for all the hardware and interfacing for this project. All this is doing is sorting the inputs & outputs needed for the front panel interfacing.

  1. ​OutTrigChannelX is the output for when a drum sound is to be triggered. Each channel number specifying a different drum sound. 
  2. ledTrigChannelX is mimicking the OutTrigChannelX but for the LED rather than the drum trigger, just for a visual indication.
  3. ledStepX is the set of red LEDs on the faceplate, these indicate which steps are active for the channel selected. A step being active means the drum sound will trigger on that step.
  4. btnStepX are the input buttons to activate/deactivate steps on the channel selected. These buttons are the 8 buttons on the interface and the main part of how this sequencer works.
  5. btnChannelSelect this is the button to scroll through the different channels. I am planning in the future to replace this with a rotary switch, but this works well as it is for now.
  6. btnTrigChannelX are 5 more input buttons, these are for triggering the drum sounds when you wish (a feature that I’ve not yet implemented in the code, but it’s not that tricky to work out.
Rhythm Stick Sequencer Schematics

You’ve probably noticed there are more holes in the front panel not yet filled… I am planning to add a mixer stage to this module at some point in the future so you can adjust the output level of each different drum sound. At the moment all you can adjust are the global volume level & the tone of the whole output. But for now, this all works & I’m pretty happy with the outcome.

Check my Instagram page for videos of the Rhythm Stick 55 in action.

Here’s the real world final product with the messiest cabling known to mankind.

Internal Workings of the Drum Sequencer

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