"Baby8" CV Step Sequencer – Part 3 – PCB Build Guide

By Kevin on September 1, 2024

Here are the build notes for my "Baby8" CV Step Sequencer PCB Design.

Warning! I strongly recommend using old or second hand equipment for your experiments. I am not responsible for any damage to expensive instruments!

If you are new to electronics, see the Getting Started pages.

Bill of Materials

"Baby 8" CV Step Sequencer PCB (GitHub link below)
1x CD4017 decade counter.
1x CD4093 quad Schmitt trigger NAND gate.
16x 1N4198 small signal diode (or equivalent).
8x 3mm or 5mm LEDs.
4x 100KΩ resistors.
3x 10KΩ resistors.
1x 500KΩ potentiometer (RV09 style - see photos and PCB for footprint).
8x 10KΩ potentiometer (as above).
1x 100pF ceramic capacitor.
3x 100nF ceramic capacitors.
1x 4.7uF electrolytic capacitor.
1x 8-way rotary switch (see photos and footprint and below for details).
9x SPDT slider switch, pcb mounting, 2.54mm pitch.
Jumper pin headers and jumpers.
Optional: 1x 16-way DIP socket; 1x 14-way DIP socket.
Optional: 4x 2-way jumper header sockets.
Optional: 1x 8-way right angle header socket.
Optional: 1x 8-way right angle header pins.

My 8-way rotary switch was called "RS17 Band Rotary Switch 1 Pole 8 Position" on an overseas marketplace. I had to design the footprint myself (see the design notes) so be sure to compare the measurements. The key dimensions being:

Build Steps

Taking a typical "low to high" soldering approach, this is the suggested order of assembly:

All resistors and diodes.
DIP sockets (if used).
Disc capacitors.
LEDs (if mounting flush).
Electrolytic capacitor.
Slider switches (if mounting flush).
Jumper headers and pins.
Potentiometers.
Rotary switch.

Here are some build photos of the first version of the PCB. The second version fixes a few issues, but the build process is largely the same.

Note: If a front panel is to be used then it might not be possible to use IC sockets as that would leave the ICs too high off the PCB.

The LEDs and switches can be mounted flush with the PCB, but if a panel is to be used they would probably need to be mounted at a suitable height to be flush with the panel.

The final stage is to mount the potentiometers and the rotary switch. It is recommended that the 500KΩ (marked "B5 04" probably, compared to "B1 03" for the 10KΩ pots) is fitted first to ensure it doesn't get confused with the others.

Testing

I recommend performing the general tests described here: PCBs.

This PCB can run off 5V or 3V3 (and to be honest, any supported voltage of the 4017 and 4093 being used).

Basic checks should be performed in stand-alone mode, i.e. the two jumpers are linked as to enable the internal clock and connect the CLKINH/CKEN signal into the reset chain (assuming the second version of the board):

Then the following can be checked:

Step switch reduces the number of steps appropriately.
The tempo switch adjusts the speed of the sequence.
The full output CV should be close to the VCC level used (i.e. 3V3 or 5V).
The pots should adjust the CVs for each step.
The GATE should be HIGH for every step that is enabled via the switches and LOW for any that are skipped. Note: there are no gaps in the GATE signal if all switches are enabled.
The TRIGGER signal should be momentarily HIGH for every step that is enabled via the switches.

PCB Errata

There were a number of issues with the first version of the PCB, but the second, currently published version, should have fixed them.

Enhancements:

None

Find it on GitHub here.

V1 PCB Patch and Workaround

There were three issues with V1 of the PCB:

CLKINH/CKEN is not wired correctly for stand-alone (non-cascading) use.
There are spurious triggers when steps are skipped.
The pots are wired backwards. Again. Sigh.

The first two have patches and workarounds. I just had to live with the pots.

The main issue with the CLKINH/CKEN signal is that for use as a standalone module, the CLKINH/CKEN pin really needs to be left pulled LOW to GND. But as designed in V1 it will receive a pulse along with the RST pin when the counter resets. When cascading to a second counter this is correct and desired behaviour. When fed back into the same counter it causes issues.

There are two possible workarounds, a simpler one and a more complex one. Both require cutting tracks on the PCB.

The Simpler CLKINH Workaround

This involves using the external RST connections on the IN and OUT headers. The external RST OUT is used to feed to RST signal back into the external RST IN rather than having it routed through the PCB. This means that the CKEN signal can be left pulled LOW to GND and not be involved in the RST itself.

There are two cuts required on the underside of the board as shown below.

In addition to the PCB cut traces it requires the following:

The "INT RST" jumper must be removed (highlighted in yellow above, shown from the underside of the board).
The OUT external RST signal must be connected to the IN external RST signal.

The RST connection can be done either with a long jumper wire from the OUT (on the right hand side) to the IN connector (on the left hand side); or by connecting the external RST signal jumpers to the adjacent "RTN" pins, which were added for exactly this reason, albeit for use when cascading modules. This second options is illustrated by the two green connections in the above photo on the two opposite sides of the board.

The Complex, but more Complete, CLKINH Workaround

In order to preserve the configurability of the module, it is possible with some care to turn the 2-way "INT RST" jumper header into the 3-way jumper as per the updated schematic in the design.

This requires drilling an additional hole in the PCB and adding some patch wires in addition to cutting traces. This is illustrated below.

Here are the steps required:

Drill an additional through-hole to make the "INT RST" jumper a three-way jumper. The hole should be just below the existing header, keeping the 2.54mm standard spacing. The other holes are 0.8mm diameter.
Any copper trace from the GND zones on both sides of the board around the new hole must be removed to prevent shorting to GND.
Cut the same two traces as for the "simple workaround" - shown in blue on the underside of the board on the left-hand diagram.
Cut one more trace on the topside of the board. This is the trace between the "INT RST" jumper and the left-most, vertically oriented 100K resistor - again as shown in blue on the right-hand diagram.
Solder two jumper wires (shown in green on the right-hand diagram): one between the rotary switch and the new centre of the 3-way jumper; and one from the right-most (as seen from below) 100K resistor to the new pin of the 3-way jumper.

Once complete, the new 3-way jumper can be used to switch between "internal" mode (with the top two pins jumpered) and "external" mode (with the bottom two pins jumpered).

Preventing Spurious Triggers

The key here is to add a small RC circuit between the CLK and the trigger circuitry as shown in the updated schematic:

On V1 of the board there is a track that connects the 4017 to the 4093 using a via. This can be cut (blue arrow below) and a 10K resistor used instead between pin 6 of the 4093 and the 100K resistor connected to pin14 of the 4017 (green arrows below).

Then the 4093 side of the resistor can be connected to GND using the 100nF capacitor (other green arrow).

Closing Thoughts

You think by now that I'd get the pins of the potentiometers the correct way round - hey ho. That isn't a major issue. The other two issues come from being able to practically try out the board and issues like this aren't unusual really in a prototype.

Taken together all three made it worth getting a V2 board built.

I would definitely like to make a panel for this module, but am not sure about the component heights at the moment. The switches do look like they might be an issue as I don't think the legs are long enough.

Kevin