Expressor

A tiny 2HP 1U utility module (Intellijel 1U format) that turns an expression pedal into a 0–10V CV source (~9.7V in practice, see How it works). Plug a TRS expression pedal into the exp jack, take CV from out, and you've got a foot-controlled filter sweep, LFO rate, FX send, or a nice macro controller for your patch. A small LED between the jacks shows you what's happening.

ADD ON MODULARGRID

Two stacked PCBs behind a PCB front panel: the Jack PCB sits right behind the panel and holds the two jacks and the LED, and the Main PCB sits behind that with the actual circuit and the power header (on the silkscreen they're labeled CtrlBrd and MainBrd).

[!NOTE] If you're in Germany / the EU, I have a bunch of PCB/Panel sets available. Shoot me a message!

Three things you should know before you build this

It uses a 3-pin power connector, not a regular Eurorack ribbon cable. There's simply no room for a 10-pin header in 2HP 1U. You'll either need the little 3-pin busboard from this repo, or the janky jumper-wire hack described in that same section.

The Main PCB fits in a few different ways, but only one way works. The power header has to sit behind the green jack, and the components are NOT visible from the back. Flip it 180° and the module won't work. Nothing shorts the power rails in any orientation, but it wasn't the wisest design choice to make it reversible and not label it :D Watch the pictures carefully.

2HP is tight. The jacks need to sit perfectly straight — the green one is exactly as wide as the panel, and if it's rotated even slightly you might not be able to fit a module next to it.

[!TIP] Join my Discord Server for build support and discussion.

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Contents

• Bill of Materials
• PCB & panel files
• How it works
• Assembly
• 3-pin busboard
• Using it
• License

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Bill of Materials

Module

Part	Qty	Notes
Front panel PCB	1	hardware/panel/
Jack PCB	1	hardware/jack-pcb/
Main PCB	1	hardware/main-pcb/, order it assembled (PCBA), parts listed below
Thonkiconn stereo jack (PJ366ST, green)	1	Expression pedal input
Thonkiconn mono jack (PJ398SM, black)	1	CV output
Jack nut	2
0402 LED, color of your choice	1	Optional, see Assembly. Bring a spare. They're tiny and tend to yeet into the void
1x3 pin header, SMT, 2.54mm pitch	2	Connection between the two PCBs
1x3 pin header, through-hole, 2.54mm pitch	1	Power connector

Main PCB SMD parts

The BOM and pick-and-place files in hardware/main-pcb/ are exported from EasyEDA with LCSC part numbers. Order this board assembled (PCBA), e.g. from JLCPCB. The Jack PCB only has the LED on it, so it's cheaper and easier to order that one bare and solder the LED yourself.

Part	Designator	Footprint	LCSC
TL072CDT dual op-amp	U2	SOIC-8	C6961
1N5819WS Schottky diode	D2, D3	SOD-323	C191023
10µF 25V capacitor	C1, C2	0805	C3853143
100nF capacitor	C3, C4	0603	C14663
100kΩ resistor	R1	0603	C25803
20kΩ resistor	R2	0603	C4184
2.2kΩ resistor	R3	0603	C4190
1kΩ resistor	R4	0603	C21190
1MΩ resistor	R5	0603	C22935

Busboard

Part	Qty	Notes
3-pin busboard PCB	1	hardware/busboard/
2x5 Eurorack power header	1	Shrouded works too
1x3 pin header, through-hole, 2.54mm pitch	1–4	One per 3-pin module you want to power
3-pin female-to-female jumper cable	1 per module
Kapton tape		To insulate the back of the busboard

What I use with it

• Expression pedal: Nektar NX-P
• 6.3mm to 3.5mm stereo adapter: this one

PCB & panel files

Everything's in hardware/:

• hardware/jack-pcb/ — gerber_jack-pcb.zip and schematic_jack-pcb.pdf. Two jacks and the LED, nothing else. The LED polarity is marked on the silkscreen (- / +).
• hardware/main-pcb/ — gerber_main-pcb.zip, schematic_main-pcb.pdf, bom_main-pcb.csv and pick-and-place_main-pcb.csv. For PCBA, upload the gerber zip, the BOM and the pick-and-place file.
• hardware/panel/ — gerber_panel.zip for the 2HP 1U front panel. source/ has the KiCad project plus the panel artwork as .svg, .ai and .afdesign, and jack-pcb-outline-reference.svg, which I used to line up the panel holes with the Jack PCB. I ordered mine with lead-free HASL and otherwise default settings. ENIG (gold) would look even cooler, but also costs more.
• hardware/busboard/ — gerber_3pin-busboard.zip for the busboard, and gerber_10-to-3pin-adapter_untested.zip, a single 1:1 adapter from a 10-pin ribbon cable to 3 pins. It needs a 1x3 SMT header and a 2x5 SMT power connector. I haven't tested this one yet.

Both PCBs are also on OSHWLab if you want to open and edit them in EasyEDA: Jack PCB / Main PCB.

How it works

The circuit is really simple. A resistor divider (R2/R1) makes a ~10V reference from +12V, and one half of the TL072 buffers it. That buffered reference goes out to the ring of the expression pedal jack. The pedal is just a potentiometer, so its wiper (tip) comes back somewhere between 0V and the reference. The second half of the TL072 buffers that, and the output goes to the CV jack through a 1kΩ resistor and to the LED through 2.2kΩ. R5 (1MΩ) pulls the input to 0V when no pedal is plugged in.

The two Schottky diodes in series with the power rails protect the module if you plug the 3-pin power cable in backwards.

The output tops out around 9.7V, not a full 10V. The reverse-protection diode drops ~0.35V off the +12V rail before the divider, so the reference ends up at roughly 11.65V × 100k / 120k ≈ 9.7V.

Your pedal needs to be wired tip = wiper, ring = reference, sleeve = ground, which is the most common layout. Some pedals have a polarity switch for this.

Assembly

1. LED (optional)

The LED is only an indicator for the CV out. If 0402 soldering scares you, just leave it off, since the module works exactly the same without it.

If you wanna add it, watch the polarity! The cathode (-) goes on the left, pointing towards the large hole between the jacks. Here's a polarity guide for SMD LEDs if you're not sure which side is which on yours. My boards in these photos don't have the - / + marks yet, but the gerbers in this repo do.

0402 is super tiny (one was lost during this photoshoot). Easiest way: put some solder on one pad (the right pad if you're right-handed), heat it up again and push the LED in with tweezers. Once it sits right, solder the other pad. Don't apply heat for too long or you'll push it off.

2. Bend the jack leads

Bend the jack leads like in the pictures. They need to be flat against the jack housing, otherwise they won't fit. Also pre-bend them to the sides as shown, otherwise they'll try to twist away.

3. Jacks and panel

Place the jacks on the Jack PCB (E = green jack, O = black jack), put the panel on and use the jack nuts to hold everything in place.

Make sure the jacks are perfectly straight. Ideally use clamps (like the Omnifixo) to hold them. Solder one pin on each jack first, check that everything is flush, then solder the rest.

4. Trim the jack leads

Use flush cutters to trim the jack leads as flat as you can, otherwise the Main PCB won't sit flush.

5. Headers between the boards

Take the two SMT headers and solder only one pin on each. Don't solder them all yet! Put the Main PCB on and make sure it fits and both headers are perfectly aligned. You can carefully heat up that one pin between the PCB sandwich to nudge a header into place.

Once everything is straight, take the Main PCB back off and solder the rest of the SMT header pins.

6. Power header

Solder the 1x3 power header onto the Main PCB. It goes on the back side, where the text and the -12V @ Stripe mark are. Make sure it's soldered at a right angle.

7. Put it together

Put the Main PCB back on. The power header sits behind the green jack! If you get it the wrong way around, the module won't work.

Solder one pin on each header, make sure everything is flush, then solder the rest and trim the pins so they're nice and flush with the PCB.

Use a multimeter to check for shorts between the power pins and GND before you plug it in.

3-pin busboard

• Solder the 2x5 power connector. If you're using a shrouded one, the notch should point towards the PCB edge. Plug a cable in to double check that the red stripe matches the white line.
• Solder one or more 1x3 headers.
• Put Kapton tape on the back of the busboard and over any unused header pins, so nothing shorts later (especially if your case is made from metal).
• Use a 3-pin female-to-female jumper cable to connect the module. Pay close attention to match the white line on the busboard with the -12V mark on the module.

Janky hack: if you don't want to order the busboard, use a 3-pin male-to-female jumper cable and push the male end into a regular Eurorack power cable. The 5 rows of 2 pins are: -12V on one end (red stripe side), +12V on the other end, and the three rows in the middle are all GND, any of those is fine. Ideally secure everything with some tape.

Using it

Connect your expression pedal and move it. The LED should go from off to full brightness and the CV output from 0V to ~9.7V. If it doesn't, check your pedal for any settings or adjustments. On my Nektar NX-P I use Mode 1 (switch on the back), and the offset potentiometer on the left needs to be at zero.

Now take the CV output and use it in your patches. It works great as a manual filter control, for LFO speed on an Øchd, as an FX send, or as a macro control routed to multiple destinations.

Only plug a stereo (TRS) cable into the expression jack. A mono cable shorts the reference output to ground. It won't break anything (the TL072 is short-circuit protected), but the module won't output anything useful and the op-amp gets warm, so don't leave it like that.

License

MIT, for everything in this repo: PCB files, panel artwork and docs.

The End

I hope this was a fun project and you got through with ease. If you're having difficulties at any point, please reach out. You can join my Discord Server for support. I'd also love to see your creations! Tag me on Instagram or send me a photo to add here. Check out my other stuff on YouTube or Instagram. You can Buy me a coffee if you've found my work helpful :)