FOC-Stim: a new approach to DIY stim hardware.

[emiumfinal]

I plugged ST's Board Design project files into Altium's online viewer for you:

Spoiler: show

Screenshot_20250131_184111.png

That's a top-down view, with USB in the bottom left.

Hope that helps. As long as you get the wiper (pin 2) right, worst that can happen is volume increasing when you turn CCW (meaning pins 1 and 3 are swapped)... which is a good reason to set software volume limits low, and not test on flesh!

Ideally use a multimeter to check which pin on your replacement pot is the wiper. Whichever pin, when checked against the wiper, shows 0 ohms when the pot is fully CCW, that's the ground. There should be 10 kOhms between two pins no matter the position of the wiper, those two pins are 1/3 (ground and Vcc).

Also the pot is a Bourns 3386P-1-103TLF which is a 10 kOhm half-watt. Datasheet doesn't say so I'm assuming linear style, not logarithmic. Try to use at least a 0.5W external pot, though most chassis mount are more than that IIRC.

Thanks, now I just have to decide if I have the requisite soldering skills to get it off the board.

I tried it this morning and it was fantastic once I got the hang of using it, but the lack of an easily accessible manual volume control bothers me.

[icehash23]

Thanks, now I just have to decide if I have the requisite soldering skills to get it off the board.

I tried it this morning and it was fantastic once I got the hang of using it, but the lack of an easily accessible manual volume control bothers me.

Me too. Switching to the right tab on restim, highlighting the Volume field, finding the Page Down key, and holding it 'til it's at 0%, too much fuss IMO. Hardware is nice for some things, even if it's just controlling software.

Maybe diglet could add a feature for scroll-wheel adjust when hovering the green volume bar? But that might increase the potential for accidents, I suppose.

For desoldering parts, I find the best way is to put it in a vice vertically, with the solder side facing your dominant (soldering) hand. Add a bit of solder to each pad/pin to refresh the joints, add flux, more thermal mass to avoid damaging the pads. Grip the part with non-dominant hand (or pliers), then heat up each pin in a quick rotation while gently pulling on the part.

As long as the pins aren't bent + clipped before soldering (often not in mass-produced things, and easily checked prior just from the shape of the joint) then it should come out with relatively little hassle. One thing of note is that any pin attached to a copper pour (1 & 3 in this case, I believe, for ground/VCC on separate board layers) then there will be more thermal mass and the joint will take longer to go liquid, meaning longer dwell time on those pins in the rotation. There are thermal reliefs on the pads though, which are meant to minimize this problem.

Once the part's out, you can use a desoldering pump ideally, or solder wick to clear the holes. Use lots of flux! Best of luck.

[edger477]

Me too. Switching to the right tab on restim, highlighting the Volume field, finding the Page Down key, and holding it 'til it's at 0%, too much fuss IMO. Hardware is nice for some things, even if it's just controlling software.

That was reason I created restim-controller...

There is even button to stop instantly (volume=0) and if you have a phone or tablet or anything you can use it as remote

[emiumfinal]

I plugged ST's Board Design project files into Altium's online viewer for you:

Spoiler: show

Screenshot_20250131_184111.png

That's a top-down view, with USB in the bottom left.

Got it swapped out, but that diagram seems to be backwards. I compared it against the installed pot about six different ways/times and on my board the ground pin is the one near the C in CE, and the 3V one was the one closest to the edge of the board. The picture f12345u posted, assuming his black wire is ground, is how mine was/is wired.

[Bouncyboingboing]

I've got my device built up and working. Seems great so far. Thanks very much diglet et al for this.

I am however getting signal lost disconnections from time to time. I think I did the firmware update via STMcube Programmer. Firmware is showing as V2J45M31. is this the correct version?

In STMCube when I "connect" the board I'm no longer able to hit the firmware upgrade button. Seems weird to me.

[edger477]

I am however getting signal lost disconnections from time to time. I think I did the firmware update via STMcube Programmer. Firmware is showing as V2J45M31. is this the correct version?

I used to get disconnections with older version and now am on V2J45M30, which works stable. I just checked STMCubeProgrammer and it doesn't show any new versions available...

[digitalparkinglot]

Hey Group,

I thought I'd share my initial impressions for those who are thinking of building a FOC-Stim. It's a fairly easy build. If someone makes a PCB for this then it will be a really easy and really fast build. Comparing the FOC-Stim with a DIY box, the FOC-Stim and DIY box feel very similar. The FOC-Stim feels smoother and cleaner. I was hoping the FOC-Stim would feel significantly different and way better. Based on only about 3 hours of experimenting, the estim feeling between the FOC-Stim and a DIY box isn't significantly different. I will be playing around with this more today and if my opinion changes I will update this post.

Big thanks Diglet and Edger for your help. "Charging only" micro USB cables are a thing and they do not work with this device. Once I got the correct cable my computer immediately recognized something new had been plugged in. I followed Diglet's instructions and was able to get my device built and up and running. Follow the instructions precisely, they are not written as an easy how to, but you can make your way through. After messing around with the FOC-Stim running Restim for a few hours I am beginng to believe I don't need a volume knob. Because every session is being run by funscripts through Restim there are no variations in the volume between sessions. Just open Restim, set the pattern generator to circle, fine tune the calibration wheel and adjust the volume % until everything feels like you like, and you are golden for the entire evening. Just keep dumping new videos into your player until you are done, you don't need to adjust the volume unless you want to bump it up of cool it down and that is as simple as page up or page down with the 'volume tab' open.

Quick question for those who have built a FOC-Stim device, does your STM32G431CB microcontroller get hot? Mine begins to warm up immediately and gets to a point were I'd call it hot. I can hold my finger on it and it feels hot but not so hot it burns me. Is this normal and to be expected or did I do something wrong? It works great estimming so maybe it's okay?

-digitalparkinglot

[longtimelurker!]

Hi,

do I understand correctly that FOC stim only works together with restim?
So is there any way to play mp3 stim files with the board or does it have to be funscript files?

[diglet]

Quick question for those who have built a FOC-Stim device, does your STM32G431CB microcontroller get hot? Mine begins to warm up immediately and gets to a point were I'd call it hot. I can hold my finger on it and it feels hot but not so hot it burns me. Is this normal and to be expected or did I do something wrong? It works great estimming so maybe it's okay?

It gets hot, the software has temperature sensors which are reported over teleplot, the board reaches around 50-60° with 100% duty cycle waveforms. This is perfectly fine, the board can handle 100° forever.

Hi,

do I understand correctly that FOC stim only works together with restim?
So is there any way to play mp3 stim files with the board or does it have to be funscript files?

It only works with restim. It's unlikely generic mp3-to-funscript will be implemented anytime soon because it is very hard and probably less useful than you think.

[Afapp]

For desoldering parts, I find the best way is to put it in a vice vertically, with the solder side facing your dominant (soldering) hand. Add a bit of solder to each pad/pin to refresh the joints, add flux, more thermal mass to avoid damaging the pads. Grip the part with non-dominant hand (or pliers), then heat up each pin in a quick rotation while gently pulling on the part.

As long as the pins aren't bent + clipped before soldering (often not in mass-produced things, and easily checked prior just from the shape of the joint) then it should come out with relatively little hassle. One thing of note is that any pin attached to a copper pour (1 & 3 in this case, I believe, for ground/VCC on separate board layers) then there will be more thermal mass and the joint will take longer to go liquid, meaning longer dwell time on those pins in the rotation. There are thermal reliefs on the pads though, which are meant to minimize this problem.

Once the part's out, you can use a desoldering pump ideally, or solder wick to clear the holes. Use lots of flux! Best of luck.

A useful product for desoldering. Chipquik or any other low temp solder. you add this to the joint with your iron. low melt point now allows you longer working time and less chance of overheating the board.

[icehash23]

I've got my device built up and working. Seems great so far. Thanks very much diglet et al for this.

I am however getting signal lost disconnections from time to time. I think I did the firmware update via STMcube Programmer. Firmware is showing as V2J45M31. is this the correct version?

In STMCube when I "connect" the board I'm no longer able to hit the firmware upgrade button. Seems weird to me.

V2J46M31 is the version I have, must be at least as recent as the 29th. Haven't had any disconnects yet, but I've only had a couple sessions since then. Make sure your USB connections/cables are good? A badly-shielded cable will pick up more noise, which can cause problems even if the connections are good.

I used to get disconnections with older version and now am on V2J45M30, which works stable. I just checked STMCubeProgrammer and it doesn't show any new versions available...

From the release note here (Rev 35, January 2025): "On ST-LINK/V2-1 boards, upgrade the firmware to the version V2J46M31 (STM32) or V2M31S7 (STM8)."

Forget how I found that PDF. Navigating ST's site is a little tricky sometimes. I wish there were actual firmware files available on each product page, but they just link to the STSW-LINK007 upgrade utility. When I run that, it shows that I can upgrade to V2.J46.M31, but I already have that. Not too sure how I'd go about downgrading...

Quick question for those who have built a FOC-Stim device, does your STM32G431CB microcontroller get hot? Mine begins to warm up immediately and gets to a point were I'd call it hot. I can hold my finger on it and it feels hot but not so hot it burns me. Is this normal and to be expected or did I do something wrong? It works great estimming so maybe it's okay?

Glad the cable sorted your problem @DPL. I haven't noticed the MCU getting hot personally, I just did the sniff test at initial powerup and all seemed fine. Datasheet says the max junction temperature for the STM32G431CB is 150 Celsius (302 F) so I would say that if it's not burning you, then it's not overheating. If you're worried about it, there are those stick-on heatsinks for Raspberry Pi boards and the like. For that chip size (48-LQFP) you want 7mm x 7mm a la these uxcell ones on amazon. But I don't know how much they'll help in a sealed enclosure with no airflow.

A useful product for desoldering. Chipquik or any other low temp solder. you add this to the joint with your iron. low melt point now allows you longer working time and less chance of overheating the board.

Ahh that's smart, I even have some paste in the fridge. Thanks!

[puste]

Are other have this issue:
temperature limit exceeded 80.02. Current temperature=79.24. Restart device to proceed
And it is good it stop. But I can not found out why this happend ?
It happend after using it for about 20 minutes. Is there some part that could be wrong that I am using. ?

[diglet]

Are other have this issue:
temperature limit exceeded 80.02. Current temperature=79.24. Restart device to proceed
And it is good it stop. But I can not found out why this happend ?
It happend after using it for about 20 minutes. Is there some part that could be wrong that I am using. ?

On some boards, the temperature sensor appears to be working poorly. You can change the temperature limits by changing this code in config.h

Code: Select all

// board temperature limits
#define MAXIMUM_TEMPERATURE 80.f    // degrees celsius.

To a higher value. I will probably increase it to 100.f next release.

[puste]

I try to put a GPU Heatsinks - 15x15mm on the STL180N6F7 POWER MOSFET that was hot and I think this help.
It has now running for much longer time. So I hope it help.

[digitalparkinglot]

Based on a couple days of experimenting, the estim feeling between the FOC-Stim and a DIY box is similar, but different in some pretty important ways. The FOC-Stim feels a lot smoother and cleaner. With the DIY box, if I move an electrode 1mm I can get a pretty big change in how it feels. With the FOC-Stim electrode placement is MUCH more forgiving and feels MUCH more consistent. With the FOC-Stim I never get little shocky or burny sensations like I do with the DIY box. I've also noticed you can move around a bit and the FOC-Stim seems to adjust to you. With the DIY box I was always afraid to move because I didn't know if I'd get a shock or loose the stim altogether. There is also something about the way FOC-Stim feels more like it's more directly connected to the nerves and muscles rather than how the DIY box feels like it's just on the skin. The FOC-Stim also feels a lot more separated between the channels. The DIY box can feel a little muddy in terms of separation, but the FOC-Stim never does. The separation is always very clean. This could be because it's pulse driven and the nerves aren't supposed to get as saturated with a pulse driven device, I don't know or really understand, I just know it feels better. I am definitely a convert to the FOC-Stim for playing funscripts.

Hey Diglet, I want to follow up on the board heating up. So the connector on my 12V-1A power supply is identical to the one for my DIY box. So turns out I was feeding the FOC-Stim 12V-5A for a couple hours. It handled it like a champ. The board got hot to the touch but not so hot it burned. With the correct power supply it only gets warm to the touch. Apparently I'm one of those people where a little knowledge is dangerous.

A warning to those of you that have cobbled a FOC-Stim together without putting it into an enclosure of some sort. I have naked boards and wires strung across my desk right now. It works and feels really great. I purchased parts to build 2 of these devices. If something goes astray with your device Restim picks it up right away and you are forced to restart your FOC-Stim device which requires unplugging the Micro USB and plugging it back in. Well one time as I was going to plug it back in I missed slightly and the connector grazed the underside of the STM32G431CB microcontroller. After that nothing was quite the same. The board would immediately get very hot (especially one chip right by the connector) and within a minute I would get an overvolt error. Luckily I had another STM32G431CB microcontroller that I could swap out.

Thanks Milovana Community, you are the best!!!
-digitalparkinglot