Low-cost (~$40-60) DIY Stereo E-stim for Audio Input (based on TroniC's MidiStim)

[Electro]

I believe that the lower the wattage tap you connect the trodes to, the higher the sensation "Turns it up". If you look at how the transformer is powered to stim, it is reversed from how it would be used for speaker distribution. The higher number of turns in the secondary compared to the primary, the higher the voltage out. The lower wattage tap has many more turns than the other taps.

I thought that too and there's conflicting information about that on the other site and I didn't understand it until I read through the forum enough to find a really old post where the lower wattage tap was expected to increase the sensation but it didn't and the math was posted about why that happened. The explanation is that the resistor being on the amp side effectively becomes a higher value which counteracts the normal voltage boost from the higher turns and the volume knob needs to be turned up higher. I've personally tested this, I have the blue wrapped Specco T7010 transformers and swapped from the default 0.5w to the 0.25w tap and needed to turn the volume up on my computer for everything, although it does feel smoother. I've ended up hitting a point where the amp can't actually produce the level of power it's being tasked to do(I set the computer volume from 60% to 100% for example and there is no real difference in the output, but with the tap set back to the original value it's more powerful than I can handle). I'm running the 0.5w tap and everything is doing just fine. If the taps aren't being changed drastically or a random potentially unsuitable audio line transformer is used instead, it seems like you could play with this a bit. I think with the other designs where the resistor is on the electrode side that this doesn't apply because the resistors are not impacted by the turns ratio in the same way.

[mrman]

Thanks for the replys, i had it completely wrong as usual. Was going to use 10w ouch
This is my first build so very exiting, just waiting for amp to arrive then build and test.
The lowest out on the tr1005 is .625 then 1.25, think ill start .625, then obviously if it not enough i can come up.
Thank you mrman

[hosenguy]

The biggest issue with the conflicting information and the results is why it is such a hard task to perfect a stim box.
Here are some variables to consider:

The transformer is being connected to signal in the reverse of it's intended use.

The fact that the intended use of the transformer does not require much quality and is often built with no concern of tolerances.

The fact that any transformer below 5 watts, in my opinion, be operating at or near saturation when it is connected to the lower settings. The lowest winding also has the most resistance and inductance which effects the signal transfer to the human.

The real low wattage windings produce much voltage and little current. The ratio of power being absorbed by the human vs. the amount dissipated in the hardware used to build the circuit is no longer matched.
This includes the impedance of human skin which is generally thought to be matched at the .65 to 1.25 watt windings as the box is designed. But different transformers and humans don't always follow the rules. Human skin is also quite capacitive and differs with hydration.
I also have a human resistance chart in the Mega link that should allow calculation of why this happens.

The electrical theory (physics law) says that if everything is optimal, then the lowest tap would be the highest signal into the human. But there are so many variables that, as you have found, this is not what you feel. I heartily encourage experimentation to determine what is best for you.

And enjoy the experimentation and keep notes for future reference to help perfect your box!

[jagshag]

Does anyone know of a good spot to buy the parts for shipping to Canada?

Parts-express wants to charge $50!! for shipping and there will be duty and brokerage fees at delivery. Normally I would get things shipped to the US and go pick it up but with the pandemic that's not possible right now.

I've looked at digikey and mouser but they don't have all the parts listed. Any help is appreciated! :D

[Ldroamer]

Does anyone know of a good spot to buy the parts for shipping to Canada?

Parts-express wants to charge $50!! for shipping and there will be duty and brokerage fees at delivery. Normally I would get things shipped to the US and go pick it up but with the pandemic that's not possible right now.

I've looked at digikey and mouser but they don't have all the parts listed. Any help is appreciated! :D

You could try Amazon or Ebay. I got the parts I needed to build a mid stim unit off of Amazon a few years ago. Not sure about ebay as I don't use it but I think you can often find things there too. Having said that I think the prices on Amazon were a bit steep but it was convenient for me so I went with it.

[CaliPhotoDude]

To satisfy my own curiosity I purchased the exact BOM from Parts Express above to test out.

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I found that the volume control has an off/on switch. The amp does not pop or produce any spike when turned on but makes a fairly pronounced one when turned off. It is not suppressed.

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I think this is a good beginner's stim box and provides a substantial experience. With a couple adds it could easily become a more advanced box.
Stay tuned.

I really appreciate all of your input on this and frankly joined the board mostly just hoping you can help me clarify a couple things since I am now spiraling down this new rabbit hole.

You mentioned the pop on volume on off and I was looking at your schematics you posted later in the thread and was thinking if I read it right I can add switches on the output side of the transformers on the positive electrode sides as basically kill switches in case mistakes are made to allow me to turn off in safety or really kill it if I feel like the box is trying to kill me.

Also for the tri-phase wiring simplification couldn't I also add a switch to basically short the common outputs together after the transformers before they go to the output.

I tried to highlight the section I am thinking about.

If that works will these switches work or am I underestimating the power? I remember about 10% of my high-school electronics. I started working with computers instead. I am at that place where I think I know what I am doing which is where I mostly make mistakes.

https://www.parts-express.com/SPST-Ligh ... ch-060-542

Thank you again though, I've learned a lot already from just reading everything.

[hosenguy]

I really appreciate all of your input on this and frankly joined the board mostly just hoping you can help me clarify a couple things since I am now spiraling down this new rabbit hole.

You mentioned the pop on volume on off and I was looking at your schematics you posted later in the thread and was thinking if I read it right I can add switches on the output side of the transformers on the positive electrode sides as basically kill switches in case mistakes are made to allow me to turn off in safety or really kill it if I feel like the box is trying to kill me.

Also for the tri-phase wiring simplification couldn't I also add a switch to basically short the common outputs together after the transformers before they go to the output.

I tried to highlight the section I am thinking about.

If that works will these switches work or am I underestimating the power? I remember about 10% of my high-school electronics. I started working with computers instead. I am at that place where I think I know what I am doing which is where I mostly make mistakes.

https://www.parts-express.com/SPST-Ligh ... ch-060-542

Thank you again though, I've learned a lot already from just reading everything.

Regarding the switch: 2 of those will work the way you show as they have the capability needed BUT ...... I use a single DPDT switch of similar specifications. With 2 switches you will not be able to abort as quickly as with a single switch.
With the DPDT switch you will turn it on after turning on the power to the amp and then you turn it off before readjusting your trodes with your hands and also before you turn off the amp.
A switch like this one would work or any with similar specification.
https://www.parts-express.com/DPDT-Medi ... ch-060-554

For tri-phase you are also correct. This is the common way to do it.

In both uses the switches require less than an amp by a bunch. So if it handles around 150 Volts, or more, it likely will work.

I am glad you are reading the available information provided here and in other related threads!
Good luck!

[CaliPhotoDude]

Regarding the switch: 2 of those will work the way you show as they have the capability needed BUT ...... I use a single DPDT switch of similar specifications. With 2 switches you will not be able to abort as quickly as with a single switch.
With the DPDT switch you will turn it on after turning on the power to the amp and then you turn it off before readjusting your trodes with your hands and also before you turn off the amp.
A switch like this one would work or any with similar specification.
https://www.parts-express.com/DPDT-Medi ... ch-060-554

For tri-phase you are also correct. This is the common way to do it.

In both uses the switches require less than an amp by a bunch. So if it handles around 150 Volts, or more, it likely will work.

I am glad you are reading the available information provided here and in other related threads!
Good luck!

Thank you so much for your help, it is appreciated. I will probably go with this switch just for design considerations. Strange they don't offer a medium or light duty DPST in that switch design.

https://www.parts-express.com/DPST-Heav ... ch-060-564

And I am happy to have access to all of the information, figure best to read enough to ask hopefully intelligent question.

Now to I guess get parts on order, looks like due to transformer backorder it will be closer to end of the month but hey more time to study and plan I guess.

[DBRob]

Slightly confused noob here, I have some Visaton TR 10.16 transformers and want to make sure I'm wiring them correctly since the pins are arranged differently than on the parts express transfomers.
Is it correct as drawn in this image?

scr1.png (54 KiB) Viewed 6221 times

[JakofClubs]

It looks right to me, but I'm no expert. I have constructed three of these, but with different transformers.

[hosenguy]

Slightly confused noob here, I have some Visaton TR 10.16 transformers and want to make sure I'm wiring them correctly since the pins are arranged differently than on the parts express transfomers.
Is it correct as drawn in this image?

Yes that matches the other drawings if "in" goes to the amplifier and "out " goes to the trodes.

[maps524]

Fun experiences with a TENS 7000 and the low cost of this DIY device have given me the courage to try to put one of these together.

I've ordered all of the parts on BOM except for the Bullet plugs, because I wasn't sure if they'd be compatible with the electrodes that I have. Is there something I can do to make this work with my current electrodes, or will I need to get new ones that will work with the bullet plugs?

Thanks for all the good tutorials on building this thing! I'm really hoping that between the content of this post and my basic soldering skills that I'll be able to make this thing work, because I'm really looking forward to it.

[hosenguy]

Fun experiences with a TENS 7000 and the low cost of this DIY device have given me the courage to try to put one of these together.

I've ordered all of the parts on BOM except for the Bullet plugs, because I wasn't sure if they'd be compatible with the electrodes that I have. Is there something I can do to make this work with my current electrodes, or will I need to get new ones that will work with the bullet plugs?

Thanks for all the good tutorials on building this thing! I'm really hoping that between the content of this post and my basic soldering skills that I'll be able to make this thing work, because I'm really looking forward to it.

I do not know for sure what you are referring to as bullet plugs. Do you mean the stim signal output connectors?
The design is with 4mm banana receptacles often labeled as speaker wire connectors.
Or do you want to match the existing cable you have from a TENS device?

Common TENS cables often have a pin plug or button connectors on the electrode end and those come in a couple sizes so you would need to either measure them or look up the electrode you use to see what they specify about them.

If you want to plug a TENS cable you have into this stim amplifier you should buy a connector to match the cable and modify the BOM to the different receptacles. The TENS usually have a coaxial connector and I would not try to find or install those. It would be much cheaper to buy a new set of leads I would guess. I often buy the recommended connector in a project to have it to look at to see what I can use with it. In this case the picture should be enough to clarify this.

Good luck!

[JBK]

I have 2 transformer related questions. I know they've been somewhat answered in different topics, but I'm looking for a complete overview of the differences between the given options. So, here they come :) :
1. Why do we use the 8 Ohm and not the 4 Ohm connection of the transformer? How would the stim differ?
2. Why do we use the 0.625W exit and not the 1.25W? How would the stim differ?

[hosenguy]

Actually the issue is matching impedances. The 8 ohm output is slightly safer for the amplifier output so it does not get overloaded. But I do not know of any reason you should not match the transformer impedance to the amplifier. If you have a 4 ohm output then it can certainly be used to hook to the same 4 ohm value on the transformer. I would not expect any change in the sensation either way.

The output of the transformer is usually .625 because that impedance transfers the signal to the human the best in most but this is a personal choice and the 1.25 watt is also useful and often used. The sensation is usually more powerful on the .625 watt tap but some transformers and some humans have different impedance so you should try various taps to see what you prefer.

A bigger issue that affects the sensation is the value of the series resister in the output circuit. The smaller the value the sharper the sensation - sorta like a stinging feeling. The larger the value then the smoother the feeling.
The reason for this is that the signal is mostly a voltage with a small resister while it becomes more current based with the larger resister value.

The best thing about building your own stim device is that you can tune it as you go until it is the best for you!