Micro-current electrical stimulation device

Discussion in 'The Completed Projects Collection' started by wayneh, Aug 15, 2016.

  1. wayneh

    Thread Starter Expert

    Sep 9, 2010
    This is a project to build a micro-current electrical stimulation device. These are also called tDCS devices and are similar to MENS devices. They are not much like the common TENS devices which use much higher voltage pulses.

    I have posted about this project before here. Since then I've built a couple of these devices for friends and they've been very pleased. So I thought I should update the circuit and post the complete information here.

    A tDCS device provides a constant current from zero to 600µA applied the the earlobes of the user. That alone can produce a therapeutic effect, but it's more effective if
    1) the current is pulsed at ~0.5Hz,
    2) reverses direction (this is an AC device), and
    3) the pulse timing varies "randomly".​

    The changing frequency feels somewhat random and prevents a "burn in" in the patient. It's more effective than a steady beat. This is a probably a small refinement, but I wanted to include it.

    I based my project off this device, described in patents US2010145410, US2010047834A1 and others. I also added TENS capability in my first unit since most of the circuitry was already in place to easily add this capability. I have since removed the TENS circuit. It's a completely different treatment and there is no reason that someone using tDCS will necessarily care about using TENS. And, you can buy very elaborate and nice TENS units for ~$30. No need to build your own.

    Circuit Overview
    The strategy for my circuit was to use a dual timer 556, with the first timer slowly changing the voltage on the second timer's control pin to change the timing. The clock pulses from the second timer go to a 4017 counter to create the proper waveform. Finally, the waveform drives a dual op-amp configured to provide a sort of H-bridge. An adjustable constant-current is applied to the output earclips.

    Schematic and Description
    Screen Shot 2016-08-15 at 11.06.05 AM.png

    The voltage on the timing capacitor C5 of the first timer U2 feeds an op-amp voltage follower U6, and then the voltage is amplified by a 2nd op-amp U7 to make a ~30-second triangle wave from rail-to-rail. It's a crude triangle, since it's really just the middle of the timing capacitor's RC curve . This is applied to the control pin of the second timer U1, which would otherwise be running at ~0.5Hz. Pulling the control pin high and low changes the frequency over a ~4-fold range from 0.25Hz to 2hz. (It also changes the duty cycle, but that's not an issue in this circuit.)

    The 2nd timer's output is fed to a 4017 counter U3 and the "0" and "2" outputs are compared. This provides a way to alternately remove and reverse the direction of current flow. Using the "2" pin against the "0" pin as outputs allows the current to reverse direction with a dead space in between when the "1" pin is hot and the 0 and 2 are both at ground. The pulse on the 3 lights an indicator LED and the counter resets on 4.

    Output from the counter controls the state of a dual op-amp controlled-current circuit. The circuit works with an LM358 for U4 and U5, but the simulation looks better using a TLV2721. The LM358 can't get as close to the ground rail, and the simulation shows the "dead space" intervals as reduced-current shoulders. But the current never goes to zero as it does (in simulation) with the TLV2721. Again, I built two successful units using the LM358. I've switched to the TLV2721 for later builds.

    The current intensity into the load R8 is adjusted using a dual gang pot, since there are essentially two channels, plus and minus. R10 and R11 in the simulation are actually each a 3KΩ fixed resistor and a 10K variable resistor in the pot. The minimum of 3K results in an output intensity of ~0µA. At the maximum of 13K total, the output reaches 800µA, a little higher than the target of 600µA but still quite safe.

    Screen Shot 2016-08-15 at 11.23.25 AM.png

    Parts List
    Following is a recent Mouser order for most of the required parts
    Screen Shot 2016-08-15 at 11.27.46 AM.png
    Screen Shot 2016-08-15 at 11.28.13 AM.png

    I had a few parts, like the LED, on hand already and so there may be a few parts missing from the order. Let me know if you find one missing and I'll add it.

    Here is where I ordered some cheap ear clips on e-bay http://www.ebay.com/itm/331870106853
    I've also ordered from Lhasa Ohms in the past:
    Ear clips: http://www.lhasaoms.com/Ear-Clip-Electrodes.html
    Wire leads: http://www.lhasaoms.com/Lead-Wires-with-3-5-mm-Jack.html

    I use these 4x6 cm prototyping boards to build the circuit on. Standard fiberglass with 0.1" hole spacing. It's a little tight on the 4x6. Use a 5x7 if you want extra space.

    My circuit design is done is drawing software. I use Intaglio. I can supply the original Intaglio file to anyone that asks but I have not attached it here because I suspect there are few other users of that software here. Instead I'll post the various layers.
    The board:
    Screen Shot 2016-08-15 at 11.45.40 AM.png

    The board plus solder traces
    Screen Shot 2016-08-15 at 11.46.03 AM.png

    Plus components
    Screen Shot 2016-08-15 at 11.46.24 AM.png
    Plus wiring
    Screen Shot 2016-08-15 at 11.46.54 AM.png

    Detail of the output and adjustment pot
    Screen Shot 2016-08-15 at 11.47.14 AM.png

    The backside solder traces, viewed from the back
    Screen Shot 2016-08-15 at 11.53.32 AM.png

    See the project linked in the introduction for a photo of my first complete build. I'll be making more of these soon and will post more detailed photos.
    Last edited: Aug 15, 2016
  2. wayneh

    Thread Starter Expert

    Sep 9, 2010
    Here is the LTspice simulation file. It requires the CD4000 library and the model of the TLV272 op-amp to be called as included files.
  3. ScottWang


    Aug 23, 2012
    Hi wayneh:
    Please make a direct link or copy the end product to this thread, it may easy to see it, thank you.