Multi channel TEMS

This page describes a proposed mult-channel transcutaneous electrical nerve stimulation (TENS) device for Dr. Mario Cabodi’s biomedical device design lab (BME). Here is some description from the customer (N. Vellanki):

"Transcutaneous electrical nerve stimulation (TENS) is when a device is used to send small electrical impulses onto the skin to override the nervous system to provide pain relief. It’s used to help patients suffering from chronic pain and is usually a single channel device and I’m trying to make a multichannel version for experimental purposes."

Figures provided by the customer below.

The customer has asked for a quote on a complete four-channel circuit, battery-powered. The existing circuits use a single 9V battery but we propose to use 8 x AAA or AA batteries to provide 12V and longer battery life.

Reference

• GitLab?: ​https://gitlab.com/BU-EDF/edf-projects/non-physics/multi-channel-tems
• ​https://www.arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf
• ​https://diyi0t.com/arduino-reduce-power-consumption/
• ​https://www.sparkfun.com/products/15140
• ​https://conservancy.umn.edu/bitstream/handle/11299/103933/1/Young_Joseph_January2011.pdf

Rev A ECOs

• Encoder knob not wired. Need a jumper from R15 top to pin of Arduino.
• Encoder A, B are wired to A6, A7 which seem to be only analog inputs. Sigh.
  • Can wire (with maybe few K resistors) to MOSI, MISO - pins 1,4 of otherwise unused J7
• R12 should be a jumper so the LCD backlight is visible

​Google Doc with details for mods to assembled board.

Implementation

2021-11-15

Installed one transformer. No noticeable output. The "555" schematic drives the MOSFET gate with 9V-12V from the 555, while on the PCB it is driven only by 3.3V. Oops. Luckily there is an op-amp (U3). Rewire it for 2x gain, still not much output. Going to change it now to ~ 4X gain so we can get all the way to 12V.

Also according to the notes on the 555 circuit the output pulse width is adjustable 40-270uS. Should set up a test with the arduino to output narrow pulses.

2021-11-12

Working on the board. Power OK. Arduino/serial OK. LED in knob OK. LCD Ok. Encoder doesn't work.

Made changes mentioned above, now all is OK with the encoder, button and backlight.

R1*C3 is a couple of orders of magnitude too small, since the PWM freq is around 500 Hz. Changing R1 to 10K would be pretty good.

Haven't tested the outputs yet.

2021-10-22

Layout ~ done, but want to tweak a few things:

• Add a GPIO connector
• Think about mechanics of FT232 thing
• Add a programmable LED
• Connect the 12V to an analog input through a divider for battery level sense

After meeting 10/15/21 with the customer, decided to move ahead with an "Ardunio-based" project.

• ​Schematic (PDF)

One concern with the Arduino shield model is power consumption. According to ​this page (unverified) an Uno R3 uses ~100mA (so 700mW at 7V input). An Arduino pro mini immediately reduces the consumption due to not having built-in USB/serial thing, and it can easily be run on 3.3V and at lower clock speeds. AA batteries are ~2000 mAh while AAA average around 1000 mAh.

Proposed parts:

• Arduino Pro Mini 3.3V ​sparkfun ​schematic
• FTDI Breakout ​sparkfun
• Character LCD 3.3V ​sparkfun ​data
• Shaft encoder ​sparkfun (maybe ​breakout board)
• Clear Knob ​sparkfun

Cost estimate

    Engineering                        Either   Option 1    Option 2
    Discussion, specifications          1 hr
    Schematic capture                            3 hr        1.5 hr
    PCB layout (either option)                   3 hr        2   hr
    Assemble, debug, test prototype              1 hr        3   hr   
                                        Total    8 hr        7.5 hr
    PCB fabrication (JLC)  $20
    Parts (wild guess)     $50 ea

    So maybe $1500