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Section 2 - Create your Board

In this section we describe how the control board is designed and how you can produce it. Rea the whole tutorial first and perhaps make a checklist so you have everything understood and together once you start the building.

Circuit Design

We designed a control board that uses an EMS signal of an off-the-shelf EMS device and attenuate the singal.

We bypass the singal from the the user and lead it back to the device. When a signal should be applied to the user, the resistance is scaled up and the current takes the pass through the users arm.

CoreCircuitEMSModule.pdf

For safety reasons we designed two galvanically isolated circuits, the EMS circuit and the control circuit. The control circuit is connected to the BLE module, the Arduino, and a 9V or USB power supply. The EMS circuit is connected to the signal lines of the EMS device and the human body, as shown in the figure above. To implement the galvanic isolation, we used an optically isolated MOSFET driver. It generates a current to drive two MOSFETs in the EMS circuit, which reduce the signal intensity. One MOSFET is used for the positive and one for the negative half-wave of the EMS signal. The resistance of the MOSFETS should be between 100 and 10 k, because otherwise the EMS device switches off automatically. For safety reasons we also included two photo relays to instantly cut off the EMS circuit when switching the module off. In the control circuit the input of the MOSFET driver is leveled by a 1 k I2C digital potentiometer, because the MOSFET driver is too fast for simple PWM. For Bluetooth communication we used a standard BLE chip.

Circuit Schematics

We designed the schematics for eagle. They are available here: Schematics comment: # (Add link to schematics)

If you don't have eagle, you can download it from the link below:

download eagle

Open eagle and go to "<your created folder>\ToolKit\Hardware\EagleSchematics". Open the file "EMS_WS_Module_Vertical_V01.sch" from the repository.

You will see the board layout and the schematics as shown in the figure below.

eagle_files.jpg

The board layout is shown on the left, in which the circuit is defined. On the right, the schematic that defines the interconnection between the single components is shown. You can edit the layout and change parts.

Build the Board

If you would like to produce the board you can export the gerber files with a cam process or use the PCB design from the repository. Below is an sample list of manufactors you can find in the web.

We built our control boards of toolkit with a LPKF mill device and later after extensive testing we chose a manufactor. An hint: Buy your PCB with a stencil. It is useful for lining up the solder paste.

Board manufactors:

The manufactors need gerber files to produce your PCB. For elecrow you can find the gerber files and the cam job in "<your created folder>\ToolKit\Hardware\EagleSchematics". If you use another manufactor than elecrow, then mind be need to create new gerber files. For this read the tutorial about how to create gerber files of the specific manufactor and create the gerber files.

Create gerber files using the cam processor

If you chose another manufactor, you have to create gerber files for the manufactoring of your board. The files describe the layout of the board in a standarized language. This section is about the creation of these files.

First open eagle and open the toolkit project. Open the .BRD file. you will see the layout of the board. Same as in the picture below.

eagle_open_camp_proc.png

Then we will need to start the cam processor of eagle. It creates the gerber files for us. So click on "File->Cam processor".

When it has opened, it should look like the following picture. The most fields are empty and the process is not configured. To configure the process we need to load a .cam file. You get this file from your manufactor. As said before it describes the settings of the gerber process. Store this file on your pc and open it in the camp processor window over "File->Open->Job...". Compare with the next picture.

eagle_choose_job_camp_proc.png

Now you should see many tabs and in each tab there are paremeters configured. Don't change them if you don't know what you do. We did an additional step to remove some text we do not like to see on the board. Therefore we removed the "tnames" and "bnames" layers from the tabs "Silk bottom" and "Silk top". Look at the following pictures. To do this select the tabs and unselect in the right list the mentioned layer.

eagle_remove_tnames_and_bnames_camp_proc.png

eagle_removed_tnames_and_bnames_camp_proc.png

When you are done with this little change, click on the "Process job" button. Now the gerber files will be created. You will find them in the project folder of the opened project.

Please read also the instructions of your manufactor. At the end upload the gerber files to you manufactor. Which files you have to upload will also be explained by your manufactor.

Parts Ordering

See quantities and hyperlinks in the partlist in partlist. We choose Mouser because Mouser has all parts we need for our prototyping toolkit on stock. You can choose another supplier and also similar parts. You need to get sure that the technical characteristics are similar.

Part Placing and Soldering

After you have ordered the parts, you can see in the eagle-".brd" file where the components have to be placed as you see in the pictures below. For mounting you have two options: solder the components manually, which might be hard, or use a reflow process.

The recommended parts of the part list have different sizes follow the link to learn more about SMD in general:

Manually Soldering

If you don't have the possibility to use a reflow oven, follow the next link to learn how to manually solder the parts to your PCB.

For manually mounting and soldering you will need the following:

  • a fine tweezers
  • soldering iron with a fine tip
  • solder
  • flux
  • desoldering braid
  • endurance and some time ;-)

If you solder the parts manually, read the next steps for reflow soldering. They might help you to place the part correctly. Regrading the size of the parts, we recommend to use a reflow oven.

Reflow Soldering

The printed or ordered PCB board is double layered. You need to place your parts on both sides. For mounting the parts and reflowing them, we recommend to start with the bottom side (1) and then with the top side (2). The parts from the bottom side are flatter, so you can place the board better in the second reflow session.

For mounting and reflowing you will need the following:

  • a fine tweezers
  • reflow oven / heat gun
  • reflow solder past
  • solder paste stenciling (mask)
  • rubber glove
  • a soldering iron with a fine tip
  • solder

Optimal and useful:

  • "helping hand" with magnifying glass
  • tissue for cleaning
  • a spatula for applying the solder paste
  • (as always) Tap
  1. Bottom side

First start with ordering all parts that you need for the bottom layer:

  • 1 x AD5252 (start placing with this part)
  • 4 x LH1546ADF
  • 2 x VOM1271
  • 4 x STD25NF20
  • 2 x Resistor 1kOhm
  • 2 x Resistor 1.5MOhm
  • 2 x Capacitor 10nf

Then place the solder paste stenciling and apply solder paste.

Read the following tutorial if you have never done this before. The general use and storage of solder paste is explained, as well as how to use a solder paste stencil and how to apply the solder paste:

Place your stencil for the bottom side on the PCB and fix it with tape. Note: The stencil needs to be flat, without bubbles on the board, and the stencil should not move when you touch it. Put some of the solder paste on the stencil and spread it using a spatula as described in the tutorial.

Remove the stencil carefully from the board.

stencil_lining_up_solder_paste_text.png

If you are done, the board should look like the following picture. You can see a small amount of solder paste on all pads. It is important that the solder paste of two pads are not connected. If you don't have solder stop on your PCB, there should be no overlapping solder paste to the ground plane or to other wires. Such shorts are really hard to find and to fix while testing.

solder_paste_on_pcb_bottom_small.png

Now place the parts on the PCB board. Again, you need the following amount of SMD parts:

  • 1 x AD5252 (start placing with this part)
  • 4 x LH1546ADF
  • 2 x VOM1271
  • 4 x STD25NF20
  • 2 x Resistor 1kOhm
  • 2 x Resistor 1.5MOhm
  • 2 x Capacitor 10nf

Start in the middle and go to the outside while placing the components. This will help to avoid slipping off the components. Use the tweezers to place the parts. Start initially with the digital Potentiometer (AD5252) as you see on the placing. It is the smallest and hardest part.

placing_parts_bottom.png

The following figure shows the placed components. The first pin of each component is labeled with a red circle. The orientation of the resistors and capacitors is equal. The ceramic capacitor is bipolar. The MOSFETs can only be placed in one direction.

placed_parts_bottom_small.png

Reflow the PCBs. Read the instructions of your reflow oven and on your solder paste. There is a short introduction on how to reflow.

This man bought a reflow oven from beta layout and shows what you get and how to use it. He explains the temperature curve and some other facts. In the end he reflows a small board:

The next picture shows how we reflowed the bottom side. We put the boards on a surface and placed the temperature sensor in the middle. Be careful and monitor the process. Ones our temp sensor falls down and we got burned boards.

reflow_bottom_small.png

Let the parts cool down, then start with the top side.

You can also use a heat gun to reflow the board. How to solder an SMD - Youtube

We still recommend use a reflow oven. Some time parts fly away or drop down when you are soldering the second side.

  1. Top

Place your stencil on the top side of your board and do the same as above. Then place the following components:

  • 1 x RN4020 Bluetoothl LE Chip
  • 1 x blue LED
  • 1 x red LED
  • 1 x green LED
  • 4 x 220Ohm Resistor
  • 1 x 470Ohm Resistor
  • 2 x 1kOhm Resistor
  • 2 x 2.2kOhm Resistor
  • 2 x 4.7kOhm Resistor
  • 2 x female 15 Position SMD Header 2.54mm

Start placing with the Bluetooth chip. Then place the small parts (resistors and LEDs). Mind the polarity of the LEDs, otherwise you have no cool status light. See the following figure:

placed_parts_top_text_small.png

After placing the parts, put the board on a surface in your reflow oven and start the reflow process. See the following figure.

reflow_top_small.jpg

  1. Last steps

Finally, solder the remaining parts (the right angled 6 position shrouded header, battery clip) manually. These parts are not made for reflowing! The next figure shows the bottom side, where the shrouded headers and the power source are connected. It shows you the channel input and output pins and the polarity of the connection to the power source. These power pins (Vin and GND) are directly connected to the corresponding pins on the Arduino. Therefore the recommended voltage has to be between 7V and 12V.

placed_parts_bottom_2_small.png

Your toolkit

It should look like this:

  • Top

Toolkit_top_resized.png

  • Bottom

Toolkit_back_resized.png

Build connectors

Build the connectors for your EMS-Device and to your electrodes. Below you see sample connectors.

  1. Connectors to EMS-Device

We used the provided cable from our EMS Device and put them into header with 6 Positions. Note that the two positions in the middle are not connected. The polarity is equal, because the device generates a positive and a negative square wave.

ems_input_cable_small.JPG ems_input_cable_wiring_small.png

  1. Connectors to Electrodes

We used ribbon cable for the connection and header with 2mm diameter, which are fitting our electrodes. Note that the two positions in the middle are not connected.

ems_output_cable_small.JPG ems_output_cable_wiring_small.png


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