Building the ModularEEG

Credits and copyrights

The following people have contributed to this document

Dan Griffiths, Nelo, Jim Peters, Andreas Robinson, Jack Spaar and Yaniv Vilnai.

This document is copyright (c) 2002, 2003, the authors and is licensed under the Creative Commons Attribution-ShareAlike license, version 2.0

Please report to the mailing list if you find any errors, if something is unclear or if there is anything you would like to add.

1. Assembly

1.1 Introduction

(TODO)

1.2 Tools and materials

1.2.1 Tools

• Soldering iron, with a fine tip.
• A cellulose sponge soaked with water. You use this to clean the iron.
• Optional, but recommended: A "third-hand" tool, to help you hold the PCB when soldering.
• Tweezers
• Flat-nose pliers (the jaws have smooth surfaces).
• Wire cutter
• Wire stripper
• Optional: Hot-air gun, with a small nozzle attached. This is for removing stubborn parts.
• A multimeter, that can measure voltages and resistance.
• Optional: A pair of off-the-shelf, strong glasses (+3.75 strength). At $5, they are much cheaper and more convenient than a magnifying glass.
  

1.2.2 Materials

• Paper tape - can be used to keep a part in place while soldering. Plastic tape does not work - it will melt.
• Electricians tape - very handy if you want to tape down the PCB to the workbench, if you do not have a third-hand tool or similar.
• 0.5 mm diameter single-strand, insulated copper wire in various colors. Your basic coupling wire.
• Solder, < 1mm, with non-corrosive flux
• Solder-wick, (copper braid, 2 - 4 mm wide, impregnated with flux), used to mop up excess solder.
• Optional: Non-corrosive solder flux.
• Optional: Heat-shrink tubing for isolating wire ends.

1.3 The workplace

Semiconductors are easily damaged or destroyed by electrostatic discharge (ESD), so you need to take some precautions.

Avoid rooms with woven wall-to-wall carpeting and/or very dry air.

Do not wear clothing that generates static electricity. Synthetic materials and wool are bad and cotton is good. Shoes with thick rubber soles are also bad because they block electric charge from leaving your body in a safe way. If you are working on this at home you might want to try being barefoot!

Sit in a chair where the seat is made of metal, wood or cotton. Synthetic materials can cause a buildup of electrostatic charge.

The smoke from the solder flux in the solder is not very healthy. Make sure the room is well ventilated, and avoid breathing in the smoke.

1.4 Parts

Onboard Parts

For a list of parts to put on the PCBs, look at the bill-of-materials (BOM). It lists several distributors you can buy parts from.

If you order parts from distributors other than those in the BOM, the following is important.

You need to pay special attention to the "Part requirements and notes" in the ordering information section of the BOM. Any specification in that column should be met or exceeded. For example, a 1% tolerance should not be replaced by a 5% part. See the FAQ for common questions on parts selection, and if in doubt, ask on the mailing list.

In addition, you should compare the dimensions of the part to the space available in the board layout to make sure the part will fit.

You can use Eagle CAD for this. The following instruction is for Eagle CAD v4 and may have changed in later versions (?):

• Open a board layout (File menu...)
• Open the View menu and select the Grid item. You may then choose your unit (inches or millimeters for example).
• Find the mark button () in the toolbar, and click on it:
• You may now place a white plus-sign shaped marker anywhere in the layout.
• When you move the cursor around, notice how the distance to that marker is printed in the upper status bar, like (R x y). You can change the grid size and measurement unit (inches, mm, mils etc) by clicking on the grid button ( ).

By following this procedure, you may take all the measurements you need.

Cables and connectors

You will have to buy or make several cables for this project:

• Board-to-board cable
• Programming cable
• Serial cable
• Electrode cables

The board-to-board cable is a 34-lead ribbon cable. You can make one from an old floppy-disk cable. It is recommended that you buy a new female connector so that you can make it shorter if you want to. The distributors that sell electronic parts usually have connectors as well.

The parts needed for the programming cable are listed in the section on building it below. From v0.07, the header pin configuration is the same as Atmel uses with its evaluation boards, which should make it possible to use other AVR programmers and tools without modification. Caveat emptor: compare with your programmer first!

The serial cable is a plain, 9-pin serial extension cable, sold by computer stores. On the ModularEEG, it should be connected to a 9-pin female D-sub connector, with solder-cups.

TODO: electrode cables

Power supply

Recommendation: Use a battery, for safety.

Electrodes

TODO (But don't hold your breath)

Enclosures

TODO (here either)

1.5 Assembling the Boards

It can be instructive to know what the ModularEEG circuit boards look like, and below you will find two photos of the two boards with most parts, except the integrated circuits mounted. The pictures show version 0.07, which is a prototype. Your boards will look slightly different if you are building a different version.

The digital board

The amplifier board

The cyan-colored numbers in the photos above mark the locations of the various connectors.

On the digital board (upper image) you find these connectors

1. Board-to-board connector: Carries power to, and signals from, the analog board
2. Power input (PWR = positive input, GND1 = negative input.)
3. Serial I/O (RxD = data input from PC, TxD = data output to PC)
4. Microcontroller programming connector

On the analog amplifier board (lower image) you find these connectors

1. EEG channel 1 input
2. EEG channel 2 intput 
3. DRL output
4. Calibration signal output
5. Board-to-board connector: Carries power to, and signals from, the analog board

Some EEG cabling notes

Each EEG channel inputs consists of two leads, and shielding.

The shielding can be connected to VGND, and the layout is designed for this setup in mind.

However, a device which connects the cable shields to the DRL output has been built. It has some advantages, such as possibly more stable DRL operation.

Note that in that device, the amplifier board itself is placed in a metal box which is attached to VGND, while the cable-shields are attached to the DRL. There is no direct electrical connection between VGND and the DRL output in this setup (or any other for that matter). Keep this in mind when you choose connectors.

Fixing board defects

Before soldering any parts to the boards, check them for production defects by shining a light from below. You should see glowing circles around each via (hole). These are clearances. Some circles will be connected to the ground plane by thin "+" shaped traces. Some will be connected by thicker traces to other vias.

Look carefully for any clearances where the circle itself is deformed. In rare cases the ground plane may be making contact with a via. You may need to scrape a clearance around that via, being careful not to sever legitimate traces. Flip each board over and check the other side the same way.

Make a cut where you want the edge of the clearance to be, first, or you may end up removing more of the ground plane than intended. You will have an easier time peeling or scraping off the copper as well.

Backlit before-image (left) and after-image (right). Courtesy of Jack Spaar.

Of course, defects can occur along straight copper traces as well, so you should check the entire board.

After fixing any defects, solder all parts and sockets, but do not insert any of the ICs in their sockets right away. Instead, follow the procedures in the testing chapter, to get your ModularEEG up and running.

Do the parts with the lowest profile first. That way they will not fall out of the holes when you turn the board over to solder on the back. You can make the parts stay put by bending the legs/pins slightly after inserting them or by taping them down with a piece of paper tape over the component body.

Again, solder the lowest profile parts first. Here is a suggested order. The parts you get may require a different order

• Small diodes
• Resistors [1]
• The microcontroller crystal
• Large diodes
• Inductors
• IC sockets [2]
• Socket strips, if used. [3]
• The 7805 voltage regulator [4]
• Ceramic capacitors
• The transistors and the voltage reference.
• Electrolytic capacitors [5]
• Film capacitors
• Trim potentiometers [6]
• Ribbon cable connectors (pin headers)
• The DCDC converter [7]

Notes

1. If you do not know how to read the markings on a resistor, verify its resistance with a multimeter before putting it in.
2. Orient the IC sockets so that the small indentation in one end is aligned with the indentation on the component outline. It marks the end where pin 1 is located. Similarily, the ICs themselves have an indentation on top, or a small dot marking pin 1, which should be aligned with the outline.
3. You can use the socket strips in the holes for the EEG inputs, DRL output, calibration signal output, power inputs and RS232 inputs and outputs. They let you attach coupling wire without soldering, during the testing phase, provided the diameter of the wire matches the socket strip. When testing is done, and you have placed the ModularEEG in a box, you should add solder, for a more reliable connection.
4. This IC is not used if you intend to power the ModularEEG from 5V directly.
5. CAUTION: Remember to check the polarity! On tantalums, the + end is usually marked with a line. By convention, the positive lead is often longer as well. (The same is true for most LEDs as well.) If you insert a tantalum capacitor the wrong way it may go up in flames when you turn on the power. If you are unsure what end is what - ask someone who knows, for example on the OpenEEG mailinglist.
6. If you are using INA114 instrumentation amplifiers (and most people do), you can replace P201 with a short wire between two of its pins. See the amplifier schematics or DRL trimming instructions below for more details.
7. Save the DCDC converter for last. It is not the tallest part, but it is tricky to remove once in place. It is a good idea to get some soldering experience from the other parts first.

You might find these images useful.

The image to the left shows the outline of an 8-pin IC. Notice the location of the indentation and how the pins of an integrated circuit are numbered counter-clockwise. The image to the right shows the outline of the programming connector, with pin numbers.

Finally, when you have soldered everything, you need to flip over the amplifier board and put some solder on two of the six small solder bridges found near the large 34-pin connector.

Bottom view of the amplifier board - the channel-selecting solder bridges.

Place blobs of solder over the gaps marked 1 and 2. If you are building a second or third amplifier board, place the blobs over 3,4 and 5,6 respectively.

1.6 Making the Programming Cable

You can either make your own programming cable, or buy a prebuilt programmer.

Olimex, the same company that manufactures the ModularEEG PCBs sells two versions: a serial port version, AVR-PG1B for 6.95 USD and a parallel port version AVR-PG2B for 9.95 USD. Either of these would require use of alternative free programming software. www.avrfreaks.net, has all the information you will ever need on finding programmers and software for the AVR family of microcontrollers. Look in the "tools" section, and then under the "programmer software" and "programmer" sub categories.

Now, to make your own cable, which is the cheapest, but not the easiest option, you will need these parts:

• A 25-pin male D-sub connector with solder cups (that is direct wire connection)
• A connector shell for the dsub connector
• A couple of 220 ohm resistors.
• A 10-lead ribbon cable.
• A 10-pole ribbon cable connector

Here is the schematic, also found in the file Cables.Sch in the ModularEEG zip-file. It shows how all parts are supposed to be connected to each other. The name SP12 below, refers to the name of the programming software.

Microcontroller programming cable.

The schematic quite simple, so the assembly details are left to you. Nevertheless, a few building hints are useful, especially if you have not done this kind of thing before.

• This is how you attach the cable to the ribbon cable connector:

  Insert the cable into the connector, aligned with the line of round indentations on the clamp on one side, and with the small metal shears on the other side. Then compress the connector closed. You will probably have to put it in a vise for this. Using a hammer or a club is not a good idea, you will just destroy the clamp.

• There is no way you can hold the cable, the connector, the soldering iron and the solder at the same time. You should either get someone to help you, or simply tape down the connector and cable to the table. Then let a resistor, which you have already soldered to the lead, rest in the solder cup of the connector. Then solder.

• The DSUB connector shell is most likely made for round cables and its strain relief will not work with ribbon cables, but this is easy to fix: Get a short piece of round cable (< 10 mm long, maybe 5 mm in diameter), or any object roughly that shape. Place this object on top of the ribbon cable, in the strain relief, as a replacement for the round cable the shell is designed for.

• The ribbon cable should have lead no 1 colored red so that it becomes easy to find.

2 Testing

Before you begin, have the schematics and PCB layouts handy, preferably on your computer screen. You will also need a copy of ElectricGuru (see the OpenEEG website).

There are currently no instructions on what to do if something is wrong, sorry. If you get problems, ask for help on the mailing list.

2.1 Testing the Power Supply

(Updated for v 0.07 or later)

To make sense of the following, you need to look at the schematics and board layout of the digital board.

Setup for people using a 5V power source

• Install D104 and use a wire in place of D103.
• Do not install IC107 (the 7805). Connect a wire between the input and output on its footprint instead.
• Install a 200mA fuse somewhere on your power cable. This is to prevent you from frying your 5V power source.

Setup for people using a 9 - 12V power source

• Install D103. You may leave out D104 if you want. (Do not replace D104 with a wire!)
• Install IC107 (the 7805).

Pre-powerup checks

• Use an ohmmeter to ensure there is no short circuit (zero or close to zero ohms) between the GND1 and the +5V/1 network. Also check GND and +5V/2 for shorts. You can pick any points on the board that belong to these nets.
• Ensure the DCDC converter and the LED (D102) are installed.
• Ensure the microcontroller is not installed, and that you have not connected the analog board.

Test

• Connect your power source (PWR and GND pads, on the right side of the board) and turn on the power. The LED should go on. If not, immediately power down and troubleshoot.
• Measure the voltage between L103 (any end) and GND1, it should be 5V.
• Measure the voltage between L102 (any end) and GND, it should be somewhere between 5V and 5.5V, and absolutely not more than 6V.

2.2 Programming the Microcontroller

Hardware setup

• Power off and install the microcontroller. Ensure that pin 1 marked by a tab or a dot on one end of the IC is aligned with the tab on the board outline which is pointing 'down' towards the 34-pin ribbon cable connector.
• Connect the programming cable to the PC and the programming header. Pin 1 is on the left side of the header.
• Turn on the power (make sure the LED goes on!)

Programming

For the sake of consistency, all programming instructions are found in the firmware distribution.

2.3 Testing the Microcontroller

• Ensure the serial cable connector is properly soldered to the holes marked RxD and TxD (and one of the GND1 holes) on the digital board. See the cable schematic (cables.sch) for details on this.
• Run ElectricGuru and open the Preferences menu.
• Select the "Serial port" menu item and select the right serial port in the dialog.
• Select the "Machine" menu item and select "RS232EEG" in the dialog.
• Finally select the "Traces" menu item and change the following settings in the dialog: The Y-axis limits should be 0 and 1023. "Show which samples" should be set to "All".
• Connect the ModularEEG to the PC using a passthrough serial cable.
• Turn on the power to the ModularEEG and start ElectricGuru by clicking the green stoplight button.
• You should see two flat-line traces being drawn. If you touch the left side of the 34-pin cable connector (discharge any ESD first!) on the digital board, you may be able to affect them a little bit.

2.4 Testing and trimming the Amplifiers

Setup

• Verify that you do not have any short circuits between the positive power rail, VGND and the negative power rail. VGND can be found on any ground via, or on pin 1 in the socket for IC201. Since we don't want to charge the electrolytic capacitors backwards, connect the COM lead (black cable) of your ohmmeter to VGND. The positive and negative power rails are then found on pin 8 and 4 respectively of IC201.
• Insert the amplifier ICs and connect the amplifier board to the digital board.
• Connect the ModularEEG to the computer and run ElectricGuru.
• Turn on the power and make sure the LED goes on. If it does not, switch off immediately and troubleshoot.

First test

Start ElectricGuru by clicking on the green stoplight button. Unless you have mounted the amplifier board inside a metal box, you should see lots and lots of 50/60Hz hum on both channels. If you have mounted the amplifiers inside a metal box, you are more likely to see a noise signal which resembles EEG (but isn't).

Coarse trimming

• Power down the ModularEEG
• Connect the U_cal and Cal_GND outputs to the inputs of one EEG channel.
  You find U_cal and Cal_GND in the upper right corner of the analog board. You can either use a shielded cable (shield goes to VGND) or two pieces of plain coupling wire. If you opt for the coupling wire, it is important that you do not touch the leads, or wave your hands near them, or you will se a lot of unwanted mains hum.
• Start ElectricGuru with the green stoplight button. You should now see a square wave being drawn on one channel.
• Get a small, flat screwdriver and start adjusting trimpot P202 or P203 depending on which channel you chose.
• Adjust until the wave is slightly less than half scale.
• Maximize the ElectricGuru window and open the traces-dialog from the preferences menu.
• Adjust the window height so that the traces become as large as possible and set the y-axis limits to 262 and 762.
• Close the dialog and adjust the gain with the trimpot until the square wave just touches the top and bottom of the graph window. If the graph is slightly off center, trim down the amplitude so that you see both the top and the bottom of the square wave.

Fine trimming preparations

Ideally, the test signal is 5 volts / 20,000 = 250uV peak to peak. This corresponds to a square wave alternating between the sample values 262 and 762. In reality, the amplitude depends on the supply voltage (namned U from now on), which probably is not exactly 5 volts.

The actual amplitude of the square wave then becomes U / 20,000 volts peak to peak. Expressed in sample values, the upper level is M + U * 50 and the lower is M - U * 50.

Fine trimming

Important: Notice that the square wave test signal may have little bumps right after its rising and falling edges. You should not look at them when you set the gain. You get best accuracy if you look at the amplitude where the wave is flat.

On to the trimming. From now on, the y-axis limits found in the traces-dialog will be referred to as Ymin, and Ymax.

• First, measure the supply voltage U between any point on the +5V/3 net (basically the digital VCC) and GND.
• Open the traces-dialog again
• If the square wave is slightly off center from the coarse trimming, adjust the Ymin or Ymax value until the whole wave is perfectly centered in the graph window.
• Calculate M, from M = (Ymin + Ymax) / 2.
• Set Ymin = M - U * 50 and Ymax = M + U * 50. The ideal is Ymin = 262 and Ymax = 762, but your number will be different.
• Adjust the trimpot until the the square wave reaches the top and bottom edges of the graph window.
• Done. Restore Ymin to 0 and Ymax to 1023 and repeat the trimming procedure with the next channel.

2.5 Trimming the DRL

The trimming is done as follows

• Power down the ModularEEG and connect the DRL output to all (four) electrode inputs.
• Connect a (preferably digital) voltmeter to the DRL output and VGND. Set it to its lowest range setting so that you can measure millivolts.
• Power up the ModularEEG and trim P201 until the voltmeter shows zero volts. You should not be more than a few millvolts off.

Note: Most people will want to use INA114 instrumentation amplifiers and can therefore skip trimming if they replace P201 with a wire going from the wiper (connected to pin 5 on IC201B) to VGND.  P201 is only required for other types of instrumentation amplifiers, e.g AD620.

Of course, if you mount P201, you will have to trim it, regardless of instrumentation amplifier type.

3 Common Problems

What do I do if I can not solder?

Well you can learn. :-) Everyday Practical Electronics (an electronics magazine) has an online guide last seen here: http://www.epemag.wimborne.co.uk/solderfaq.htm

I have trouble with the small gaps between solder pads and ground plane. The solder tends to stick to the ground plane and form bridges to the pads.

If you etched your own boards, you may have problems with this, and will need to use a solder braid a lot to break up bridges. However, if you have factory made PCBs with (usually) green soldermasks, this should not be a problem at all. If you apply solder and heat to the pad, the solder should not flow over to the ground plane because there is no thermal connection that would heat the ground plane.

So, if you are experiencing this problem, try switching to a soldering iron with finer tip, and finer solder. Make sure the solder has channels of non-corrosive flux which helps the solder flow. You may also need to increase the heat. Solder has a natural surface tension which tends to break up bridges unless there is an excessive amount of or the solder iron is too cool.

I soldered in a part in the wrong place, how do I get it out?

I assume you use sockets for the expensive parts (except perhaps for the DCDC converter)?

In that case, whatever you are trying to remove is probably a lot cheaper than the PCB: Cut away the component, and buy a new one.

When you cut the leads, leave enough wire so that you can pull them out easily. Apply heat on one side of the board and pull them out with tweezers from the other side.

I managed to plug a hole on the PCB with solder, how do I get it out?

First remove excess solder with the solder braid. You should be able to see the outline of the hole afterwards.

Then you have two options. One way is to drill a hole through the solder – just be sure not to damage the plating. For this you need a drill that is thinner than the hole. If you do happen to damage the plating, not all is lost, you will simply have to solder on the side(s) where the copper trace leaves the hole, rather than just the bottom side.

Another way, is to melt away the solder, like this:

• Strip off about 10 - 15 cm of the insulation of a single-strand coupling wire.
• Straighten out the wire. There must not be any kinks on it.
• Hold it on the insulated end, and let the other end gently touch the centre of the hole (where the solder is).
• Apply the soldering iron to the wire, as close to the hole as possible. The wire will conduct the heat from the iron to the PCB.
• When the solder has melted, start pushing the wire through the hole, without applying any force.
• Once the wire end has passed all the way through and protudes on the other side, remove the soldering iron but keep pushing.
• If you do it right, the solder will attach itself to the wire in small droplets, but since you are moving the wire, the solder will not stick to the inside of the hole. If the wire gets stuck, just cut the part that has gone through the hole, apply heat, pull out the end you are holding, and try with a new piece. Chances are, you got rid of some of the solder in the first pass.
• Finally you should have the wire hanging loose in the hole, but you will not be able to pull it back up again. That is ok; just pull it up as much as possible, and cut off the rest. Done.

It is important to keep the heating period as short as possible, so if the solder doesn't melt within a few seconds, the iron is too weak or the tip needs cleaning.

If you are having problems with the above method, try this one, which also attempts to keep the part intact (no wire cutting).

Buy yourself a desoldering pump.

• Take your time. There is no quick and easy way to do this.
• Position the nose of the desoldering pump as close to the solder weld as you can. Melt the solder with your soldering iron and then cover the weld with the nozzle. Push the button on the desoldering pump to suck up the solder. The faster you cover the weld and press the button the hotter the solder will be and the better your results will be. You don't want to leave the heat on longer than required. Repeat if necessary. It may be beneficial to practise the action a couple of times before applying heat.
• You will be left with an almost clear hole. Usually there is some residual solder holding the component in place. Use the soldering iron to push the component as far through the board as you can. Proceed carefully, it is very easy to snag and lift the tracks of the PCB. Make sure none of them are sticking to the component as you move it.
• Hold a pin (or a cut off leg from a previous component) with long nose pliers and heat it with the soldering iron. You may need to tin (put a bit of solder on) the end of the iron first to get optimum heat transfer. Whilst heating the pin you can use it to push the component out the rest of the way.
• If the hole is not completely cleared you can tidy it up with the heated pin or try the desoldering pump again.

CAUTION: Make sure you press the nozzle against the weld, or the plating and copper traces may be ripped off the board when you push the button.

I tried to unplug a hole like you said, but the solder won’t melt. Now what?

The main cause: not enough heat. Try “wetting” the iron with some solder before putting it to the wire. This will give you better thermal contact. Trying to unplug a via - a hole placed in the ground plane - without any leads going to it - is trickier. The ground plane conducts heat very well so you will need a bigger soldering iron, or a hot-air gun. If you use a hot-air gun as heat source, make sure the nozzle is as small as possible. Heating a larger area than necessary, or longer than necessary, can damage the PCB.

I think these IC's / transistors I bought are broken. Why? They were new!

They may have been defective from the start. It does happen. Electronics manufacturers routinely have to repair their products even before they leave the factory.

Another very common reason for failure is electrostatic discharge (ESD) damage. The instrumentation amplifiers are particularily sensitive and will produce garbled signals if damaged.

You should minimize the physical handling of the semiconductors and keep them in their bags as long as possible. The silver-coloured plastic protects them from ESD.

Before handling the semiconductors, discharge yourself. You can do this by touching ground (earth). Common places connected to ground are the kitchen sink or a nearby (metal) radiator. You can also wear a conductive wrist-band and connect it to earth.

One EEG channel is working correctly, but the other is unstable or sometimes working, sometimes not. What has happened?

There are two known factors that might cause this:

1. There might be very a thin short circuit between the shielding (or VGND) and one or both electrode connections. Check that the connection from the electrode to the INA114 is correct and does not interfere with anything else. The signal wire and shielding wires are quite close, so applying silicone to them might help to prevent short circuit problems in this part. If this did not help, you might want to check the rest of the signal path to the amplifier board output.
   
2. The worst problem could be a damaged INA114 as they are very sensitive to electrostatic discharge (ESD). If you are not sure, just switch the two chips. If you then experience a similar problem on the other channel you can be quite sure it is damaged and needs to be replaced. Always be connected to ground when touching this chip to prevent ESD, especially in dry weather.

The software is receiving data, but I only see flat lines.

1. Remove the ribbon cable (while the power is off!). Fire up your ModularEEG and software. Ground yourself e.g by touching a radiator. This should discharge any static electricity you might be carrying around. Then put your finger on top of the left side (the side where pin 1 is) You should see the flat lines move up or down slightly, when you do that.
2. Make sure you put solder on the solder bridges on the back of the amplifier boad.
3. Is the flat line located near level 512? If not, you may have a problem with the voltage reference or virtual ground generator. Make sure that the voltage between pin 21 on the microcontroller and GND, is 4.0 volts. Also check that the voltage between VGND and GND is 2.0 V

My ModularEEG does not work! Help!

1. Calm down. :-)
2. Join the mailing list and ask for help.

4 Safety

TODO (some day)

This chapter is supposed to be about how to maintain isolation distances when you add connectors and put everything in a box.