Olimex Support Forum

Others => Biofeedback - EEG, ECG, EKG => Topic started by: Kris on October 25, 2012, 12:42:47 pm

Title: Correct use of the active electrodes
Post by: Kris on October 25, 2012, 12:42:47 pm

I was wondering if you needed paste/gel to apply the active electrodes on the scalp?
Title: Re: Correct use of the active electrodes
Post by: LubOlimex on October 25, 2012, 01:19:35 pm
It is highly recommended for better contact and overall better readings. It also prevents the skin from possible damage (not that I have seen any but you never know).

If you aim at high accuracy it is good idea to not touch anything connected to regular power supply (120V/220V) since it might issue a 60Hz/50Hz hum/distortion to the graphic. If powering the board from laptop it might be good idea to disconnect it from the electric network and use only the laptop battery for power.

Best regards,
Title: Re: Correct use of the active electrodes
Post by: zackbam on November 15, 2012, 12:28:34 pm
One week ago I received EEG-SMT along with 5 passive and 4 active electrodes. I have been experimenting with OpenVibe and BrainBay since then. The main task was to observe clear alpha activity (around 10 Hz) increase around the occipital lobe and around the vertex. When using just passive electrodes the signal is pretty clean, and the alpha activity clearly increases when eyes are closed. This is a clear indication that we are measuring pure brain activity. Nice! I was also able to detect P300 (11 out of 20 accurately detected P300 in the online scenario. Random would be 3.33 out of 20).
When using the active electrodes thought, the only thing I was able to get is Muscular activity. The signal is extremely noisy and unpredictable.
In any case I use conductive Gel.
Has anyone been able to get clear alpha activity using the active electrodes shipped by olimex when eyes closed?


Here are the screenshots with clear alpha activity when eyes closed and the P300 speller (using passive electrodes).
Title: Re: Correct use of the active electrodes
Post by: zackbam on November 15, 2012, 03:18:22 pm
EEG-SMT stopped working with passive electrodes. I only get this weak signal that does not responding to any muscular or whatever activity. I have the impression that there must be sth wrong with the usb slot of the EEG-SMT. The wires seem to be pushed inside... Sometimes once I connect it, the red light flashes once and then turns off. I tried to reinstall the drivers, but this did not fix anything. With the active electrodes I still get the noisy signal (where I can only capture muscular activity). Nothing happening on the electrodes seems to affect anything. As if NO AMPLIFICATION of the passive electrodes TAKES PLACE. Here is a screenshot when using passive electrodes:

I would open the EEG-SMT to see if there is any bad connection, but there is a paper stating "warranty void if removed". So what should I do? send it back to olimex and expect a replacement? The same problem happened two days ago, but the next day, it just worked again. It does not seem to be an overheating problem, as it did not work from the beginning.
Title: Re: Correct use of the active electrodes
Post by: LubOlimex on November 15, 2012, 03:44:40 pm
Hello Zack,

Can you take a picture of the USB connector for me? There should be four "bumpy" metal pieces situated symmetrically in the middle of the connector. 

I can't advise you to open the case, since that would void the warranty. The reasons for this are two: 1)some of the customers of EEG devices are not really experienced in electronics 2) the EEG-SMT is quite sensitive device with a lot of components.

Still if you believe the device was working before and you are capable of fixing the problem, it might be a better deal to check it yourself (it will save you travel expenses at least). If you go for that path mind that it might be a little tricky to remove the device from the case (especially around the USB). Arm yourself with patience beforehand.

Sometimes it is possible that the whole USB connector gets unsoldered because of improper transportation/handling. If you open the device you can also clean dubious joints (if any) with a bit of alcohol.

Best regards,
Title: Re: Correct use of the active electrodes
Post by: zackbam on November 15, 2012, 05:05:56 pm
The problem was indeed in the USB port that was problematic from the first moment I received the EEG-SMT. Two wires are pushed deep inside and when I connect the cable the contact is problematic resulting in unstable signal, or no signal at all or freezing of the flaw. Now I managed to pull out a bit the wires using a small tool -without opening the case of EEG-SMT- and it works, but still I have to be careful and keep everything very stable.
I would say that Olimex should be a bit more careful and checking better the devices it sends... Now I have to either replace the usb port myself and VOID WARRANTY or send it back and pay for the shipping cost myself.
I think all this is pretty unfair, when I received a problematic device from the beginning...
Title: Re: Correct use of the active electrodes
Post by: LubOlimex on November 19, 2012, 01:39:19 pm
Hey Zack,

Unfortunately, big part of our devices are cheaper than the transport costs to the customer. We used to cover those device "trips" but it was easily exploited by some customers - free repair for obviously damaged by high input voltage devices or damaged after custom soldering work, etc.

What I did was to inspect the USB ports of all EEG-SMTs we have at stock here. They seemed fine. I will keep my ears open for other complaints related with the four wires of the USB connectors.

Anyway, at least the problem is identified, sometimes it takes quite a while.

Best regards,