Can ya hear my hearbeat?

•November 11, 2007 • 1 Comment

Ok, I decided to setup the basic single channel circuit and test it out to see if i could atleast register my own heartbeat. Easier said than done. A little bit of theory…

INA118 gain is determined by a single resistor Rg. And the Gain formula is : Gain = 1 + (50kilo-ohm/Rg)

Now, I thought i could manage to record my heartbeat with a nominal gain of 100 or so, but I actually had to crank up the gain to get a decent recording by using a 100 ohm resistor for Rg. Anyways, got that all setup and now, for the electrodes. I haven’t got down to creating the active electrodes yet, so I decided to simply use a pair of home-made ones. Each passive electrode simply consisted of a plastic cap with an office clip (yes! the all-useful office clip) and a sponge soaked in saline water. Here’s what it looks like:

Passive electrodes

A word of advice: Watch out for all that chest hair when you tape the electrodes to your chest. It can get excruciatingly painful when you try to pull the electrodes off! Don’t ask me how i know! 😀

Electrode positioning: Main electrode (V+) on the left chest close to the heart, other electrode (V-) on the right chest and the Right leg electrode on the right ankle (duh! obviously!!). The electrode cables were simple single cores with shielding.

OK, time for Testing! As I suspected, the circuit worked better with a dual power supply rather than a single 5v supply. (just hook up a couple of 9V batteries to 7805 & 7905 with supporting components and you’ve got your dual supply). Also, the right leg driver didn’t really improve the quality of the readings. In fact, I got better readings by simply connecting the Right Leg electrode to Vref which, in the case of a dual power supply, would be the Ground. The reading I got was pretty noisy but not bad, considering my electrodes were home-made. Recorded my heartbeats through the soundcard using Adobe Audition. Here’s a screenshot for y’all:

testing INA118 - heartbeats

It’s definitely not the cleanest of recordings… which brings to me to the next stage… Filtering!

See y’all in a while!

EEG for the braindead

•November 11, 2007 • 12 Comments

I was hoping to start this blog yesterday but I couldn’t find the time coz I went for a rock show. This little project is a resurrected version of something similar I worked on a few years ago. A lot of my inspiration comes from the OpenEEG project. So, if you’re ready for the ride, let’s begin!

I hope to be able to provide a few useful guidelines to those who’d like to build a cheap and affordable EEG recording circuit for themselves. My idea is to build a basic EEG circuit based on the venerable INA118 (precision instrumentation amplifier) and then record the data output either through a :

a) RS232 serial port (optoisolation achieved using 6N139)


b) Soundcard (signal is first FM modulated, optoisolated using CP Clare’s LOC 110 which is then fed into the Mic input and finally software demodulated into the original EEG signal )

One thing I definitely want is to have atleast 8 separate channels and I’m planning to employ Active electrodes (16 of them + 1 for the Right Leg Driver). The difference between a Passive and Active electrode is that the latter uses an op-amp combined with the electrode itself. This provides better readings and also better noise reduction. Now, the best part about this entire thing is that i plan on using ONLY ONE INA118 inst. amplifier for all 8 channels. This will be achieved by routing the signals from the active electrodes through an isolation analog multiplexer such as MAX4558.

It’s gonna take a while before i complete the entire thingamagic, but it’ll be worth it. If all works well, I’ll use this for my own neurofeedback training purposes.

Here’s a pic of the basic EEG circuit. What you’re looking at is the basic building block based on INA118 (the chip on the left) with a Right Leg Driver (RLD) running on TLC272 (to the right).

INA118 basic EEG circuit with RLD

Keep checking in every now and then. Once the circuit is completed, I will post the entire schematic. Will definitely keep you posted on any progress made in the meantime…