Is a sound card capable to record
biologic signals, like ECG signals? Sound card are now cheap,but they often
bring a respectable 16 bit A/D and D/A converters!
However, many biologic signals
have very low frequency, so the 20 Hz bandwidth limitation of most cards can
be a problem. It can require to increase the input capacitors, or at work
out a frequency shift up. Safety is the main issue on medical devices,
so isolation is unavoidable.Optoisolation of analog signals requires an accurate
tune-up of the related circuitry. As an alternative, off the shelf linear
optoisolators are available as well (e.g. from Harris). For sake of safety,
the non-isolated power track is supplied by very low voltage disposable batteries
or by approved medical use power supplies
Another big issue is noise, and correct grounding and circuit layout is essential to get optimal results.
Developing the software in Java and using a standard browser for the graphical interface can open new possibilities for your machine. Connect it to a LAN and the experiment status can be easily monitored from another room. Connect it to the Internet and the same experiment can be easily monitored from anywhere in the world, using nothing but a web browser! This can be of great value for emergency rescue, or for little villages where there are no doctors. Nowadays, web interfaces are an hot topic. Follow on of the links in the technology compass section for inspiration.
From Alexander Nickolsky:
You wrote - " Is a sound card capable to record biologic signals, as ECG signals?
Sound card are now cheap,but they often bring a respectable 16 bit A/D and
D/A converters! "
The answer is - no. There are several reasons for this:
1) ECG needs at least 3 channels. 6 is better. (EEG) needs even more.2) ECG needs freq. range starting from 2-5 Hz or even DC. These problems can be solved using, for example, intermediate modulator with software demodulator.
3) ECG device should be isolated from AC power and this isolation should hold at least 4000 volts.
Two years ago I've done ECG device.
There we solved the problem (3) easily - our device was intended to use with
battery-powered notebook or Newton computer. We strictly prohibited using
device when connected to the AC outlet. Our device used its own ADC and RS232
interface. I think today such a schematic can be easily made on Atmel's AVR
processor. (10 bits is quite enough for ECG ADC)
The optoisolation could be used to achieve safety. If there should be connection to a PC, the simplest solution is to make the optocoupled RS232 connection.
All I can say about my device is that we used Burr-Brown's instrumental amplifier (not very cheap one). The most essential parameter of an ECG amplifier is the coeff. of reducing of synphase signal. This is why dual op-amp can be used, however it is not the best solution.
I tried an IR LED and a remote control module receiver, as used in TV. It work nice up to 4800 baud and up to a couple of meters away (but it sinks significant peak current).
Wow, I think this is a project of great educational value. I remember of a cheap ECG amplifier built around a quadruple op-amp connected as differential amplifier, do you have any suggested schematic to send to Jeff?
Another nice variation would be to connect the amplifier out to an ADC microcontroller, sending samples out via infrared or RF for sake of safety. That way our sensor will be wearable.
Alexander Nickolsky: Connecting crocodile clips to parts of your body is not very good practice. Skin resistance can be quite large, so to obtain good contact (and not cause pain from crocodile clips) it is better to have two (or three) stainless steel plates (3x5 cm approx.), cover them with salt water and attach them to hands using rubber band. Usually 3 electrodes are used - two on hands(+ and -) and one on foot (ground).
Other ideas for a heart-beat monitor are - take IR LED/photosensor pair and place it over a blood-vessel and monitor changes in output current. - use microphone placed over a chest to directly 'hear' the heart beat.
DISCLAIMER: The information above is provided as an idea only. Use it at your own risk. Components involved can damage your equipements, cause injury to people and even be lethal. Always keep safety first.