OK ! Interesting thoughts ! Now I see what you mean with skin reactance. The tric would most likely be quite sensitive, as this probably could detect changes very early in the "sweating phase". Sven -----Ursprungligt meddelande----- FrŚn: Robert K. Johnson Till: PICLIST@MITVMA.MIT.EDU Datum: den 30 juni 1999 10:52 €mne: Re: SV: Re: SV: Re: Help Researching Galvanic Skin Response Circuit Designs >At 11:00 PM 6/27/99 +0200, you wrote: >>Best Robert, >> >>Most likely someone else wrote "...For one: You want either a WELL-isolated >>power supply, if you're going to plug this unit into AC power (And I don't >>mean a typical "Wall >>Wart", I mean something that's certified for medical use!) - OR, you >>want to use a deep-cycle lead-acid battery (probably a better choice!)..." >> >>which makes me suggest batteries is best bet for this application. Very much >>so, as the application suggests electrodes attached to a human body. I am >>sure you are familiar with these issues. Kindly understand that my comment >>is aimed towards any reader who would make his own electronic circuits, and >>attach it to a body. >> >>2nd comment: Well - I did read the original question - maybe a bit fast. The >>reason I suggest accelerometers is not to detect a seizure; that is too late >>- yes. That was the event to be avoided. What I mean is that there might be >>other ways to detect a situation that would precede the seizure. Sweating is >>one, but possibly the occurence would be preceded by an increase in "tossing >>and turning", which would be easier detecable as an increase in >>accelerometer activity over - say - a 3 minute period or so. Another guess - >>but remember that this also is just a guess as we know very little of the >>situation - is that signals from a microphone to a PIC could assist. >> >>I don't believe that 2 to 5 Volts DC would result in just a few microamperes >>of current through electrodes, and I suggest that 2 to 5 Volts DC is way too >>much for monitoring purpose over - say a whole night. I also suggest that >>"just a few hundred nanoseconds" is too short time to measure anything but >>reactive componens. Sweat is a lot of ions, and you should let them get a >>move on to be able to measure by applying a small voltage for several >>milliseconds. So I suggest that a fairly low frequency is applied for skin >>counductivity measurements. BUT it may NOT be DC. The ions that are moved >>under the skin, must go back again. Rephrased: the capacitor in series with >>the output of the exciting oscillator (or whatever) is important; making >>current resultant 0, like you said. >> >>Threshold of feeling is about 500 microamperes under 100 Hz. Currents >>smaller are harmless, but must not have a DC component. This is all just >>clearifying the DC issue, that you seem aware of. I am not at all an expert >>on GSR, but hope that the presented problem can be solved without getting >>into too much trouble with GSR. A change in GSR as an indicator ? well - if >>applied right and the change is very distinct - maybe...:=) >> >>Anyway - most imortant is of course that this person can be helped. I hope >>that the assistance progresses, and a solution emerges. >> >>Best Regards >rest of message deleted for the sake of brevity... > > My suggestions were not intended as any method of making an absolute >measurement, Rather as a way of detecting a change in conditions. As to >the small pulses >my intent was to make reactance measurements. I am not concerned with >subdermal ionic activity in this suggestion. I do believe that there will >be significant changes in skin conductivity when the body begins to sweat. >This effect will change the capacitance of a pair of electrodes coupled to >the subject. and seem to be a nonintrusive method of detecting the sweating >condition using insulated non contacting electrodes. The µProc should >generate a variable width pulse stream and adjust its pulse width to center >the input at the detection threshold (logic level)... This will require a >voltage follower with a fast rise time as the input mpedance of a typical >PIC is in the order of 20K ohms with 20 to 50 pf of capacitance... >(these figures are from memory so dont shoot me if I am off in my estimation). > > There are many possibilities here for a measurement system that should be >easy to code and cheap to build... The WDT in a pic16lc711 can be >progbrammed to operate at a 2.18 secomd (approx) period turn itself on >(housed in a watch type case) make a measurement, compare it against the >previous sample... seems to me that the ram comtents are stable in sleep >mode and if the error condition is detected could ring an LC circiut with >short pulses to be detected by a close am radio... > > Robert K. Johnson > rkj1@ix.netcom.com >