Log Delta Ohm Meter for Physiology Study

Thread Starter


Joined Apr 27, 2008
Attached is a page from my old engineering notebook, dated 1976.
The instrument was used for several years
in my department at the Univ. of Tennessee (Medical Units), Memphis, TN.

In application, the physiological 'alert' response from a subject varies post-stimulus in a proportional manner. Using the skin resistance (as one of four different responses) as an indicator of 'arousal' post-stimulus, we plotted the responses for 30 seconds post-stimulus. The signals varied negatively from the immediately prior baseline, rapidly for 3 seconds, slowed for 10 seconds and gradually returned to a near previous baseline. During a long test session, with repeated stimulus the baseline drifted sometimes downward by 70 percent. Using common polygraph linear amp techniques and equipment, the baseline indication had to be adjusted prior to each stimuli. The repeated adjust of baseline indicator was necessary because the polygraph equipment was linear, and the sample signal was varying as a proportion of the immediately prior baseline.

The purpose of this instrument was to eliminate the adjustment for baseline drift.
The reason for eliminating this adjustment was related to the sensitivity that subjects (people)
have to being 'connected' to electrical devices, and seeing those devices 'adjusted'.
This distraction was suspected to affect the response we were measuring.

Attached you should find pieces of the notebook page in GIF format.
The project was designed, built, tested, paid for,
and used many years ago.

Note on application:
This circuit has a resistance RX which was generally used as the input signal.
A subjects finger/skin resistance was placed/connected at this point.
Skin resistance would range from 10 K Ohms to over 1 M Ohm.
Variations in Skin Resistance occured quickly and could be negagive 10 percent per second for 3 seconds.
The Skin Resistance would slowly drift back towards the previous baseline.
The baseline was continually drifting, as the subject became acclimated or agitated by the stimulus.

Varying resistance at RX basically controlled gain on a reference signal VR at the '+' input.
As RX changed rapidly, the results were monitored for 1 second, 10 second, 30 second levels.

The point RX would also accept a voltage signal, such as controlled by a piezo-crystal.
We used the crystal by applying it to a small artery on the outside of the thumb,
in order to monitor heart function. We could watch the heart waveform, systolic, etc.,
as it varied during pre-stimulus to post-stimulus. Interesting, but more than we needed.

The whole polygraph approach is an interesting study in itself.

Your comments are welcome.



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Thread Starter


Joined Apr 27, 2008
dThanks. Was not looking for assistance.

The moderator has moved this finished, working project
from the Collections Forum into the Projects Forum.

This is a finished, working project. It had a specific application.

Temperature drift is not a significant factor, in this application.
741 Opamps have not proven to be a problem in this kind of application.
I designed other Log Amp circuits using 741 with external temp comp.
One circuit had Seven Decades of usable range.
In a controlled environment and a limited time frame, Temp Comp may not be a critical factor.

I designed a Bipolar Active-Diode using 741, then modified it to use LM308, LM310, LM318.
This was interesting to me, and left me with a question which I may post and request a comment.

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