photo plethysmograph


A simple photo plethysmograph



The following pictures and drawings describe a simple photo plethysmograph which is ideally suited for simple psycho-physiological experiments and bio feedback. The circuit design employs standard components only and uses a single supply voltage.

The most critical part of a plethysmograph is the sensor which is quite expensive if bought from a medical parts supplier. Fortunately a simple sensor can be built for a few dollars with some effort. The main part of the sensor is a reflection light barrier which can be bought for about 1 dollar. The housing of the sensor is made of a few centimeters of plastic pipe as used for electrical installations. A part is cut out of this pipe in a way that it can easily be attached on a finger of the test person. (The way this sensor was built was an idea of Michael Juris of the institute for experminental and applied psychology of the Johannes  Gutenberg University  Mainz where I was working when I was a student. :-)


The following pictures show the home built sensor from both sides:



The following picture shows the interior of the experimental version of the plethysmograph:


Visible on the front are the following switches an jacks (from left to right): The power switch with a power indicator LED, the connector for the sensor, a switch to select between to coupling capacitances, the gain potentiometer, the heart beat indicator LED and finally a BNC connector to pick up the amplified signal and display it for example on an oszilloscope.

The signals obtained by this simple plethysmograph are quite good as the following picture may show (these are some of my heart beats :-) ):



Click here for the schematics of the plethysmograph - it is just a hand written schematic, but it should be sufficient to build a device like this. :-)