The mercury-in-rubber strain gauge is the detector of breathing. In the model device, the strain gauge produced by Medimatic, Demark was used. The change in the length of the strain gauge wrapped around the chest during breathing causes a varying electrical resistance of about 0.2 0/cm. The constant current passing Rl, R rand R4 gives the constant component on the output of Ul differential amplifier. The change in the resistance of the strain gauge, which results from breathing, produces proportional varying in output voltage of Ul amplifier.
The voltage follower, based on U2, separates the output voltage of the filter from the input stage amplifier. U3 works as a noninverter amplifier with regulated gain. This device should be supplied with a stabilized constant voltage of Â±15 V.