Infrared-sensing Cardiac Monitor

Built as course project in Biomedical Engineering elective, Boston University Hospital, 1980. Used TRW/Optron OPB-706 infrared reflective optocoupler as blood flow sensor held to fingertip. Readout via sound (beep tone), front-panel light (LED), and calibrated meter (tachometer). Triggered on R-wave (highest amplitude component of heartbeat coincident with strongest blood flow) via reflective amplitude discrimination using Schmitt trigger.


Schmitt trigger: circuit used when an input signal to a digital circuit doesn't directly fit the description of a digital signal. For various reasons the input signal may have slow rise and/or fall times, may have acquired some noise that could be mis-sensed by later circuitry, or could be an analog signal (such as in this heart monitor) whose frequency we want to measure. All of these conditions, and many others, require a specialized circuit that will "clean up" a signal and force it to true digital shape. The required circuit is called a Schmitt Trigger. It has two possible states just like other multivibrator circuits. However, the trigger for this circuit to change states is the input voltage level, rather than a digital pulse. That is, the output state depends on the input level, and will change only as the input crosses a pre-defined threshold.

Right-side view. Design used 9 TTL IC's (integrated circuits), plus a number of discrete semiconductors (transistors, diodes) on two printed circuit boards.

Left side view. I hand-traced, etched and drilled the printed circuit boards from raw copper-clad phenolic board stock.  Not in focus, but I etched “SMS 80” into the PC board (at arrow).