The IR rays thrown by the transmitter diode is reflected from the finger tip blood content of a person and is received by the receiver diode. The intensity of the reflected rays vary at a proportion determined by the heart pumping rate and by the difference in the oxygenated blood levels inside the blood content.The sensed signals from the infrared diodes is processed by the shown opamp stages which are in fact a couple of identical active low pass filter circuits determined to cut-off at around 2.5 Hz. This implies that the maximum attainable heart rate measurement would be restricted to about 150 bpm.
We use the IC MCP602 for the processing in the form of IC1a and IC1b in the proposed heart rate sensor and processor design. The IC is a dual opamp manufactured by microchip.
It's designed to work with single supplies and thus becomes extremely favorable for the discussed circuit which is supposed to operate from a single 9V cell. This also means that the output of the opamp would be able to produce a full positive to negative voltage swings corresponding to the sensed heart rate signals from the IR diodes.
Since the ambient conditions may be polluted with plenty of stray signals, the opamps needs to be immunized against all such spurious electrical disturbances, therefore blocking capacitors in the form of the shown 1uF capacitors are positioned at the inputs of each opamps.
The first opamp is set to produce a gain of 101, the second one being identical to the first IC1a configuration is also set at 101 gain, however that implies that the total or the final gain of the circuit at the output is rendered at an impressive 101 x 101 = 10201, such high gain ensures a perfect sensing and processing of the extremely weak and obscure input heart rate pulses delivered from the IR diodes.
An LED can be seen attached across the output of the second IC1b opamp which blinks in response to the received heart rate pulses from the IR diode stage.
The application presented here is for reference design purposes only and is not intended for any life-saving or medical-monitoring use.
How to Set up the Heart rate sensor circuit
Setting up the proposed heart rate sensor, processor is actually very easy.
As we all will understand that the difference between the oxygenated blood and de-oxygenated blood could be hardly distinguishable and require extreme precision in all respects in order to enable the processor to judge the subtle differences within the blood stream and yet be able to convert into a swinging voltage change at the output.
In order to ensure a perfectly optimized IR beams from the IR Tx diode, the current through it must be restricted to a well calculated proportion such that the oxygenated blood offers a relatively higher resistance for the rays to pass through but allows relatively lower amount of resistance for the rays during the deoxygenated state of the blood making it easier for the opamp to distinguish between the beating heart pulses.
This is simply done by adjusting the given 470 ohm preset.
Keep your index finger tip over the D1/D2 pair, switch ON power and keep adjusting the preset until the LED at the output begins to develop a distinct flashing effect.
Seal of the preset once this is achieved.
Positioning arrangement of the index finger over the enclosed photo diodes
It may be done by soldering the diodes over the PCB at some calculated distance apart that becomes just good for the index finger tip to cover the radiating tips of diodes completely.
For an optimal response the diodes must be enclosed inside an appropriately sized opaque plastic pipes, as shown in the following figure:
In the next post we'll learn a simple heart rate monitor and alarm circuit specially designed for the elderly citizens for keeping a track of their heart critical rate.