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Heart rate monitor with programmable SoC

14 Oct 2013  | Asha Ganesan, AnandaGanesh M S

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Photoplethysmography is of two types – transmission and reflection – using IR waves based on the place from which the light sensed.

Type-I: IR reflection method
Many IR transceiver chips with an IR LED and photo-diode/photo-transistor are available in the market specifically designed for heart rate monitoring systems, where the conduction of the photodetector varies depending on the amount of light reflected back on it.

Let us assume that the IR LED is excited by a constant source. When the blood volume at the position where the IR transceiver is placed changes, the amount of light reflected back also changes. The change in output of this optical transceiver translates the heartbeat into the electrical domain, which then has to go through a signal conditioning process. Finally, digital logic is needed so as to count the number of pulses per minute.

Figure 1: IR reflection method.

Type-II: IR transmission method
The IR reflection method is often the best one to choose when a finger tip is chosen as the source of heartbeat measurement. However, it is tough to place a similar device on the earlobe. In this case, one side of an earlobe clip contains the IR LED, while the other side of the clip contains the photodetector. When the blood volume increases, the amount of light received by the photo-transistor decreases (i.e., the reverse behaviour of the reflection method).

Figure 2: IR transmission method.

Design requirementsThe IR transmitter consists of a cleanly powered IR LED.
The IR receiver comprises a phototransistor whose base to emitter voltage (Vbe) changes depending on the light falling on it. To detect a change in Vbe, the collector of the phototransistor is pulled to 5V through a resistor.

Figure 3: IR transmitter/ receiver.

 • Since the change in the IR receiver output per change in volume of blood flow is very small (on the order of 50-70µV, depending upon the diode-transistor pair used), there is a need to amplify the signal so as to bring it up to a measurable voltage range. Hence the amplifier gain has to be on the order of 50,000 to bring the voltage into a measurable range.
 • There are many possible noise sources to be considered when designing such equipment, a few of which are: measurement (or body contact) noise, electromyogram (EMG) noise (muscle contraction), and movement artefacts (common while doing physical activities). These noises from high frequency sources have to be eliminated using a 1st or 2nd-order low-pass filter.
 • Obtaining a gain of 50,000 typically requires cascading two amplifiers (e.g., of gains 250 and 200). Thus two op-amps can be used to design a 2nd order low-pass filter with an overall gain of 50,000.
Finally, to generate a square wave train so as to count the number of pulses, we need to feed the output of the two-stage amplifier to a comparator with a suitable threshold. Note that the threshold depends on the IR transmitter and receiver used.

Now, the comparator generates a series of pulses of the same period as our heart beat. We need to feed this output to a digital block or an MCU (microcontroller unit) to count the number of pulses per minute and thus display the resultant output on an LCD. Also, the MCU has to store personal data like height and weight to be able to perform calories-burned calculations.

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