Heart Rate 6 Click Board™

The photo elements inside the BH1790GLC sensor are located behind the IRCUT filter, which reduces the influence of the IR spectrum of the light. There is a green filter also, narrowing down the green light responsiveness even further, which helps to achieve accurate readings, even by using common green LEDs.

Two LEDs are driven by the BH1790GLC sensor, which provides a constant, programmable current. The choice of LEDs is not critical at all since the integrated light filters on the sensor allow only a narrow band of green light in the range from 520nm to 560nm with 0.8X reduction in respect to the centre frequency (about 540nm). The complete monolithic solution IC with the two constant current LED drivers, narrow pass-band for the green light spectrum, low-power consumption, and a high integration ratio that allows a very low number of external components, make the Heart Rate 6 Click Board™ a perfect solution for development of various wearable health-related devices, smartphones, tablets, and similar space-constrained applications.

Heart Rate Monitoring (HRM)

While the blood passes through the capillary blood vessels, they expand and dilate. Their light reflectance index changes accordingly. This is the basis of the photo-plethysmogram (PPM), a method used for the volumetric measurement of an organ, or in this case - blood vessels. The heart rate signal is calculated according to the changes of the reflected green light, sensed by the PD element. The Heart Rate 6 Click Board™ can provide the HRM readings by simply placing the index finger over the optical sensor.

How Does The Heart Rate 6 Click Board™ Work?

The Heart Rate 6 Click Board™ is equipped with the BH1790GLC, a monolithic integrated sensor for heart rate monitoring, from ROHM Semiconductor company. This IC is a highly integrated optical sensor, very well suited for performing PPM measurements. Due to the large integration scale of this sensor, as well as its low-power consumption, it is perfectly suited to be used on a wearable IoT device. However, being a Click Board™, Heart rate 6 Click Board™ allows easy evaluation and rapid application and firmware development.

Two green LEDs are driven by the integrated LED driving section of the BH1790GLC sensor, with the programmable pulsating frequency of 64Hz or 128Hz. The current through the LEDs can also be programmed in the range from 0 to 60 mA. Finally, there are two settings for the pulse duration: 0.3ms and 0.6ms. These two values affect the duty cycle of the LED pulses. Optimal readings can be achieved by balancing these three parameters: the current amount through the LED (brightness), the speed of the light pulses (LED frequency), and the pulse width (0.3ms or 0.6ms).

The reflected light burst is detected by a sensing element in a form of a photo-diode, sampled by a low noise 16-bit A/D converter. The photo-diode is located behind two light filters which pass only a narrow band of green light in the range from 520nm to 560nm, with 0.8X reduction in respect to the center frequency. The top filter is an IRCUT filter, that prevents the influence of the IR light, while the second filtering layer only passes the green light. This allows even broader color range LEDs to be used, reducing the overall cost of the design. However, Heart rate 6 Click Board™ uses the KingBright super bright clear green LEDs, with the spectral response that is closely matched to the passband properties of the optical filter. This allows most of the LED energy to be used, further improving the power consumption profile.

Two output registers contain the 16-bit measurement in a form of two 8-bit words. The upper and the lower 8-bit registers contain the measurement data, which can be retrieved over the standard I2C interface. The host MCU can read these registers in cycles of 1/32 sec, or 1/64 sec, depending on the BH1790GLC settings. The datasheet of the BH1790GLC contains the correct algorithms, which describe the measurement process with more details. However, the Click Board™ comes with the library which contains functions that allow measurements to be performed with minimum efforts.

The I2C pins of the BH1790GLC sensor are routed to the respective mikroBUS™ I2C pins. The I2C bus lines are already equipped with two pull-up resistors, which together with the two external LEDs are the only components required by the BH1790GLC sensor. Pullup resistors are connected to the 3.3V power rail so that the Heart Rate 6 Click Board™ can be used only with MCUs that use logic levels up to 3.3V for the communication.

SPECIFICATIONS

PINOUT DIAGRAM

This table shows how the pinout on the Heart Rate 6 Click Board™ corresponds to the pinout on the mikroBUS™ socket (the latter shown in the two middle columns).

Notes	Pin					Pin	Notes
NC	1	AN	PWM	16	NC
NC	2	RST	INT	15	NC
NC	3	CS	RX	14	NC
NC	4	SCK	TX	13	NC
NC	5	MISO	SCL	12	SCL	I2C Clock
NC	6	MOSI	SDA	11	SDA	I2C Data
Power supply	3V3	7	3.3V	5V	10	NC
Ground	GND	8	GND	GND	9	GND	Ground

ONBOARD SETTINGS AND INDICATORS