Hardware
The Heart Rate Hat is composed of
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​Mbed Microcontroller
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Adafruit Bluefruit LE UART Friend
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NTE74HC154 4-16 Line Decoder
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Optical Pulse Sensor
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330 ohm resistor
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Battery Power Supply (6V)
Optical Pulse Sensor
The pulse sensor uses an ambient light sensor and a green solid state lamp to measure the light reflecting off the user's temple. An on-board active filter and op amp clean up noise and boost the chip's output. The sensor's power is tied to the mbed's 3.3V VOUT pin and it's output is tied to the mbed's P15.
There are 15 LEDs in the hat (10 red, 5 green), each of which are tied together to a 330 ohm pull-up resistor. The LEDs' grounds are then tied to output pins Y0-Y14 on the 4-16 line decoder. The mbed output pins P21-P24 are wired to the decoder's select pins A0-A3. The enable pins E1-E2 on the decoder are tied to ground to disable the IC's multiplexing functionality. The decoder is attached to a small printed circuit board (PCB) via a soldered in plugboard. VCC, ground, and all other connections are soldered to this board.
LED ARRAY
The Bluefruit UART breakout board is wired VIN to 6V battery supply. CTS and GND are wired to ground. The board's serial pins (RXO, TXO) are wire to the mbed's own serial pins (P13, P14).
Bluefruit UART
Datasheets
Pulse Sensor
NTE74HC154 4-16 line decoder