Continuing the work with analog sensors on the Raspberry Pi, I decided to test the PulseSensor‘s behavior in building a pulse sensing application. The Pulse Sensor is a well-designed plug-and-play heart-rate sensor for Arduino but considering that it’s a simple analog sensor that sends values between 0 and 1023, it’s easy enough to use  with the MCP3008 ADC on the RPi.

Here’s the python code that can be used to make a simple pulse sensing application:

#!/usr/bin/env python
import time
import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
# read SPI data from MCP3008 chip, 8 possible adc's (0 thru 7)
def readadc(adcnum, clockpin, mosipin, misopin, cspin):
if ( (adcnum > 7) or (adcnum < 0) ):
return -1
GPIO.output(cspin, True)

GPIO.output(clockpin, False) # start clock low
GPIO.output(cspin, False) # bring CS low

commandout = adcnum
commandout |= 0x18 # start bit + single-ended bit
commandout <<= 3 # we only need to send 5 bits here
for i in range(5):
if (commandout & 0x80):
GPIO.output(mosipin, True)
else:
GPIO.output(mosipin, False)
commandout <<= 1
GPIO.output(clockpin, True)
GPIO.output(clockpin, False)

adcout = 0
# read in one empty bit, one null bit and 10 ADC bits
for i in range(12):
GPIO.output(clockpin, True)
GPIO.output(clockpin, False)
adcout <<= 1
if (GPIO.input(misopin)):
adcout |= 0x1

GPIO.output(cspin, True)

adcout >>= 1 # first bit is 'null' so drop it
return adcout
# change these as desired - they're the pins connected from the
# SPI port on the ADC to the Raspberry Pi</h1>
SPICLK = 18
SPIMISO = 23
SPIMOSI = 24
SPICS = 25

GPIO.setwarnings(False)
#set up the SPI interface pins
GPIO.setup(SPIMOSI, GPIO.OUT)
GPIO.setup(SPIMISO, GPIO.IN)
GPIO.setup(SPICLK, GPIO.OUT)
GPIO.setup(SPICS, GPIO.OUT)
#pulse sensor connected to adc #0
pulse_adc = 0
#Threshold for pulse sensing (half of values between 0-1023)
THRESH = 512
#pulse detection
pulse = False

while True:
#read the analog pin
analog_value = readadc(pulse_adc, SPICLK, SPIMOSI, SPIMISO, SPICS)
#draw the equivalent number of points in an attempt to draw a vertical pulse sensing graph
for i in range(analog_value / 100):
print ".",
#detect beats
if (analog_value > THRESH):
if (pulse == False):
pulse = True
print "Beat"
else:
print ""
else:
pulse = False
print ""
#hang out and do nothing for a tenth of a second
time.sleep(0.1)

The application simply processes the values received in the ADC’s channel #0 and if the value is greater than a certain threshold (in this case 512, considering that is half of 1024 values) it senses a beat. It also ignores subsequent beats above that threshold and it only considers a new beat once the values go beneath the threshold again.

Running this code should produce a similar output to this (depending on your heart beat):

. . .
. .
. . .
. . . . . . Beat
. . . . . . . . .
. . . . . . . . .
. .
. . .
. . .
. . .
. . . .
. . . . . . . . . Beat
. . . . . . . .
. .
. . .
. . .
. . .
. . . .
. . . . . . . . . Beat
. . . . . . . . .
. . .
. . .
. . .
. . .
. . .
. . . . . . . . . Beat
. . . . . . . . .
. . .
. . .
. . .
. . .
. . .
. . . . . . . . . Beat
. . . . . . . . .
. . .
. . .
. . .
. . .
. . .

You can also create a more interesting application by having the RPi send the raw pulse sensor data through some sort of channel (web server, sockets, serial, etc…) and graphically present the results on another computer.