DEMO: IR Heartbeat detector
Beau Schwabe
Posts: 6,566
Theory of Operation:
The two 2n3904's, 1M and 10k resistor form an auto balanced transistor bias
with an output at the 1M and 10k node. The RX in this design is reverse biased
and utilized as a capacitor which has a "leakage" proportional to the amount
of IR light that falls upon it resulting in a detectable signal. The 10u
serves as a DC block only allowing an AC signal to pass. The 1k, 2.2k, and
0.047u serve as the necessary components for a single pin Sigma Delta ADC.
The 330 Ohm resistor and TX provide the source IR for the RX detector.
Program:
This program maintains an ongoing average of the incoming data, and responds
when the incoming data is substantially different than the average data. A
maximum value with decay, meaning that it slowly migrates back to zero, is
maintained to auto calibrate to the incoming heartbeat pulses. Each pulse is
measured and the current beats per minute are displayed serially.
Enjoy!
Schematic:
Video:
https://www.youtube.com/watch?v=jGTHaZLhGjg
OBEX:
http://obex.parallax.com/object/698
The two 2n3904's, 1M and 10k resistor form an auto balanced transistor bias
with an output at the 1M and 10k node. The RX in this design is reverse biased
and utilized as a capacitor which has a "leakage" proportional to the amount
of IR light that falls upon it resulting in a detectable signal. The 10u
serves as a DC block only allowing an AC signal to pass. The 1k, 2.2k, and
0.047u serve as the necessary components for a single pin Sigma Delta ADC.
The 330 Ohm resistor and TX provide the source IR for the RX detector.
Program:
This program maintains an ongoing average of the incoming data, and responds
when the incoming data is substantially different than the average data. A
maximum value with decay, meaning that it slowly migrates back to zero, is
maintained to auto calibrate to the incoming heartbeat pulses. Each pulse is
measured and the current beats per minute are displayed serially.
Enjoy!
Schematic:
Video:
https://www.youtube.com/watch?v=jGTHaZLhGjg
OBEX:
http://obex.parallax.com/object/698
1024 x 680 - 68K
Comments
"I think it should work with the cheap Norcomp Pulse-Ox sensors" - Perhaps, the caveat is that the IR sensor is reverse biased and used as a capacitor. The detected IR proportionally controls the leakage of the capacitor resulting in an output signal.
Based on the schematic here, yes, it looks like it could be configured either way. I would be willing to bet that this sensor would also work just fine, but since I don't have one to actually test, that assumption is just speculation. Anyone want to try it and let me know? that would be great.
Reference:
http://learn.parallax.com/spo2-sensor-propeller-spin-demo