Hi all, I'm a third year Computer Science student & I've decided for my third year project I want to do something with pulse oximetry. I'm trying to build a pulse sensor and then do something with the results such as build graphs and analyse real time. Although I'm more concerned about the programming of software I'm also interested in trying to build a device like this.

I've got no background in electronics but thought it might be fun.

I'd been following the schematic which is attached and I've been able to ensure it works (kind of) by checking the serial monitor from my Arduino interface. The problem is, when I try to place my finger between the LED and LDR, the LDR is not absorbing any light and I'm unsure why.

Here is the tutorial I've been following:
http://www.instructables.com/id/Homebrew-Arduino-Pulse-Monitor-Visualize-Your-Hear/?ALLSTEPS

 

Generally speaking, that receiver circuit doesn't seem sensitive enough for this application. An LDR usually has a fairly high off resistance, meaning it takes a lot of light to make it pass enough current to turn on Q1. Note that the MAKE article referred to uses a matched infrared emitter and photodiode. Also, that circuit is not as sensitive to the absolute light level as this circuit is; it detects changes in light level.

Instructables projects usually get bad reviews around here. I recommend abandoning it and going with the MAKE article for hardware guidance.

ak

 

Generally speaking, that receiver circuit doesn't seem sensitive enough for this application. An LDR usually has a fairly high off resistance, meaning it takes a lot of light to make it pass enough current to turn on Q1. Note that the MAKE article referred to uses a matched infrared emitter and photodiode. Also, that circuit is not as sensitive to the absolute light level as this circuit is; it detects changes in light level.

Instructables projects usually get bad reviews around here. I recommend abandoning it and going with the MAKE article for hardware guidance.

ak

Thanks for replying I got a little deterred from the MAKE guide as I seen some of the comments saying that the circuit does not work, any thoughts?

 

I checked a disposable pulse rate module, a red LED & a small Si photo cell. Using another red LED & 1.5 V scale VOM, I could barley detect my heart beat, maybe 10 mV. Maybe an AC coupled amplifier with a gain of 1000 might give results.

 

? the circuit uses a positive feedback - that means - if you tune it wrong

1. it won't work at all → not enough . . . whatever there's not enough
2. it will work similar to provided example
3. it will oscillate (too strong feeback) and thus the results are not credible

depending on transistors and LDR you use your R1 R3 values may differ a lot from what is shown on a schematic
small about : http://www.electronics-tutorials.ws/io/io_4.html

 

? the circuit uses a positive feedback

Don't think so. D1 is an indicator that the LDR signal is within the detection range of the circuit. It is not the illuminator for the LDR.

ak