Introducing Easy Pulse: A DIY photoplethysmographic sensor for measuring heart rate

[snishanth512]

I did not understand how they designed the low-pass filter.

How voltage variation is sensed from the photo-transistor?

https://embedded-lab.com/blog/?p=5508

 

[diegobb]

Hi Nishanth !!
It's a classic (non inverting) integrator. For high frequencies the capacitor acts as an open circuit, while the resistor provides the needed feedback in DC to make it stable.

Best regards

 

[snishanth512]

How does the capacitor acts as an open circuit at high frequency?

I didn't get.

 

[diegobb]

How does the capacitor acts as an open circuit at high frequency?

I didn't get.

Hi Nishanth, the capacitor acts as an open circuits because it's capacitive reactance (Something like It's resistence in AC) depends of the frequency of the signal in AC: \[X_{c} = \frac{1}{w{\cdot}C}=\frac{1}{2{\cdot}{\pi}{\cdot}f{\cdot}C}\]. As the frequency gets high in the previous equation also the admitance of the capacitor gets high.

Best regards