What is the cut off frequency of the RC filter in this circuit

Question

I have the circuit of a heartbeat sensor module from pulsesensor.com but I can't seem to understand the kind of filter in the schematic. They Said it contains a low pass RC filter with R = 100 ohm and C = 47uF. But from my analysis I see a hpf. What kind of filter is formed by R2, C2, C1, C3 and C4. what is the cut off frequency of the filter.

Answer 1

They Said it contains a low pass RC filter with R = 100 ohm and C = 47uF.

No, they said:-

we designed a fairly universal Low Pass Filter for the output (passive RC. R: 100 C: 4.7uF)

but...

We made some changes to the original Pulse Sensor circuit

There are two filter stages in this circuit. The first is a passive low-pass filter formed by R2 and C2/C1. It has a cutoff frequency of ~5.64Hz and stop-band attenuation of 20dB per decade. However the load impedance in combination with R2 and C2 will form a high pass filter, so the practical result is a band-pass filter.

(Note: the 14K resistor in the simulation below is a simplification of the complex load impedance in your circuit).

^{simulate this circuit – Schematic created using CircuitLab}

The second stage (C3, C4, U2 etc.) is an active band-pass filter with gain. In isolation it would have 47dB gain at a center frequency of about 2.3Hz, but with the two stages combined the overall result is 37.5dB gain at a center frequency of about 2.6Hz.

Analyzing a multistage filter circuit like this is not easy, due to the complex interaction between stages. Rather than trying to do the calculations by hand I simulated your circuit in LTspice. Here's the schematic:-