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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. schematic

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  • \$\begingroup\$ Why don't you contact them for clarification? \$\endgroup\$
    – Andy aka
    Aug 15, 2016 at 7:17
  • \$\begingroup\$ There are no components marked 100 Ω or 47 µF in your schematic. I don't think we can help. \$\endgroup\$
    – Transistor
    Aug 15, 2016 at 7:30
  • \$\begingroup\$ @Transistor I mean. What kind of filter is formed by R2, C2, C1, C3 and C4 \$\endgroup\$
    – oziomajnr
    Aug 15, 2016 at 7:39
  • \$\begingroup\$ Well edit your question and ask that! \$\endgroup\$
    – Transistor
    Aug 15, 2016 at 9:46

1 Answer 1

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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).

schematic

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:-

enter image description here

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