I am working on a university project at the moment. Creating a pulse oximeter by using a red and an infrared led to shine on a phototransistor coupled common emitter. The result is that when no finger is present = ~ 0V on the output When finger is present =~ 4.9V oscillating with +/-200mV.
This signal (the pulse) I need to amplify and offset as input to my ADC/Microcontroller. I have limited space to the circuitboard (40x40x30mm). Looking a calculations for RL or RC circuits result in physically large components which is not an option.
Question: How can I design a Highpass or Bandpass filter with common uni components in this reduced space? My primary goal is to remove the DC component for easier offset and amplification.
Question: Can I use electrolyte or tantal capacitors for this?
Edit: Signal is heartrate in the range of 2/3Hz to 5Hz. Edit: Since the microcontroller only accepts analog input in the range 0-5V I want to cut away the dc components before the op amp so I can use the 10 bit ADC fully
Description: LED1 is the phototransistor, C1 and R6 form the highpass filter, R2 is voltage offset. Non-inverting Op amp with gain of (1+((R4+R5)/R7) = 150. C2 and R7 form a similar highpass filter.
What I was hoping for: Output of Phototransistor when fully illuminated ~ 2.5V due to the voltage divider of R2+R6. Output of phototransistor when finger is detected => 2.5 +/- AC signal of heartrate/pulse between 0.67 and 5Hz roughly.
Op amp's job = amplify the AC (~200mV peak to peak) to 2V peak to peak. Since the Op amp serves as input for the ADC/Atmega microcontroller, is impedance still an issue? And why is this not working as intended? Output has massive noise from 10Hz to 3kHz. and voltage when "no signal" is not sitting at 2.5V as hoped
Edit:I was able to find the components needed to create a highpass filter, even a ceramic capacitor since the electrolyte ones as I read had other issues.
Testing the parts separately (Sensor I/O), Highpass filter and amplification circuit worked, although together did not show the desired results.
Going to rework a simple solution which will result in less bandwidth of the ADC input.