Hello there, can anyone help? Here are three sections, here as follows.
1. A 15V supply (for testing will be 9V for battery use) supplies a resistor (1K2) and a bright red led (1.2V across) and connected to ground.
2. The red light triggers a phototransistor which is in series with a resistor (100k).
3. The output current rises and voltage drops (oscilloscope shows ideally a 0V to 15V drop which is what I want, scale on oscilloscope is 5V per square).
4. The signal on a digital oscilloscope show and output once my finger is placed inbetween the red led and the phototransistor (in close proximity i.e. close peg with drilled holes).
5. My altering blood flow allows light intercepted by phototransistor to vary i.e. current rises and falls.
6. The output signal is sinusoidal with noise, it calculates at 1.25Hz which is like 800ms i.e. about 1.25 x 60s is like 75beats per minute.
7. The high frequencies can be ridden by a passive low pass filter before the signal is intercepted by a schmitt trigger for a pic to pic up.
Now I am finding it hard to design the low pass filter, I have the noise at 20mV so do I now need to calculate the signal and obtain a signal to noise ratio in order to calculate the values of resistance and capacitance for the Low pass filter?
It sounds easy but that part is doing my head in ha, if you can help that would be great or perhaps give a better alternative like an IMGF filter.
Using a bandpass filter combing high pass filter and a low passfilter may help rid some noise for a nice signal to be intercepted by PIC microcontroller.