Principles of the heart rate sensor with photo diode
and infraredlight LED:
The principle of measuring the heartbeat with a light source is based on the transmitted and absorption characteristics of oxygenated and de-oxygenated hemoglobin. Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through while deoxygenated hemoglobin absorbs more red light and allows more infrared light to pass through. Red light is in the 600-750 nm wavelength light band. Infrared light is in the 850-1000 nm wavelength light band.
- The first version of the heart rate sensor, I tested with the oscilloscoop with a light sensor (...) and an infrared light (...) (...) showed signals. Unfortunately it was not my heartbeat. It had been just very exceptional as it would operate at once... I decided to add an amplifier, which should enhance the signal. I decided to use the circuit previously found on the internet, because in that example, an amplifier was used.
This is the image of the circuit and this is the link: http://www.johnhenryshammer.com/TEChREF/opAmps/opamps.html
I have used the eagle software to enter the design in order to mill it out on the MODELA later on. The pictures
shows the results of the eagle-software.
nothing. I could not program the chip. Here came
1) I checked the soldered and the schematic again.
- The potential meter was connected wrong in the schematic
- I soldered the LM358 other way around. I learned more about the different packages of the components.
- I think I used a wrong capacitor. I probably had misread. Instead of 0.01 uF I used 0.1 uF
- At the end I found a missing trace of the voltage regulator. At first I didn't connect the board to the 9Volt battery, but to the external power supply to the header to set the board to 5 volts. I used a oscilloscope to measure its behaviour. Any movement changed the signal, but it was not my heartbeat.
I had a
presentation of the fab academy. Time pressure... I wanted
to show something
to what was my design. I decided
to make make
again. Moreover, you
Below is the
into five parts: The chip, the amplifier, the LED, the sensor and
The board of the chip include connections for the batteries, a voltage regulator and an external crystal and some capacitors and resistors.
The board of the amplifier include two potentiometers and two high pass filters.
The board of the LED include two LEDs and some resistors
The circuit board of the sensor and the infraroodled was made in a ring. I believe the ring would imply less movement. I used copper foil and the vinyl cutter to cut out the circuit. The picture above shows copper foil pasted in the ring and soldered components. This part of circuit include the first high pass filter.
Now debugging really came into play.
I learned more about active high pass filters. The reason ....
ligt in de versterker. vermoedelijk is het filter
juist. high pass filter is anders aangesloten en gaat meteen van in
naar de out.
high frequency laat hoge frequenties door. een hartslag geeft .... frequentie
gebruik pin 13 om een potmeter op aan te sluiten die de functie krijgt van als A REF (external voltage reference)
meet met de oscilloscope je hartslag en zet de weerstand van de potmeter op het voltage wat je maximaal hebt gekregen met de oscilloscope. zet in het programma AREF = 1voor de ATtiny 44
Vinyl cutting the traces in the copperfoil
1) Open eagle PNG document in Illustrator and then choose live trace and expand. You can still modify the traces a little. For example, I made the lines a little thicker.
2) Choose print from the illustrator menu: place the object in the lower left corner of the screen. Further operations of the vinyl cutter on the device itself. The width of your file appears in the display.
3) Lift the handle at the back and slide the copper foil under the wheels. The white marks indicate the possible position of the wheels.
3) Navigate through the menu Select roll. The force and speed I used for copper foil 1cm = 60g / s
4) Click OK (PC) to print
Copper foil fresh from the vinyl cutter.
Carefully removing the residual material using tweezers.
Ready to transfer the traces.....
with transfer foil.