Mock up testing of Sensors

Several things bugged me.

1. Do the sensors work if powered without the base device

2. Will my extension connectors work i.e. soldered correctly, no significant leaks

3. What signals will return from the +sig and -sig outputs

4. What scale are the return voltages

I used a battery voltage supply of 12V as it was what i had lying around and guessed a swing of 6V with respect to ground wasn't too much more then 5V.... I then powered the sensors and just used a multimeter to read what returned from sensors. The respiratory sensor uses stretch sensors to change a resistance, therefore it works with relative voltage changes when the band is expanded. As it requires no calibration it works pretty well.

The BVP device looks as if it needs a little more calibration and care with what supply voltages it is given. The sensor returned voltages that did move between 120mV and 160mV in a pattern that did look as if it followed a typical heart rate. This needs clarification by using faster signal sampling. This would be provided by the Arduino. It was encouraging however to see that if you removed the sensor off the finger it stopped returning voltages out its signal channels.

Scale

Is a relative change of 20-40mV from the BVP enough to work with bearing in mind the Arduino's input pin resolution. The maths (5/1023) suggests a 4mV change can be detected however when you begin working within Max small changes like this are unstable and scaling them doesn't tend to yield great results.

It would be favourable to remove the need for any gain stages after signals are returned from the sensor however this may be inevitable. This would open up a whole new chapter of signal conditioning and require low noise op amps and a cleaner power supply. We will see!