My project sounds very similar, it provides biofeedback but is more target toward stress then depression. However, it still fits into the same mental health/wearable category. Check it out!

https://hackaday.io/project/6701-bioloop

Hi,

This is an interesting idea! You could also pull data from someone's browsing habits (choice of video on youtube shifts with mood) and possibly a webcam. How can we get involved? As someone who's quite good at spotting when I'm getting low, I could start keeping tabs on things like heartrate?

Hi mr.jp.swe. I would expect depression to reduce facial emotion expression (e.g. http://www.sciencedirect.com/science/article/pii/S0165178100001682) and using automated systems to detect changes in emotion expression may help with detecting downward swings. Wiring up people's houses with webcams for such a system probably would be more effortful than just using a range of other sensors to detect other aspects of 'downward swing'.

I am willing to bet that many people will find this statement somewhat grating "And finally why not deal with the problem?" Do see https://www.youtube.com/watch?v=XiCrniLQGYc

The "black dog" youtube indicated "truth liberates"...which is a very long journey..I'm not so sure about the sensor part. ( I guess you lose me when you give an impression of wanting to stay positive ).  I would prefer the word "balanced" ( as in the end of black dog )... a sensor that measure if I lose "balance" would be interesting. ( I I'm sick and tiered of society afraid of the serious and sometimes negative part ). A body that can't feel pain is on a very dangerous path.

I think a personal EEG + FFT to see equilibrium between brain halfs, combined with ancient knowledge from India ( like Yoga Sutras of Patanjali ) could be a start. My guess is the path is personal you can point out directions to others ... but you travel "alone". 

Monkeys meditate for marshmallows

http://www.newscientist.com/article/dn20989-monkeys-meditate-for-marshmallows.html#.VQ1Driy4pqE

Neurobiologists Find that Weak Electrical Fields in the Brain Help Neurons Fire Together

http://www.caltech.edu/article/13401

The sensor you will need is some kind of EEG ( not easy to apply just by
entering a room )....next is face expression analysis ( the truly sick with fanaticism is easily detected by signs, dress code, body language ). And finally why
not deal with the problem ?

What you want sounds like  "Somebody Else's Problem field" !?

http://hitchhikers.wikia.com/wiki/Somebody_Else%27s_Problem_field

I think in order to be "positive" you have to deal with the negative.... ( truth liberates ;-)

/JB

A society based on doublethink & SEP (somebody else problem) is sick...just my opinion