Hmmm..... blood group tester ?
I recall in the school days, some students put blood drop on some paper
(not litmus paper i guess) or maybe it was some chemical. I was afraid so never went close. but anyway the idea was that the color of the paper/chemical changed according to the blood group. U can build a color detector on top this idea and u will have a device to tell the blood group.
Thinking on similar lines, you can build a circuit for pH level detection
and use it for medical tests :idea: