Small/Tiny RPM Sensor

Still working on my RC project and the next challenge;

I want to monitor 6 driveshafts in the end. However I cannot find sensors small enough to do this. Working with IR and black/white paint wont work because of dirt.

Hall (magnetic) will be difficult because if I attach a magnet I will unbalance the shafts. I might be working with magnetic tape but 4 shafts are only 3 of 4 mm in diameter.

Shafts directly attached to a wheel might be done with magnetic tape inside the wheel I guess.

How would you guys solve this?

And do I need a pin for every sensor or is there a smart way with a shift register something?

How small is small enough?
What else is that shaft attached to? You may be able to sense the speed a bit further down the drive train.

I will post some pictures later so you have an idea

How exposed to dirt will it be?
Maybe you could give some details to what you are doing and what you are driving with the shafts?
What resolution do you need?

The first car I want to use is a Truggy (1/8). Dirt is wet grass, dust, gravel and sometimes mud. I know that I can waterproof a hall sensor.

What I want to achieve is:

Record RPM driveshafts from diff to front and back to see difference in rpm (so i can fine tune diff)
Record RPM front left and right to see differents in shock/spring setup
Record RPM rear left and right to see differents in shock/spring setup
Record overall speed and acceleration times.

Resolution, i dont know. as accurate as possible so I can draw out a graph in XLS against a time frame. RPM from the motor is max 50.000 which is approx 833 rpm per second. 1 rpm is about 1.2ms if I do the math quickly.

Later on I might want to implement this on a 1/10 onroad as well but that is a lot smaller to test and tinker with.

I guess hall sensing will be the way to go.
I would 3d print small rings to slip on your shaft. If its a 3mm shaft, make it 2mm thick and 6mm diameter. Add a 1x1mm neodym magnet on either side for balancing (from the inside!).
If the fit is tight enough you can just press it on the shaft. Half revolution resolution will easily be enough, and hall sensors will be fine with 50k rpm at 2 magnets.

Is the driveshaft open or enclosed in a tube?

Does the driveshaft have u-joints and are they open enough for a light beam to pass through and be interrupted by the sides of the u-joint?


Both Thanks.

@Paul, the driveshaft is open but does not have open joints. I will post some pictures tomorrow.