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Repair / hack STW sensor.
#1
   
Yes i used to be a hand model
   
I am attempting to repair a old STW sensor from my boat. i thought it had a magnetic switch as when i measure the paddle spinning i would get beeps out of my meter in diode mode.
The paddle has 2 magnets in it with opposing poles  and is mounted in a though hull fitting.
with a black and red wire leaving the though hull.
today i decide to test for ac. after looking at the signet site when i spin it i get an ac voltage.
I assume the frequency of the signal equals the rpm of the paddles * 2
I don't have a scope.
Resistance is about 1.8K
flicking it with my finger i get about 10V AC max
How should I read this device?
What is inside the though hull a coil?
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#2
Is the voltage related to how fast it spins?

You might use a diodes to convert this to a digital signal within the range an uC can sample it.
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#3
I was thinking of a bridge rectifier and a voltage divider. That would give me 2 pulses per rotation. I am concerned about the voltage drop when it slows. I don't know how low before a esp would fail to detect the pulse. i could then just find the time between pulses to determine the speed. a square wave would be best. I don't have a oscilloscope so this is all guess work.
Signet claims there sensors are self powered.. I doubt i could get enough current to power a nano. i think for a esp i needs in the .08 amp range. It would be nice to have self powered sensors. The old analogue display is on the boat i think i am going to have a peek. it is -7 today and -25 tomorrow and -stupid till next week. May be if it is calm i will get brave and go to the boat.
I think i need a Radioisotope piezoelectric generator to solve my energy issues on the boat. You can get 80ish years before you have to refuel.

The other thought is the adc if the voltage is linear or reasonable so.
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