Magnets on a spinning thing near a hall effect latching switch chip make pulses that a counter counts to count the turns the spinning thing spun.
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Step 1: Latching Hall Switch?
Sorry, this is not a very complete instructable. Just that all these coils reminded me of what I used for a turns counter when I wound some coils.
A latching hall effect switch turns ON when it sees one pole of a magnet.
Pull the magnet away and it stays ON.
When it sees the other pole of the magnet, it turns OFF.
And stays OFF untill it sees the first pole again.
More info at:
Step 2: So What?
A normal (momentary) switch would give a short blip as the magnet goes by.
But a latch would give a nice square wave as the alternating magnet poles go by.
Less false switching and like that.
Step 3: But...
I had problems with noise from the drill motor. Not only did the crusty old drill emit grinding noises and burning electric smells, but it was spraying out interference that was being picked up by the long unshielded cable.
An RC filter at the counter helped some and it worked good enough for what I was doing. But shielded cable and a non-half-dead drill motor would be the way to go.
R1 is a pull up resistor.
R2 and C1 are a filter.
Sorry, I don't remember the values. Maybe start with 10k ohms and .01uF.