RPM Meter

Dookie · 2010-07-28T01:30:29+00:00

I have a contact/photo rpm meter but it is acting funky and would liketo create a simple meter for some testing.I've created the pic code and seem to have gotten it to work but needsome sensor to debug it. The pic code basically counts the timebetween pulses. I do not use the ADC but rather the digital inputs andmonitor when the pin goes high. The code does have built in dead timeto prevent multiple countings.My idea was to stick a magnet on the device I want to measure therpms(flywheel) and use a coil of wire as the sensor.The problem is that I'm getting much lower voltage than I expected.Faraday's law says the voltage on the coil is the number of turnstimes the change in flux. The change in flux depends on the rpm's butI'm simply using my hand to move the magnet across the coil. I getsomething like a sinusoid damped(or alternatively the derivative ofthe erf function). I have about 200-300 turns and the coil is gettingunwieldy.This is fine but it most I get about 30mV(surely to be much largerwith the flywheel at full speed). What I'm thinking of doing is simplyusing an opamp or bjt to act as a switch/comparator but the onlyproblem I see is that the noise may add extra pulses.Is there a simple way I can increase the noise floor, say, byoffseting the ground of the op amp and effectively setting making dcnegative relative to the opamp. I'm thinking that I could add aresistor to the -V of the op amp. I only have a + supply.Or is there any better way that doesn't involve to much work? Itdoesn't have to be completely perfect as I am taking an average forthe rpm's and can through out outliers. The main thing is to get theinput to the pic to act digitally.I know there are a god awefull number of ways to do this but somethingsimple with minimum and common parts is what I'm looking for.

RPM Meter

Jul 28, 2010 102 Replies

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markp 15 years ago

Yes dear, you keep taking them pills. Your condition will soon clear up I'm sure.

BTW, when John talked of charge he, like most electrical engineers, was talking about 'charge separation', which is *not* net charge and cannot be used in calculations regarding charge conservation. It is convention when talking about 'charge on a capacitor' that this really means 'charge separated on a capacitor'. I too initially thought he was talking about the latter, but realised he wasn't and apologised accordingly. I fully understand that charge is conserved, I don't think you can point to a single post where I didn't.

Formal definition of capacitance taken from

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: "The capacitance (C) of an electrostatic system is the ratio of the quantity of charge separated (Q) to the potential difference applied (V)."

So every time you mention conservation of charge in attacking others you should understand that it is you who has made an incorrect assumption about what they are really saying.

Mark.

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markp 15 years ago

Actually I not going to let that one go. There was no intention to make a threat (directly or implied) for the reasons given above, and I have taken some offence to the suggestion that it was.

By your logic, you have perceived a threat are therefore one of those who I 'never know how' might react, so maybe I should assume you are threatening me?

You have to be careful making accusations that are not true Grant. And, just to be clear, no threat is intended or implied by this post.

Mark.

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John Larkin 15 years ago

So what electronic guys call "charge" the physicists call "charge separation." Both seem to agree that the units are coulombs, namely the charge on one plate, equal to the time integral of applied current (and not twice that.)

Peace in our time? Or do we want to argue over whether a capacitor is an "electrostatic system"?

John

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