Re: Cheap tach - was r.p.m. kit??? / 4 digit under $20

Oh, I am not suggesting this as a replacement for a proper circuit and
readout, just thought I'd mention it for those odd times when you
could use some kind of a tach for whatever reason. I would use a
circut like yours if I needed a permanent display on a machine.
Dean

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Hans Wedemeyer <xyz@h...> wrote:
> Yes it's a simple idea.... but nothing like looking at some digits
and not having
> to bother with coupling to a moving parts and the danger associated
with it.
> Fingers and eyes can be difficult to replace.... ! Now if you make
it a permanent fixture
> sure it's great...
> I'd say the idea has been around about 100+ years ! :-)
> The eddy current driven version has been around almost as long, in
most cars it known as the speedometer !
> If you goto a car junk yard you may even be able to find one of the
older eddy current version speedo's and
> adapt it directly to the mill.... just a thought... give me LED's
and LCD's any day... ! :-)
>
> Best Regards
> Hans W
>
>
>
> "deanc500 " wrote:
>
> > Hi, I just tried one of the suggestions on another group regarding
> > this subject. I was intrigued by the simplicity of the idea and the
> > fellow mentioned that this is an old trick thats been around a while.
> > You just need any small DC motor, a potentiometer, and a volt meter.
> > The first potentiometer that I found in the junk drawer was a 50K
> > although other values will work. I hooked it up as in this picture.
> >
> > http://www.members.shaw.ca/axxus4/cheaptach.jpg
> >
> > The motor is connected across the pot and the meter is connected from
> > the center of the pot to one side. To calibrate, spin the motor in the
> > chuck, and adjust the pot to read your known rpm (in mV or whatever).
> > I was spinning it at 1460rpm in my mill drill. I adjusted the pot to
> > read about 1.460V on the 20V scale. I walked over to the drill press,
> > and spun the motor. I read 0.354V. I was amazed to see that the drill
> > press was set to 360rpm. Very neat trick. I will use this method to
> > measure servo and stepper rpm.
> >
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