Tacometer from mains voltage

[miles_hot]

I want to have a true shaft revs read out on my lathe. I have some memory of this being done on kit cars etc by having a magnet attached to the shaft and some sensor being placed neaby to count the pulses - one pulse = 1 rev. I expect that I can sort this out on a 12v supply but does anyone know how it can be done on a mains supply?

Any help with pointers to kit would be greatfully received!

Many thanks

Miles

 

[cambournepete]

Don't take this the wrong way, but why not just use a transformer etc to reduce the mains voltage to 12v (or even a battery)...?

 

[CHJ]

Miles, this one of the best links I know for info. I use just the discs to calibrate my Invertor Control pot. as I don't need accuracy within the odd few tens of revs for my wood turning.

 

[OldWood]

Miles and CHJ - that's a useful link. Two ideas, both devices I've got off Ebay some months back and it's reasonable to assume they are still both available.

As CHJ implies, and doesn't actually say it ( :roll: ), who needs a really accurate measure of the speed. There's nothing wrong with a table on the wall showing the RPM against the dial of whatever is setting the speed. To measure the RPM initially somewhere on Ebay you will find RPM meters for a tenner or less; perform adequately and a useful tool to have for other motor applications.

The link from CHJ includes a circuit at the end for a digital readout using the frequency range of a DVM - that of course just locks the DVM into that application. Again from Ebay I got a small LCD readout module that does the frequency measurement for you at again well under a tenner, and that is therefore your RPM meter and can be built in.

Rob

 

[miles_hot]

Don't take this the wrong way, but why not just use a transformer etc to reduce the mains voltage to 12v (or even a battery)...?

Pete
I'm open to all sorts of ideas The reasons that I didn't immediately go for this method was 1) attempting to minimise the number of boxes involved and 2) uncertainty as to what sort of transformer to go for etc
Miles

 

[miles_hot]

Miles and CHJ - that's a useful link. Two ideas, both devices I've got off Ebay some months back and it's reasonable to assume they are still both available.

As CHJ implies, and doesn't actually say it ( :roll: ), who needs a really accurate measure of the speed. There's nothing wrong with a table on the wall showing the RPM against the dial of whatever is setting the speed. To measure the RPM initially somewhere on Ebay you will find RPM meters for a tenner or less; perform adequately and a useful tool to have for other motor applications.

The link from CHJ includes a circuit at the end for a digital readout using the frequency range of a DVM - that of course just locks the DVM into that application. Again from Ebay I got a small LCD readout module that does the frequency measurement for you at again well under a tenner, and that is therefore your RPM meter and can be built in.

Rob

Oldwood - the thing with ebay is knowing what to search for! Can you give me any more details on the stuff that you used? I read the article from Chas and it looked very good but I must admit that it has been many, many years since o level physics and I started to gibber a little at the thought of deconstructing a DVM!
I was hoping to find something which had a sensor I could position near a magnet inserted into the shaft of the graduate which would pick up the pulse (or looking at the article something which looks at a black mark) and just counts the pulses/min = RPM. I'm sure it's possible I'm just not sure what to look for

Miles

 

[miles_hot]

Miles, this one of the best links I know for info. I use just the discs to calibrate my Invertor Control pot. as I don't need accuracy within the odd few tens of revs for my wood turning.

Chas
Thanks for the link - the disks will be very useful and I agree there is no need for absolute accuracy when wood turning however if it's worth doing... It would be nice to have a digital read out and if I manage it that would be more "elegant" than some pencil marks on my nice new lathe Assuming of course that IU don't end up with a mess of cables and frankenstein bits hanging out of my lathe
Miles

 

[CHJ]

Miles, I would not get too hung up on gold plating your lathe, better some pencil marks on the lathe and and concentrate the brain power and effort on spinning bits of wood. That's what the machine was designed for, no two bits of wood are going to be optimum as far as cutting speed required and personal judgement and feel are far more relevant than the spindle speed.

I for one am happier looking at what a person produces than ever I worry about how shiny their equipment is, some of the most highly skilled craftsmen I have met in my life produced their pieces using very basic and in most cases home made tooling and home built basic machines.

 

[cambournepete]

...some of the most highly skilled craftsmen I have met in my life produced their pieces using very basic and in most cases home made tooling and home built basic machines.

And others have all the gear and no idea (as a man I beat at golf said to me last year)