# DIY dro from fridge magnet and 3 sensors



## greenail (Aug 13, 2021)

The value of a DRO is amazingly high in terms of saving time but the cost of a DRO has always bothered me since they are very simple devices.  I tried and failed to make one few years ago, this time I had better luck.  I have a big role of the fridge magnet material so I can make them up to around 3' in length.  I wish I could find a cheap source for the linear scale magnetic tape.






I considered going deep into all of the details but I wasn't sure anyone would care.  If you are interested let me know and I can go deeper into how this works.


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## Bi11Hudson (Aug 14, 2021)

My background in the subject goes back a ways. (Old school) I have a couple of LVDTs some 30 inches long. A tube of 1/2 inch(m/l) diameter and a non-magnetic rod with a piece of iron on the end. Who said "digital" was the ultimate solution?

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## greenail (Aug 14, 2021)

Bi11Hudson said:


> My background in the subject goes back a ways. (Old school) I have a couple of LVDTs some 30 inches long. A tube of 1/2 inch(m/l) diameter and a non-magnetic rod with a piece of iron on the end. Who said "digital" was the ultimate solution?
> 
> .


It isn't digital, the TMR sensors produce a linear analog signal. Can you make a LVDT controller and trans/receiver for less than $7?  how accurate would a home  made version be?  I've always thought LVDT is interesting but I've not yet tried to make one.


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## hman (Aug 14, 2021)

Thanks for the video!  Yes, a fridge magnet isn't made to industrial standards, and a DRO probably uses a different type of sensor ... but you nicely illustrate the working principle.


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## Bi11Hudson (Aug 14, 2021)

greenail said:


> It isn't digital, the TMR sensors produce a linear analog signal. Can you make a LVDT controller and trans/receiver for less than $7?  how accurate would a home  made version be?  I've always thought LVDT is interesting but I've not yet tried to make one.


Having just admitted to a *serious* mistake on another thread, I question my current ability to fabricate much of anything. I could build such a controller from parts I have on hand if my  mind were up to it. The LVDT itself would be a handful on a good day. The term, LVDT, means a "Linear Variable Displacement Transformer". To wind such a device by hand would take considerable work. The finer the positioning measurement, the finer the wire necessary. The moving part is simply a magnetic slug on a non-magnetic rod. The transformer is the "bug-a-boo". To wind a "Tesla" coil would be simple compared to an LVDT. The electronics is simply a matter of measuring the difference between two signals. To make the electronics with purchased parts it would cost well over $20-25, at today's prices.

I had not intended to imply that an LVDT was "better", more a matter of questioning the "digital" solutions being the ultimate answer. Now you tell me that the measuring device is linear, only the numeric display is digital. I concede the point and will stand down. 

As an "old school" electrician, I was involved in the early transition from purely magnetic control to "first generation" PLCs for industrial control. At the time, there were no "digital scales" in that environment. We built our own with hydraulics and home brew (in house built) electronics and a digital volt meter. When a salesman came along with a commercial version, he had to tune the instrument to fit the accuracy of what we already had. His scales couldn't stand up to a "2 shift" (20 hrs) production schedule. 

In those days, a pressure transmitter was a modified guage with an LVDT attached where the needle was. The only true digital was in displays. I hold that analog still has its' place with controls. It just takes less smarts and money to make everything digital. There is some loss of accuracy but in most cases is not that important. Only in highly precision instruments does that difference come into play.

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## Gazz154 (Aug 16, 2021)

Great idea ! It would be nice to find some accurate magnetic tape but I like the idea of using a digital caliper as they have an o/p port and you can extend the pcb for  extra length.
Gary


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