Need some help finding a potentiometer

[727737Nut]

Hey Guys,
I have searched for a few hours for a special precision potentiometer that I need for my altimeter project.
It needs to meet the following specs,

5K resistance
20 turn (could be 25 turn)

1/8" shaft (1/4" could work)

7/8" to 1" body diameter

The only ones I could find where 3"+ in diameter!  Yikes.

Thanks for any input,
Rob

[jonesthesoftware]

Hi Rob
don't make life difficult for yourself. 10 turn pots 1/4 shaft are standard and easy to come by so think along the lines of 2:1 gears.
http://www.potentiometers.com/select_multi.cfm
http://www.precisionsales.com/potentiometers/multiturn/MW10B-wirewound.htm

Why do you need a special precision pot? The electronic card that you connect it to will define how precise a measurement you can make e.g 10 bit analog input or 12 bit.
10 bits is 1024 so for a 5K pot thats a precision of 4.8 ohms or 12 bit= 1.22 ohms using standard pots
Maybe you could use an encoder instead.... continuous rotation and direction detection built in
hope this helps
if not give us some more info on what you are trying to achieve and maybe the combined brains on this site can solve your problems
kind regards
geoff

[727737Nut]

I have a real Altimeter that I gutted out the air chamber parts and am know trying to retrofit the guts of another gauge inside it. It uses a geared dc motor to turn the pointer shaft. The original pot turned 1 turn for each 360deg needle movement.  I am building a 0ft to 20K ft altimeter. Hence the need for a 20t pot.  Each revolution would = 1000ft.  Theses gears are very tiny and would require precision drilling to align a different set-up.  I was trying to replace the original 1 turn pot with a 20 turn then it would be a cakewalk to interface.  I planned on using a gutted servo and drive it that way. Worked well on the real VSI gauge I interfaced.  If I go with a 10 turn then I would need to find a way to mesh a new larger gear so each 1000ft only turns the altimeter a half rotation.  Doable but not as easy.
Rob

[fsedano]

Hi,

Might be easier use same technique real servoed altimeters use: one coarse pot, which will turn once from 0 to max, and one fine, which will turn once every 5k ft.

[Perik]

Hello Rob

Edoardo has converted a real Altimeter which seems to work great.
http://b767sim.blogspot.com/2010/12/altimeter.html

A video as well:
Cockpit initial state

Cheers
Per-Erik

[Edoradar]

Hi,

Might be easier use same technique real servoed altimeters use: one coarse pot, which will turn once from 0 to max, and one fine, which will turn once every 5k ft.

The problem is that while the resolver gives values for 360 phisical degrees, a pot give only for 340 electrical degrees that make a 20 degrees with no readings in...It woulded be my aim but till there is no interface to give back a resolver signal, the way I want to is to replace the other one that makes the half turn for the whole range (bouught over ebay and manufactured by Helipot, if you look for this brand over ebay I am sure you'll find a suitable one  ) and linked to a leo bodnar card so at least it has a 12 bit resolution givin back a precision of more or less 25 ft. Not perfect but good enough to use it...

[727737Nut]

I went with a 10T precision pot, which i geared so every 1K ft gives me a half turn. That should allow me to have my 0-20K altimeter with usable resolution.  Will post some pics soon.
Rob

[AK Mongo]

Rob,

Are you using the pot for servo feed back?  I would be very interested in how you are doing this. 

Reid

[727737Nut]

Rob,

Are you using the pot for servo feed back?  I would be very interested in how you are doing this. 

Reid

Hey Reid,
That is correct, the Pot is the feedback which I have wired into a gutted servo.  Motor is this nice little gem from Servocity http://servocity.com/html/90_rpm_micro_gearmotorblocks.html  Gears are from there as well.  Let me see If i can get a pic up for you.
Rob

[727737Nut]

Here a couple of pictures for you. It is now all together and in my sim working like a champ.  Coded with SIOC of course.  In the little wood plate I made there is bearing that the shaft slides through for support on the bottom of shaft.  Goes through original bore on top.

Rob

[727737Nut]

Here is a pic of the altimeter taken today in action.

[AK Mongo]

Rob,

I am guessing that we are looking at the back end of the gauge?  How are you running the pointers?  are they already correctly geared somehow off the brass looking shaft?

Is this thing adjustable for pressure off the knob on front?  If so, how did you accomplish that?

Reid

[727737Nut]

All I did is replace the 'bellows and internal connections that went to the gear that drove the upper needle gearpack' and install the shaft and feedback pot.  The gearing for the needles and such is located in another part that sits on top of the silver geared disk you see in the photo.  That remains all original and works as normal including the Baro adjustment.  Basically, buy a standard GA altimeter off of EBAY for 25-35.00   gut the internals below upper gear pack and replace with what you see in my pic's.

Rob

[iwik]

Hi Rob,
Very nicely done, simply awesome. Have you thought of doing a more in depth article on converting such a gauge. As this one seems the more
compilcated of all the gauges to convert. I think many a GA builder for love
a few more pics and a little more detail on how it was done.
You are an inspiration to us all. Even just afew more pics,maybe of the top
would help.
Thanks
Les

[727737Nut]

Hi Rob,
Very nicely done, simply awesome. Have you thought of doing a more in depth article on converting such a gauge. As this one seems the more
compilcated of all the gauges to convert. I think many a GA builder for love
a few more pics and a little more detail on how it was done.
You are an inspiration to us all. Even just afew more pics,maybe of the top
would help.
Thanks
Les

It actually wasn't bad at all. All the gearing and pointers were left untouched. All you have to do is connect the inner brass shaft (see Mike Powell's books) to the original gear which I used epoxy.  The rest is shown in the pictures.   I still think a 20 turn pot would be better but they are very expensive, over 100.00!  As it was the pot I used was almost 20.00.  A gutted $8 hitec servo, another $10 in gears and a small geared motor is all you need.  I would be more than willing to help someone or even build one for someone for a small fee.  I still need to work on my SIOC code as for whatever reason unknown to me, there is a small difference in steps between some altitude's.  Could also be the cheap servo board.  May upgrade to a digital model??

Rob

[AK Mongo]

That is incredible stuff, Rob.  Thanks for sharing it.

Reid

[bnepethomas]

Awesome work, converting an altimeter is on the bucket list, nice to see it can be done.

Cheers

Peter

[AnneCourtois]

Try Spectrol products. I have used them before in different projects. Top quality, industrial duty but the price reflects that.
http://www.digikey.com/scripts/dksearch/dksus.dll?FV=fff40004%2Cfff80335%2Cfffc02b5&vendor=0&mnonly=0&newproducts=0&ptm=0&fid=0&quantity=0&chp=0&PV1175=16