How to Convert a real Cessna Altimeter for Sim use.

I recently purchased a Cessna Altimeter off of Ebay for less than 20.00. (they have several more) I gutted the insides below the main gear plate. The main gear plate holds all the gearing for the pointers and the pointer mounts. This means all you have to do is have one shaft going to the original to make the altimeter work. So, use the methods found in Mike Powell's Building Gauges for Flight Sim's, I went forward with this little project. My goal was a servo based 0 to 20,000ft altimeter. Each turn of the main shaft gives you a 1000ft on the altimeter. I wanted a 20 turn pot for the feedback but found they were to cost prohibitive. I instead opted for the much cheaper 10turn 5K pot. So now I have to gear it so it turns only 180 degree's for each 360 deg turn of the main shaft to keep my 0 to 20k ft. I put a 14t on main shaft and a 28t on the pot. For the motor, i used a nice little geared motor from www.Servocity.com, It runs on 5-12vdc and turns 40rpm with loads of torque, way more than needed here. I have a 12t pinion on it and it drives a 55t gear on the main shaft. (see pics) I gutted a cheap Hitec servo, soldered the pot in place of the servo's pot and did the same with the motor. Voila, a servo driven real GA Altimeter. Works well, very smooth and quiet. This was actually much easier than anticipated. Using Mike's methods for making the shaft and how to build it up for the gears worked great. I could have had a fancy metal plate made for the bottom support but to svae time and money, I used some hobby grade 1/8" light ply. Not pretty but it worked and was cheap. I have a total of about 80.00 in this including the gauge. Any questions feel free to ask.

Rob

Hi Rob,
Thanks for the detail. Can you give servo citys part numbers usued for all gears.
Regards
Les

Here are a few of the parts you will need. You also will need a servo to gut to use the control board, a precision 5K 10turn pot, assorted brass tubing sizes, and a good magnifying glass! lol
Rob

ServoCity.com parts
SPBD48-23-18
SPBD48-24-36
SPBD48-24-54
KPC48-32-15
MGB-250

Thanks Rob,
Have you got a resolution to the slight altitude errors that you were getting?. Are you driving this from O/C servo card or something with greater resolution?.

Les

I am using the 10bit OC usb servo card. I think it may be more in the servo board than anything else. I am going to buy a digital servo and gut it and see what happens. If I command the servo to a given altitude it drives the altimeter to dead-on the commanded altitude. If I let SIOC drive it, it commands the correct servo output bu the servo sometimes stops 100-200ft off. Then if manually command it through IOCP, it goes right to it. Intermittent at best but I really think it's the cheap servo pcb. Any deadband shows up really bad do to the number of turns in the servo compared to needle movement. With a digital servo you have pretty much 0 deadband area and that would work much better here.
Rob

Rob,
What actual Altimeter did you use. Ive looked at mine and unfortunately cant seem to reconcile your inner workings with my one. I have a Cessna and it only seems like half the length of yours.
What was its model number and do you have any pic of before your mods?.
Les

Mine is an Encoding Altimeter so that could explain the extra length? Here is the one I have and the seller has many more he said. http://www.ebay.com/itm/200696719837...84.m1439.l2649

Rob

Rob,
Thanks for the link. I have gutted my one, all i have now is the gear plate,if this is removed
im left with a spindle(which houses the big pointer) sitting on a bearing about 1" below the gear
plate. By the looks of your one you only had one bearing at the top. How did you fasten the
spindle to the brass tube u used as the spindle seems pretty small. I suppose i could put
varying size brass tubes inside each other till i get near enought to the spindle diameter. Is this what
u did?.
Would there be any chance of having your code for this. I struggle with Sioc every time i try to do something?.
Thanks again and appreciate any comments.

Regards
Les

Quote:

---

Originally Posted by iwik

Rob,
Thanks for the link. I have gutted my one, all i have now is the gear plate,if this is removed
im left with a spindle(which houses the big pointer) sitting on a bearing about 1" below the gear
plate. By the looks of your one you only had one bearing at the top. How did you fasten the
spindle to the brass tube u used as the spindle seems pretty small. I suppose i could put
varying size brass tubes inside each other till i get near enought to the spindle diameter. Is this what
u did?.
Would there be any chance of having your code for this. I struggle with Sioc every time i try to do something?.
Thanks again and appreciate any comments.

Regards
Les

---

That is correct, I used different size brass tubes to change the shaft size where needed and for the spindle to inner brass tube shaft connection I used epoxy. I will post the sioc code this afternoon after i get home from work. ;)
Rob

Thanks and no hurry.
Les
P.S what did u use for the bottom bearing?.

Quote:

---

Originally Posted by iwik

Thanks and no hurry.
Les
P.S what did u use for the bottom bearing?.

---

I have several small flanged bearings saved from dismantling old instruments. I drilled a hole in the ply and pressed the bearing in. The flange sits on the ply and a brass tube rides on this. It keeps all the vertical movement out of the inner shaft. Servocity has them as well.

Rob

Here is a copy of my code. I have the altimeter working as good as it can get using the cheap servo board. I would go with a digital servo for future versions.

Var 0022, name Altimeter, Link FSUIPC_IN, Offset $3324, Length 4
{
L1 = &Altimeter * -0.0533
L2 = L1 + 1010
&Altitudeservo = L2
}

Var 0023, name Altitudeservo, Link USB_SERVOS, Device 1, Output 2, PosL 100, PosC 555, PosR 1023

Thanks Rob,
I presume you have a collar on the inside of the ply base that rides against the bearing.
I must get some from Servo city at the same time as the rest of the stuff. Will get a flanged bearing at the same time. They really sting you for Int postage. Gears and the motor they want 42 bucks to send via post. Still if one needs it you just have to pay.Thanks also for the code.

Regards
Les

Just a quick update, I ended up using a Pololu Jrk motor controller and got great results. Altimeter is still going strong and works great. It is with-in 20ft of sim altitude.
Rob

Hi Rob,
Thanks for the update,havent built mine yet but have all the parts. How are you I/f that to FSX as
before you had o/c servo card to drive it. Arnt these controllers serial input?.
Les

Quote:

---

Originally Posted by iwik

Hi Rob,
Thanks for the update,havent built mine yet but have all the parts. How are you I/f that to FSX as
before you had o/c servo card to drive it. Arnt these controllers serial input?.
Les

---

Les,
The Jrk accepts, serial, analog, and servo signals. I use an OC servo card to drive mine. Rob

Rob,

Happy Independence Day.

How does the pressure scale interact with the needles? Does turning it change the altitude like the real thing? If so, how do you provide the feedback loop to let fs know where it is set?

Reid

Rob,
Sorry for the reply, i only had a quick look at the specs and didnt see the r/c application.
I know you had accuracy problems with the salvaged card from an old servo but you thought a
digital one may be better. Did you try one before going to the jrk?. Previously i think you had problems with it returning to the same pos is this now ok?.
Thanks
Les

Quote:

---

Originally Posted by AK Mongo

Rob,

Happy Independence Day.

How does the pressure scale interact with the needles? Does turning it change the altitude like the real thing? If so, how do you provide the feedback loop to let fs know where it is set?

Reid

---

I just hit 'b' and then adjust my real baro adj on the altimeter. Not perfect but works for me. :)

Quote:

---

Originally Posted by iwik

Rob,
Sorry for the reply, i only had a quick look at the specs and didnt see the r/c application.
I know you had accuracy problems with the salvaged card from an old servo but you thought a
digital one may be better. Did you try one before going to the jrk?. Previously i think you had problems with it returning to the same pos is this now ok?.
Thanks
Les

---

No worries :) I never tried a digital gutted servo as I had the JrK laying around. I do think a good digital servo would work as well, like the programmable Hitec ones.
Rob

Thanks,
Im going to try a digital servo i have and will see how it goes. Is nice to know the jrk works well.
Les

Quote:

---

Originally Posted by 737NUT

I recently purchased a Cessna Altimeter off of Ebay for less than 20.00. (they have several more) I gutted the insides below the main gear plate. The main gear plate holds all the gearing for the pointers and the pointer mounts. This means all you have to do is have one shaft going to the original to make the altimeter work. So, use the methods found in Mike Powell's Building Gauges for Flight Sim's, I went forward with this little project. My goal was a servo based 0 to 20,000ft altimeter. Each turn of the main shaft gives you a 1000ft on the altimeter. I wanted a 20 turn pot for the feedback but found they were to cost prohibitive. I instead opted for the much cheaper 10turn 5K pot. So now I have to gear it so it turns only 180 degree's for each 360 deg turn of the main shaft to keep my 0 to 20k ft. I put a 14t on main shaft and a 28t on the pot. For the motor, i used a nice little geared motor from www.Servocity.com, It runs on 5-12vdc and turns 40rpm with loads of torque, way more than needed here. I have a 12t pinion on it and it drives a 55t gear on the main shaft. (see pics) I gutted a cheap Hitec servo, soldered the pot in place of the servo's pot and did the same with the motor. Voila, a servo driven real GA Altimeter. Works well, very smooth and quiet. This was actually much easier than anticipated. Using Mike's methods for making the shaft and how to build it up for the gears worked great. I could have had a fancy metal plate made for the bottom support but to svae time and money, I used some hobby grade 1/8" light ply. Not pretty but it worked and was cheap. I have a total of about 80.00 in this including the gauge. Any questions feel free to ask.

Rob

---

Why did you not use the original servo motor? I love the topic, thanks!!

The Cessna altimeters are the mechanical air chamber type. I removed the air can and gear and arm and replaced with the electronics and nylon gears and brass tubes as axles to make it work. I can say that If I adjust the Alitimeter to match FSX at Startup, it is dead on for the rest of the flying trip. I am very satisfied now with the results.

Unsterstood. Im just not sure why you replaced the Hitec servo motor with another motor? Im sure there is a good reason.

I do understand why you use the multi turn pot on the servo.

Quote:

---

Originally Posted by dc8flightdeck

Unsterstood. Im just not sure why you replaced the Hitec servo motor with another motor? Im sure there is a good reason.

I do understand why you use the multi turn pot on the servo.

---

I wanted a geared 6v motor that had more torque and only turns 12rpm. :) The servo motor is to fast and to hard to adapt to the gears I used.

Quote:

---

Originally Posted by 737NUT

I recently purchased a Cessna Altimeter off of Ebay for less than 20.00. (they have several more) I gutted the insides below the main gear plate. The main gear plate holds all the gearing for the pointers and the pointer mounts. This means all you have to do is have one shaft going to the original to make the altimeter work. So, use the methods found in Mike Powell's Building Gauges for Flight Sim's, I went forward with this little project. My goal was a servo based 0 to 20,000ft altimeter. Each turn of the main shaft gives you a 1000ft on the altimeter. I wanted a 20 turn pot for the feedback but found they were to cost prohibitive. I instead opted for the much cheaper 10turn 5K pot. So now I have to gear it so it turns only 180 degree's for each 360 deg turn of the main shaft to keep my 0 to 20k ft. I put a 14t on main shaft and a 28t on the pot. For the motor, i used a nice little geared motor from www.Servocity.com, It runs on 5-12vdc and turns 40rpm with loads of torque, way more than needed here. I have a 12t pinion on it and it drives a 55t gear on the main shaft. (see pics) I gutted a cheap Hitec servo, soldered the pot in place of the servo's pot and did the same with the motor. Voila, a servo driven real GA Altimeter. Works well, very smooth and quiet. This was actually much easier than anticipated. Using Mike's methods for making the shaft and how to build it up for the gears worked great. I could have had a fancy metal plate made for the bottom support but to svae time and money, I used some hobby grade 1/8" light ply. Not pretty but it worked and was cheap. I have a total of about 80.00 in this including the gauge. Any questions feel free to ask.

Rob

---

Any idea how much voltage these servos can handle? I have 28v DC motors.

Justin

Hi Rob, could you please give me your email ? i would like to contact you for the altimeter...
pablo@dimobili.com
Thanks
Pablo