How to Convert a real Cessna Altimeter for Sim use.

[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

[iwik]

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

[737NUT]

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

[iwik]

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

[737NUT]

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

[iwik]

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

[737NUT]

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

[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

[737NUT]

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

[iwik]

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