OEM Standby altimeter build

Hello. First post here.

I have a standby altimeter/airspeed indicator for my Boeing 737 simulator. This is the real gage, not a sim product.

Originally, this is operated by both dynamic air pressure from a pitot tube for airspeed and static air pressure from a static air pressure port.

It operates from a bellows that is driven by each type of pressure. The airspeed unit uses about 45° of travel on an input shaft attached to the bellows and I can drive it from a servo using a rod, ball link, and output arm.

This servo must be stepless and cogless. Recommendations? Doesn’t need much force.

The altimeter is a whole different beast. It expands the bellows motion by 20 times for full operation. Looks like a clock or very fine watch inside.

I can partially gut this unit and use a small continuous-rotation servo to drive the altimeter. But there is a barometric pressure preset knob that offsets the altitude to zero based on the current air pressure at ground level. How to keep this function?

Please, I need help with this.



Hello, Kevin.

I’m not clear on if both devices have this 45 degree range of motion that you need to manipulate, but I can give you a bit of feedback that should apply regardless of the answer. Basically, you are unlikely to get away from some kind of minimum step size or cogging, so you really just need to know what is a small enough step size. For instance, if you want 6-degree resolution on your needle (to achieve the equivalent of one-second increments on a second hand), and then need that over 20 revolutions, you need to be able to have 1200 distinct positions, whether that’s over 45 degrees or the 160 or so that is typical of most servos. (To get the most resolution in your final system, you should make your linkage such that the full range of the servo translates to the range of your mechanism.)

1200 distinct positions is quite a bit but possible. Our Maestro USB Servo Controllers can generate signals with 0.25 μs resolution, which over one millisecond gives you 4000 distinct positions. That will be beyond the limit for most servos, and unfortunately, servos usually do not have such good specifications for their exact resolution. You could look at [our selection of servos] and try some of them with a Maestro to see what kind of fine control you get. Please let us know if you get any interesting data.

- Patrick

Hello, Patrick,

Please see my video at B737 Standby Altimeter/Airspeed Indicator Overview

B737 Standby Altimeter/Airspeed Indicator Overview

An inside look at this instrument prior to converting for sim use

The movement in question for the servo is for the airspeed indicator. It needs to be smoooooth… no cogging.

Please let me know what you think.



Like I said before, you are unlikely to get away from some kind of minimum step size or cogging, so saying you need the servo to be smooth is not a productive way to figure out what you need. It is up to you to define what step size will provide motion for your system that is smooth enough, at which point you can follow the procedure I described to figure out how many distinct positions you need from your servo system in order to achieve it.

- Patrick