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Setting up the synchro gauges

Started by kattz, May 26, 2021, 03:01:38 PM

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Can anyone here please send me the information on what I would need to set up my two gauges that use synchyos. What card to use, basically how to interface with the Sim and with the PC. I have two gauges, packs ducts and flaps.

Any and all help appreciated! Thanks.



Hi, Kevin—

I, too, am looking for interfacing options for synchros/resolvers for my B727 project and stumbled across this the other day:


It looks pretty straight forward with minimal fuss.  I'm not sure if this will help in your quest but wanted to at least get it out there!!!

Let me know if it works for you as I haven't had a chance to do anything more with my interfacing projects.

I made the decision and jumped-in with X-Plane 11 and the FlyJSim 727 and have been focusing all my attention to learning how to operate X-Plane after all those years with FS9 and FXS.  HUGE learning curve!!!



It looks like the synchro units are attached directly to the gauge and you have to move them externally using a servo card and a servo. So there's two channels of synchro, A and B, and they go to the gauge as well as the correct voltage, and you would turn the shaft on the servo on the synchro transmitter for the gauge to work. Doesn't look like other than a servo card and a servo anything else would be involved.


I'm in the same boat, also looking to drive a flaps gauge.

I've found two videos that I'm planning to investigate further to get pointers on how to drive these.





If you are building your own you could base it on a dual-shaft stepper motor from a car dashboard. Here's an illustrative link, but they are available from AliExpress and other sources:

Search for Juken x40 for more information (Juken is the original manufacturer, x40 is the dual-shaft version, but there are other versions for other applications).

The steppers are easy to drive and can be hooked up directly to Arduino output pins. If there's nothing out there already then you need a program that gets the flaps position from the simulator and runs the stepper motors in the right direction until the physical pointer matches the simulator's position.


Hi guys,
the traditional way is to connect a synchro, inside the gauge, to a synchro outside ( So the synchros are physically connected: movement in one end is detected on the other end), and physically attach then the synchro to a stepper motor that can be driven by any analog card input.
Currently, Simulator solutions from Australia are producing a card that seems to read directly the synchro and provide the 400 Hz needed by the synchro, but I just ordered the card, so I can not report yet.


Please keep us updated on the progress if you would. I think we're all probably interested in that!



June 07, 2021, 11:23:19 AM #7 Last Edit: June 07, 2021, 11:24:03 AM by AAnnillo
Yes, please update us when you've received your boards and have done your testing.  I will pull the trigger and order at least five (5) to start!

Hoping all is well.



November 23, 2021, 10:17:31 AM #8 Last Edit: November 23, 2021, 10:22:06 AM by amcalle
I'm in the process of interfacing an old DME indicator from a 707. 3 synchro receivers are in charge of moving the indicators and I'm trying to drive them using a microcontroller. In theory this is possible but requires some electronic to translate the digital angle from the micro to analog signals that can be sent to the stator inputs. I came across a few ICs that can do this e.g.
AD2S1205, but they are quite expensive.
Has anyone have experience with synchro converters?

Thanks in advance!

P.S. I don't want to replace the synchro by stepper motor, at least not for now. I would like to keep as much as possible from the original instrument.


November 23, 2021, 07:55:11 PM #9 Last Edit: November 24, 2021, 06:20:06 AM by Mike.Powell Reason: added detail
Creating the analog synchro signals is certainly doable. There are commercial modules, but as you've found, they are pricey. It's also possible to take the DIY approach which is not as expensive, though not exactly cheap depending on the performance you need. I'm currently prototyping a digital to synchro converter that has 12 bit precision and will be capable of driving simple engine gauges or more complex instruments like ADIs. I have a hacked together prototype running and will be having prototype v2 PCB made in the next few days. I post project updates on my site mikesflightdeck.com

My approach is to take a sample of the 400Hz power then scale it by the sin of the synchro angle using a multiplying DAC  (LTC1590) then amplify it to a useable level. The LM1875 20 watt audio amp works for that. You need up to 11.8 volts RMS so a basic opamp won't do. There's a parallel channel that scales the 400Hz sample by the sin of the angle plus 120 degrees. Fortunately the LTC1590 is a dual MDAC since it costs about $15.  I'm also using a pair of isolation transformers. They are not strictly needed as you can drive a synchro with one phase grounded. The synchro equations define the phase to phase voltages, not the voltages relative to ground. Transformers are useful in that they also provide a bit of protection to the instrument as they don't pass direct current and a catastrophe in the DIY electronics is less likely completely ruin your day.


Thanks Mike, very valuable information and very interesting blog.
I think I'll take the DIY approach, let's see how it goes.

I am thinking about using a microcontroller for the scaling e.g. STM32 64MHz with at least 2 DACs and DMA, so there is no need for pricey multiplier DAC.
The amplifier stage is something that I don't know how to implement it while keeping cost as low as possible. I'll have a look at the amplifier you suggested, thanks!


The Teensy 3.5 looks attractive for a software approach, It's got two DACs and what looks to be a good audio library.


November 25, 2021, 02:42:33 AM #12 Last Edit: November 25, 2021, 02:43:56 AM by _alioth_
About synchro resolvers:
If you need to drive a synchro, you can mount a synchro transmiter outside the gauge and rotate it with a stepper motor/ homing sensor.

If you need to read it, what I usually do is to replace the synchroresolver with an small, moderm and precise absolute encoder like this one:
So you can read it with arduino/teensy using just one pin.

I usually 3d print an adaptor to fit exactly in the gauge like the original synchro:

I have interfaced Vors, and HSIs in this way.




What a great solution.

I'm in a position now where I'm ready to start 'playing' with my ADIs, HSIs, RMIs, DMEs, etc.  Of course, all are syncro/resolver-driven and I have the proper power supplies along with numerous 5-30VAC 400Hz transformers.  These will particularly helpful as some syncro/resolvers require 26VAC, 11.5VAC, 12VAC.  I've tested my newly-acquired transformers and they work perfectly.

I've seen a few syncro/resolver boards and really want to get started with the interfacing process with X-Plane and the associated data refs/XPUIPC...but have no clue how/where to start!

I have the wiring diagrams/pin outs for all of my instruments and gauges and they're already wired and labeled--just need to figure out the programming to get these incredible pieces of history to move!



Hi Mike, do you know how much current your synchro demanded?

Hola Arturo, encantado de conocerte virtualmente  :)
That's a great solution! Do you have an estimation of the cost for driving one synchro receiver? So you need 1 synchro transmitter + 1 stepper motor + position sensor. I'm trying figure out the most economical system, as I would like to use the driver in many instruments.
Thanks guys!



I measured 188ma rms maximum on one phase and a 9.0 VAC rms maximum phase to phase voltage. I'm running this at a slightly lower voltage than the spec 11.8 VAC rms. This gives 1.7 VA assuming the voltage and current maximums occur at the same time (not a given as this is a highly inductive three phase device). An example in "The Synchro/Resolver Conversion Handbook" calculated 1.23 VA for a specific model of a small synchro receiver so what I measured doesn't seem to be unreasonable.

These currents can increase dramatically if the rotor cannot rotate freely. A frozen rotor can become a transformer coupling the amplifier output to the 400Hz power supply output. Without current limiting, things burn out.

Power ratings of 1.5 and 4.5 watts are standard for commercial digital to synchro converters. (I would guess there are others, but I have seen reference to 1.5 and 4.5) I assume the 1.5 watt units are intended to drive synchro control transformers while a 4.5 watt unit would drive a synchro receiver.

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