Mike Powell

I can't make an acurate assessment of that. I've been interested in aviation all my life, but mostly on the general aviation side as a private pilot until having children sucked my wallet dry. My time in the Air Force Reserve sadly (or maybe luckily) kept me on the ground.

SOIC I can do, but I haven't tried MSOP. I'm looking at several approaches. There are interesting trade offs between sampling rates and numbers of bits. With the low cost of PICs, it may even be best to use multiple PICs and go PWM. One of the largest costs is printed circuit board area. I think basic, single pointer engine gauges in military and commercial A/C are simply synchro receivers with a needle on the shaft. More complex gauges with mechanical rolling-digit displays under the needle are servoed with a synchro transmitter used as a position sensor. The general aviation engine gauges I've pulled apart often have a moving coil mechanism.

The first version of the DTS used a pair a MAX543 MDACs and no inverter stage. The price of the MAX543s jumped up and I redesigned to use the less expensive LTC1590 dual MDAC. I had some friends whose application required more precision, so the extra bit from using the inverter stage was attractive. Now I am considering lower precision, lower cost approaches. Many MDACs are only available in difficult to use SMD packages. A pair of discrete R-2R DACs coupled with firmware to synchronize to and track the 400Hz supply is a possibility. Most synchro engine gauges do not need 12 or 13 bits. ...perhaps a single board that provides the synchro signals and enough 400Hz power for a single gauge...?

Very nice! Where did you get the servo-mounted gear, ServoCity?

The PF35T-48L4 stepping motor is a good choice for gauge experimentation. It's often available as a surplus item. For example see:http://www.allelectronics.com/make-a-store/item/SMT-108/STEPPER-MOTOR-6-WIRE/1.html It's easy to drive with inexpensive electronics. I've used it in a few projects based on PIC micro controllers. I wouldn't be surprised to find that a stepping motor shield for an Arduino would do fine as well. The brass gear shown in the All Electronics picture is a 48 pitch gear which mates with the 48 pitch Delryn gears from Serv-O-Link http://www.servolink.com/gears.htm A non-geared approach is to half step a 400 step-per-revolution motor. However, I haven't seen a 400 S/R motor at a surplus outlet for some time.

Sweet!

I think the circuit worked for me due to my own dumb luck. The signal levels and the normal spread of component parameters just happened to fall together in my favor. Clearly, building more units and testing them under a variety of conditions would have been of benefit. Their feedback called my attention to the transistor. I understood what they did in general, but not, I think, the specific details of their solution. In any case, all kudos to them. I was unaware of the problem until they found it. I think my answers to their questions were of limited (if any) use, and they developed a fix themselves. The revision I posted is the result of me taking a fresh look at the application in general and FETs in particular.

I will be posting a revised, but untested, board layout which incorporates the circuit changes. A possible construction revision is to mount the circuit board on short stand offs to a aluminum plate which can be used as a heat sink for the LM1875s. The leads from the LM1875s can be inserted from the bottom side of the board so the 1875s can be bolted to the plate.

Yes, I believe the transistor was the source of their problem. They generously kept me up to date with their progress. The update is a direct result of their feedback. The symptom was the inverter stage was inverting when it should not.

Check toward the bottom of this page: http://www.mattssimulator.com/sim_020.htm Matt has been working on simulators for a long time and has tried and implemented quite a variety of interfaces to real A/C instruments.

There has been an upgrade to the digital-to-synchro project described in Building Recreational Flight Simulators. If this project is the approach you're planning, please check my site for the changes.

Thank you for the kind words.

Scott has put a great deal of thought and effort into it, and it shows. He's also built a number of other instruments as well as writing a lot of interface software.

http://home.exetel.com.au/flightsim/hsi_project.htm