If I can?t get RPM info from the D-180 then I guess maybe my best alternative is to sense physical throttle position with a limit switch. I?m thinking about adding a small microswitch mounted to one of the carburetors to sense full open throttle position. I?m interested to see if anyone has a slick way of doing this?
You
can get RPM from the D180, just not in a convenient manner. The D180 spits serial data (including RPM) out on a port pin. So you have two choices that I can see. First, you could use a throttle position switch to sense when the throttle is opened past a certain point (like, more than you would use for taxi). It's more wiring and a mechanical installation, but relatively easy to do for normal people.
Second, and the easier path easier for slightly twisted nerds like me, is to grab the serial data stream from the D180 and use that. That means no mechanical installation or extra wiring. I designed it to fit into a short enclosure with a DB-25 on each end, so it just plugs between the D-180 and the cable - so no wiring hassle and it's easily removable.
So why go to all that trouble when it would be easier to just replace the spar warning input? I debated that. I think the D180 spar pin input is less that completely useful. If it alarms in flight, it means your switch is flaky... there's no way those pins are coming loose on their own, right? And if you haven't put the pins in right you'll know it when you try to start the engine - even if you miss the big red glowing light. Still, I try to do things like this with a thought to others using it. I didn't want to spend time having that discussion with every single person who might want one. I also wanted to leave the plane doing everything it did before -- even things that are of debatable value. Plus, adding RPM sensing required a microcontroller, and once that's in place it's two lousy lines of code to add the logic to merge the spar pin and canopy switch functions.
I can't say when I'll have a chance to install and test it. I am guessing it will take me a couple of tries to debug the firmware, since I have not dealt with the D180 data stream before... it's documented, but I have found that such documentation is often not completely accurate after a few years of software revisions. But, when I do have it working right I'll have several circuit boards left over in case anyone else wants to try it out.