cgzro wrote:
Now if you wanted to offer constructive arguments you could argue about what happens with the component's temperatures are below their minimum rated and suggest that it not be used below about -15C or above 40C. No doubt Photofly would agree with that.
Temperature is the obvious one. I put one in the freezer for 3 hours to -18°C and it still told me the power line frequency was 3600±2RPM. I haven't warmed one up - my hot air gun has been stolen by my two year old to heat-shrink some stuffed animals together in a macabre echo of some horrific surgical procedure.
Or you could argue that the device could read incorrectly if an LCD segment goes toast. No doubt Photofly could add a few lines of code to cycle the LCD segments at power up to avoid that problem, perhaps he already has
Nice idea. It will be adequate I think just to display 8888 at startup and easier to spot something missing - I'll do that now.
Or if you wanted to you could ask about the code, has he ensured there are no sources of spurious interrupts that could jitter the code timing? He would likely respond that yes he has grounded the unused interrupt pins but if he hasn't he'd likely say oops good catch.
There's only one interrupt, and that's on the photosensor. Everything else is an output pin. Interrupts are locked out to avoid a race condition for a few microseconds of code but that only runs after one measurement is completed and before the next starts so I think that's safe too.
Biggest error is where you point the thing. If you let the "beam" drift over 1/8 of the prop arc between the start and end of a timing cycle that's a change in frequency of ±20rpm. Easy to avoid when you're close to the prop but harder when you're in the pilot seat 6' away. You do have to hold it still.
Also remember we're trying to assess a tach. The engine is just a machine for turning the tach, and the prop is redundant. A big piston engine with an aerodynamic bar of Aluminium on the end isn't a very good constant speed device. It's sensitive to every breath of wind, and it has no speed governor at all, so you're at the mercy of whatever tiny fuel and oil flow variations produce. Of course you can see this in the tach too, but a digital display makes small variations much more obvious. You can still assess gross errors (4% is about 100rpm) but stressing over whether a crystal oscillator is "good enough" because it's not calibrated to better than 50 parts per million is looking in the wrong place. In my opinion.
DId you hear the one about the jurisprudence fetishist? He got off on a technicality.
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