Detect a keep-alive signal
1.does this do what I assume it does?


I do believe it does. Subject to a few tweaks, of course.


  2.is R1 well-chosen?
  3.is R2 even necessary?


Yes to both, but I would do this a little differently. My concern is that your charge current to the timing cap depends on the amplitude of the pi pulse. Although this is probably consistent, my ingrained paranoia suggest that you do this







simulate this circuit &ndash; Schematic created using CircuitLab

ETA: Carp! R6 is your 40 k. Sorry. Stick with 40k.

This way, the charge current is independent of the input pulse amplitude as long as it is more than ~1 volt.


  4.are R11 and R23 the right values? (I don't mean the setting, but their value of 5kOhm)


Yup


  5.is LM741 the right choice?


Nope, on 2 counts. First, a 741 is not rated for use as a single 5 volt amplifier. Second, its output swing is not what you want. I'd suggest LM311s.

And while we're on this section, I'd advise each comparator use this circuit:







simulate this circuit

ETA: Sorry, R1 should be 1k - 10k.

This will provide clean switching when a level is reached. And do not forget decoupling caps if you value your sanity.


  6.is 50uF for the capacitor a realistic value? Is the RC circuit and its charge resistor well-chosen for that matter?


Yes to both.


  7.is it reliable: •interference, should I even worry about it?


Yes, and good on yer for thinking about it now. You can probably get away with using twisted pair to connect the pi to your circuit.


  •can it handle temperatures between -30°C and +50°C?


Excellent question. Yes it can, but you'll need milspec (or at least industrial) parts, particularly for the comparators. Commercial temp ranges are usually only 0 to 70 C. But check all the part ratings when you buy them. 


  •age, will this run for 10 years?


I don't see why not, as long as you do things like cleaning all the solder flux from your pc board. The weakest link is probably your 50 uF cap, followed by your 5 volt supply, but there's not much you can do about that.


  •tolerances in values, what should I pay attention to?


1 % is normal in resistors these days, and caps will run about 10% to 20%, but you'll adjust for that in setting your pots. Not a major concern.