Maybe I'm stating the obvious, but you can of course also use the
alternator warning light for registering engine hours.
True, if the alternator fails the hobbs will stop, but without
alternator it won't take long before the hobbs stops anyway. ;-) Also,
the same logic could be applied to using the oil pressure as source for
the hobbs.
Using an oil pressure switch is just introducing another part with the
potential of leaking, getting loose, etc. And it also comes with a
weight penalty. And costs some money.
The alternator warning light is free, without risk or weight penalty. I
can't really see the benfit of an oil pressure switch for a
non-mission-critical part like a hobbs...
And no, you don't need a relay for it to hook up the hobbs to the
alternator warning light. Just put the hobbs in series with the bulb,
and connect the alternator warning output to the joint between bulb and
hobbs. The other end of the hobbs should go to the opposite part of the
power supply (that is, if the bulb is connected to ground, the hobbs
should go to the 12V, if the bulb is connected to 12V, hobbs to ground.
Yep, I don't remember whether the warning light is switched to 12V or
ground, am too lazy to look it up. ;-) ).
The bottom line is that the bulb is used to complete the circuit of the
hobbs. As the hobbs uses far less power than the bulb, the bulb won't
lit, and the hobbs will see the full voltage. When the alternator light
comes on, the hobbs will now not get any voltage difference accross its
terminals, and thus stops counting.
I certainly would have done it this way, but I already have an engine
hour hobbs in the Smart prop controller.
Frans
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