Re: [Autopilot] LM2907 Tach Question

[Trammell H. <hu...@ro...>]

On Thu, Jun 26, 2003 at 09:21:15PM -0700, Jake McGuire wrote:
> Regarding the LM2907 tachometer and Hall effect sensors: on the 
> sourceforge website you mention that the output of the Panasonic Hall 
> Effect sensor needs to be AC coupled for the LM2907 to work properly.  
> This makes sense to me.

I'm not sure why they designed it that way, since wouldn't it still
be triggered by an AC input (such as reluctor on a crank shaft) if
an appropriate bias voltage was used?

> But on the most recent schematic (rotoschsh1.pdf) it looks like the 
> data line from the header is connected directly to the input of the 
> LM2907.

That's correct.

> Is this because the requirement for ac-coupling is no longer there, or 
> is the AC coupling done in whatever plugs into the tachometer header?

I didn't want to exclude someone from using something like a magnetic
pickup or other traditional tach sensor, so the AC coupling has to
be done in the sensor head.  For the OC Hall effect sensor, it is just
two resistors and one cap, as described here:

	http://sourceforge.net/mailarchive/message.php?msg_id=3884810

We do not have any code in the tree right now that uses the tach
input as a govenor.  Eventually someone will get around to writing it.
Until then Dennis and I are using the throttle curves from his radio.

Good luck,
Trammell
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