X-Mozilla-Keys:                                                                                 

I updated this drawing with a skematics of how to connect it
to a hand held VOM/Counter combination.

Paul,

Three notes on your schematic.

1) At 180 ohms and 12 volts you draw 0.8 watts and 0.0667 amps.  While the A/C
electrical system will hardly be affected, that's a lot for instrumentation.

That is only when the transistor is on and it ignores the drop
across the transistor. Average current drain is less than 
half that. That is also for TTL compatability. If you are 
driving a CMOS part or feeding it into a high input impedence
counter you could go up on the resistence. The beauty of an
open collector circuit like that is the data is transmitted in
current loop mode and therefore is more immune to interference.
Well worth the current required. I used that trick for 20 years
on my road test equipment and it was probably used for
50 years before that by the teletype industry. 

2) The alternator actually runs over 14 V. This may heat your sensor.

The Hall sensor has a built in voltage regulator. It will accept
anything
between 4.5 and 16 volts. Sure it will run hotter but the internal
current drain is so low you won't notice it.

3)  For completeness, the meter also needs grounding (and probably an internal
battery).

Gary Moir

All hand held VOM/CPS (I am going to quit using the stupid term Hertz)
counters  
have built batteries and you connect the black lead out of the VOM to
ground
of course.
 
Paul Lamar
 
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