# [EVDL] tach pullup resistor



## EVDL List (Jul 27, 2007)

Peter,

I am not familiar with your exact application but generally a pull-up
resistor will go between the "data" or "signal" line and the positive
voltage (Vcc, +12v, etc). So in your example the resistor would connect
the red and green wires.

(A pull-down resistor would connect the green and black wires...assuming
that when you say 12 volt negative you actually mean "ground", as
opposed to an actual negative 12 volts.)


Theory: Some sensors will connect the data line to the ground when
activated....but they do NOT connect the data line to the +12v when NOT
activated. The purpose of the resistor is to always connect the signal
line to the +12v at a limited current. When the sensor connects the data
line to the ground, it does not have a resistor, so has much more
current to ground, and "wins" the "fight" with the resistor bridging the
data line to the positive side, pulling the data line down near zero
volts. But when the sensor is NOT activated, the resistor pulls the
signal/data line up to close to +12v.

It is important that the resistor has enough ohms so that you are not
passing too much current from +12v to the signal line. IMHO your 10K Ohm
resistor should have plenty of ohms and won't "fight" too hard.


Jay



> SLPinfo.org wrote:
> > Hi there,
> >
> > Am trying to get my hall-effect tach working. It has 3 wires which
> ...


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## EVDL List (Jul 27, 2007)

Jay,

Thanks much.
- Peter



> Jay Summet <[email protected]> wrote:
> 
> > Peter,
> >
> ...


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## EVDL List (Jul 27, 2007)

Jay,

To follow-up. I installed the pull-up resistor as you suggested this
weekend but there is no change. Looks like I will have to take part of the
dash apart and find a separate 12V power source for my tach. The mechanic
who installed it for me connected it to the same power lines as go to my amp
guage and (I guess I shouldn't be that surprised) the movement of the tach
needle looks very much like the movement of my amp guage. I'm guessing
there is signal bleed-over (not sure if that's the technical term). As
Roland suggested to me some time ago, I'll probably need to run a separate
line for the tach.

Thanks for trying to help.

- Peter Flipsen Jr
http://www.evalbum.com/1974



> Jay Summet <[email protected]> wrote:
> 
> > Peter,
> >
> ...


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