My next project is to have the MS operate my Auto Meter tack. In the car at this time I am using the yellow/ blue wire from the trailing coil for the signal wire for the Auto Meter tack which was the original wire for the factory tack.

I am a bit lost to why the drawing I see having 12V power in through a resistor to the tack (See Attachment). I always thought that the signal was only a ground signal to the tack signal wire to get them to operate. But then again my level of electronics at this point is not the highest so I am improving. Can some one shed a little light on to this subject? Also if this is a correct set up what effect will changing out the 1K resistor to lets say a 10K have on the tack function? Also is this 12V power from the battery or from some other sorce?

Thanks All.

 

The circuit looks like a pull-up circuit, but using a transistor (2N222A) rather than a switch. See this website for more info about pull-up resistors. http://www.seattlerobotics.org/encod...97/basics.html



The MS PCB is supplying a low-power signal, which should be a 0V/5V pulse. Most automotive tachometers need a 0V/12V pulse (0V, then 12V, then 0V, then 12V...), so that's why you need to connect +12V to the (1k-10k) resistor. The transistor (2N222A) is being used as a switch, shorting the tach output to ground.


When the PCB output is 0V, the transistor will be open (no current flow), and the tach signal will be 12V. There should be a very small voltage drop (less than 0.1V) across the resistor.

When the PCB output is 5V, the transistor will close (short), pulling the tach signal down to 0V. There will be a 12V drop across the resistor.



The 12V power is ignition power. It should only be on when the ignition key is in the ON position.

-s-

 

Thankyou,
Scotty the link you gave me was very help full. Now I understand the use of the 12V in and that resistor inline. Also why I was getting a faint power signal from the MS unit. I will start with the 10K resistor as showen in the drawing and used as an exsample from the link that I read. Great info and help!!

Thanks Again!
SCCA RX7 RACER

 

You're welcome. Please let us know how things turn out.

-s-