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Lightspeed III & DPS smart Tach, signal issue help?

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gmcjetpilot

Joined: 04 Nov 2006
Posts: 170

Posted: Tue Apr 22, 2008 11:16 pm Post subject: Lightspeed III & DPS smart Tach, signal issue help?

<?xml:namespace prefix = mailto />Does any one have experience getting a DSP smart tach to work with a LS Plasma III?

   

  I am helping a guy out with his new Lightspeed III ignition. My analysis the LS signal is not compatible in stock form to drive the DPS tach.

    

  He wants to use his DPS smart tach (they are out of business BTW). Here is the issue. 

   

  The new ignition LS Plasma III has a tach signal of a square tooth wave from 0 volt to +10 volts amplitude with a duration of 1.5ms. It fires two times per revolution. OK its a hall effect signal a digital OFF/ON.

    

  Here is the issue, the tach was triggered with a sensor in the access hole of a magneto. It detected the internal rotating magnet in the magneto. Unlike a typical magnetic inductive signal (alternating) it states in the installation manual, voltage sits at a constant +5 volts and when the magnet passes the sensor, the voltage momentary drops to 0.70 volts. How it looks on a scope I don't know, probably rounded or spike.

    

  I need to go from the LS's positive square wave signal to a volt drop signal that is not so square wave, basically inverting the wave.

   

  Is there some easy circuit off the top of your head to convert these two wave forms, condition the signal? I'm thinking  an Op amp, diode and a cap and R or two.

   

  I'm drawing a blank. The DSP tach does have selection of pulse per rotation. Taking the tach apart to mod it is not on the table (yet). Access to the schematics is not likely. May be it is time he bought a new tach?

    

  Thanks George



           Be a better friend, newshound, and   [quote][b]

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nuckolls.bob(at)cox.net
Guest

Posted: Wed Apr 23, 2008 5:42 am Post subject: Lightspeed III & DPS smart Tach, signal issue help?

At 12:12 AM 4/23/2008 -0700, you wrote:



 	  Quote: 	
		  <?xml:namespace prefix = mailto />Does any one have experience getting a 

DSP smart tach to work with a LS Plasma III?



I am helping a guy out with his new Lightspeed III ignition. My analysis 

the LS signal is not compatible in stock form to drive the DPS tach.



He wants to use his DPS smart tach (they are out of business BTW). Here is 

the issue.



The new ignition LS Plasma III has a tach signal of a square tooth wave 

from 0 volt to +10 volts amplitude with a duration of 1.5ms. It fires two 

times per revolution. OK its a hall effect signal a digital OFF/ON.



Here is the issue, the tach was triggered with a sensor in the access hole 

of a magneto. It detected the internal rotating magnet in the magneto. 

Unlike a typical magnetic inductive signal (alternating) it states in the 

installation manual, voltage sits at a constant +5 volts and when the 

magnet passes the sensor, the voltage momentary drops to 0.70 volts. How 

it looks on a scope I don't know, probably rounded or spike.



I need to go from the LS's positive square wave signal to a volt drop 

signal that is not so square wave, basically inverting the wave.


	


    Was the original DSP sensor a two-wire or three-wire device?

    I've seen through-the-housing aircraft magneto tach sensors

    in both formats. The three wire devices are always hall effect

    sensors that output a digital friendly, square waveform. If

    it was a two-wire device, then it's probably a variable reluctance

    sensor . . . a coil of wire wound on a central bias magnet.

    The the output from these devices is a digitally unfriendly,

    one-cycle per event ground referenced waveform not unlike that

    illustrated in Figure 2 of this document.



http://cache.national.com/ds/LM/LM1815.pdf



    Usually, the input circuitry for a device expecting a

    variable reluctance input signal are capable of

    properly responding to a single pulse having

    nicely squared edges. But there may be a DC bias

    issue presents an offset issue.



    Put a 1K resistor across the input to the tach and

    then look at DC voltage on both ends of the

    resistor. If there is no obvious DC common mode

    voltage, then check resistance of each end of

    resistor to ground. Hopefully, one end is tied to

    ground. If so, then the OTHER end is your real

    signal input lead.  If there is a measurable

    DC voltage and/or neither end of the sensor input

    runs at ground, then there's a possibility that

    the tachometer expects a floating signal sensor

    with some DC continuity and that unlike the

    tach chip described above, needs to be spoofed

    into believing that the ground referenced, digital

    pulse from LS ignition is a variable reluctance

    sensor.



    The easiest way to craft an interface box is

    on the bench. Set up a signal generator to

    emulate the output of the LS ignition.



    The 555 timer based r/c servo tester is also

    suitable. See page 7 of:



http://www.princeton.edu/~mae412/TEXT/NTRAK2002/292-302.pdf



    . . . except in this case, you want to vary

    the period of the timing pulses (Adjust R3 in

    diagram) as opposed to adjusting pulse width

    (R2).

 	  Quote: 	
		  

Is there some easy circuit off the top of your head to convert these two 

wave forms, condition the signal? I'm thinking an Op amp, diode and a cap 

and R or two.


	




    If the DSP tach expects a dc-continuity through

    a sensor with floating ground, you might experiment

    with transformer coupling a buffered version

    of the LS tach signal. Something like the top sketch

    at . . .



http://www.aeroelectric.com/_temp/LS-DSP_Interface.pdf



    . . . offers a foundation for a working solution.

    If all it needs is an inverter, then the lower

    sketch is a starting point.

 	  Quote: 	
		  

I'm drawing a blank. The DSP tach does have selection of pulse per 

rotation. Taking the tach apart to mod it is not on the table (yet). 

Access to the schematics is not likely. May be it is time he bought a new tach?


	


    That is probably the least expensive in terms of

    $time$. Even after you solve the electronic problem,

    there are issues of packaging, installation and

    the specter of unanticipated future issues with respect

    to EMC, etc.



    It's up to all the participants involved to put

    value on their $time$ and to assess return on

    investment.



    Bob . . .

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