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need to build vss signal divider


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firedemonsic 
Member - Posts: 23
Member spacespace
Joined: September 09, 2011
Location: Virginia, United States
Posted: August 26, 2013 at 6:21 AM / IP Logged  
Hey everybody.
I have an 85 Z28 Camaro with a TPI engine I recently installed an intellitronix digital gauge cluster into. The one remaining problem I am having is that the cluster requires you to eliminate the speedometer cable by installing their VSS at the transmission. The issue here is that the ECM receives its VSS signal from an optical buffer at the back of the speedometer head. With the new cluster obviously I am no longer getting a VSS signal to the ECM which is causing a check engine light, lack of cruise control and worst of all idle problems.
I have determined the ECM in my car is expecting a 2000 pulse-per-mile signal in squarewave form which the VSS for the cluster outputs an 8000 pulse-per-mile squarewave signal.
What I need to do is build an IC controller to divide the VSS signal by 4 so the ECM will be able to read it properly. Does anyone have a diagram on how to build this? Looking forward to visiting the parts drawers at radioshack.
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: August 26, 2013 at 7:13 AM / IP Logged  
Use a 4017 decade counter. With outputs numbered 0, 1, 2, ...9, #5 goes to the reset pin, and you output can be any of 1, 2, 3.
firedemonsic 
Member - Posts: 23
Member spacespace
Joined: September 09, 2011
Location: Virginia, United States
Posted: September 03, 2013 at 10:22 AM / IP Logged  
Thank you I have been studying this IC. Just had a couple questions.
Can the clock pulse voltage be 12V or does it need a resistor?
Do the outputs have a voltage equal to the VSS supply voltage if not using a resistor?
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: September 03, 2013 at 7:18 PM / IP Logged  
Yes. The 4017 typically is powered from a max of 15V and inputs can be up to the Vdd supply (plus about 0.3V or 0.6V if memory serves correctly).
But often an input resistor is used (1k?) with reverse biased diodes from the input pin (eg, Clock pin 14) to Vdd & Vss (+V & GND) - ie, diode lines towards Vss - to prevent voltages higher than Vdd or lower than Vss.
Output voltages are dependent on Vdd - not any inputs.
Note that outputs can only supply about 1mA (for Vdd = 10V) but that should not be a problem in this case.
Your input may need a pull up resistor if the VSS signal is a typical "Open Collector" output. But OC (ground switching) outputs are the norm for such circuitry since it allows interfacing between circuits of different voltages (eg, a 5V VSS to a 14.4V 4017 etc).   
[ Ironically, the 4017 is an exception to OC outputs, though it can sink more current when low than it can source current when high. ]
Don't forget to tie unused inputs - eg, Clock Enable to 0V (GND).
And don't get caught with recalcitrant info that numbers the first output as 1 (it should be 0), but in that case, the Reset pin is connected to the "n+1" output - ie m"output 5" for a divide by 4 counter.
And of course normal automotive hardening - eg, IGN +12V through a diode (1N400x) and resistor to a Zenor (say 14V?) to limit supply spikes to under the 4017's allowed 15V.
Plus maybe a PSU cap (10uF?), and definitely a 0.1uF or 0.01uF across the 4017.
But data sheets or other sources should have appropriate recommendations. (I have that stuff somewhere...)
PS - I corrected some Vdd & Vss references. Having worked more recently with other circuits (FETs etc) I've used Vs = +ve & Vd = -ve or GND. But for CMOS chips, Vdd is the +V supply and Vss is 0V/GND.
But beware, I might still be confused!
firedemonsic 
Member - Posts: 23
Member spacespace
Joined: September 09, 2011
Location: Virginia, United States
Posted: September 05, 2013 at 4:36 PM / IP Logged  
Spark, thank you very much for the input. I have assembled the 4017 to a project pcb and it appears to be properly counting. Now, forgive my ignorance here but TBH this is the first IC I have ever built and my knowledge is very limited.
I am very good with wiring and relays I installed an entire viper 5901 with remote start into my car myself with solder and shrink-wrap however building circuits is something I have never done before however next year I am planning on applying towards a degree in electrical engineering so this is not only a ways of getting a signal to my ECM but also a "feet wet" project so to speak.
I have tied the reset and latch enable inputs directly to ground with no resistor.
I probed the pins on the 4017 with multimeter and it appears to be properly advancing to the next pin every time a pulse is received on the CLOCK input. I also grabbed a 100uF capacitor for decoupling.
Now, you might be thinking that 100uF is way too high of a capacitance value however my reasoning for going with this large of a capacitor was not only to suppress any spikes coming from the CMOS but also to suppress any voltage spikes coming INTO the CMOS. This vehicle is 28 years old with primitive by today's standards wiring. All sorts of relays without spike suppression diodes and not to mention I am running a 50k volt HEI ignition so I went with the 100uF to "play it safe".
TBH I am not worried about the voltage protection diodes because the alternator should never output anything higher than 14.8v except in the case of a freak regulator failure and not to mention the voltage inside the cabin is roughly a full 1V LOWER than seen at the battery.
Now one thing that does concern me is where you said that the outputs can only sustain 1mA. In addition to the output pulse to the ECM I would like to add two 20mA LEDs to the PCB one at the clock input and one at the Q4 output so I can see the 4017 in action on the test bench. Using the equation R = supply voltage - LED voltage / LED current I came up with a pack of 470 ohm resistors. However something is not computing because you say the outputs can only sustain 1mA and the LEDs are 20mA. I see illustrations of the 4017 used to drive LEDs and probing the outputs with the multimeter shows they are positive voltage. What did I miss?
firedemonsic 
Member - Posts: 23
Member spacespace
Joined: September 09, 2011
Location: Virginia, United States
Posted: September 05, 2013 at 4:59 PM / IP Logged  
Disregard reset to ground obviously we need that to go to output Q5.
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: September 05, 2013 at 7:32 PM / IP Logged  
Phew - that did worry me!
I too have seen LEDs directly driven off the 4017, however my datasheets do claim output sourcing capability of a few mA. I cannot find any info re internal output current limiting so I assume the LEDs have resistors to limit their current, and the low current is enough to illuminated them (20mA LEDs are usually reasonably illuminated even with a mere 1mA).
You should be able to connect LEDs (with resistors) to the clock input (probably between +V & the VSS signal output) and outputs.   
As to the cap, though you theory is sound, in practice it fails due to electrolytic capacitor behavior. You'll need a small 0.1uF ceramic etc cap to suppress short switching spikes caused by the chip itself.
Usually CMOS circuits do NOT need the big caps because they don't care about "typical" DC voltage variations (ie, not "transient types that can cause false clocks or triggers), but IMO DC smoothing is nice even if not needed.
And fair enough for skipping the Zener clamp of V-reg... though never underestimate the voltage that can be output ESPECIALLY from an old alternator (if it uses an external body/chassis mounted voltage regulator, voltages can go VERY high - eg, 16V and higher - whereas newer internally regulated alternators can be clamped to a max of (say) 15.5V etc except if that protection fails....).
I'd include the reverse polarity protection diode, hence the 4017 should handle a system voltage of ~15.5V.
And if it does go higher and blows the 4017, just build another one (and then add a Zenor clamp etc).
firedemonsic 
Member - Posts: 23
Member spacespace
Joined: September 09, 2011
Location: Virginia, United States
Posted: September 05, 2013 at 8:16 PM / IP Logged  
TBH with you, I am not too worried about protection/longetivity measures with this converter because I am getting ready to build a stroker motor for this vehicle am will be going with what is called an EBL flash ECM which uses a totally different signal interpretation (I believe it is 4k ppm sinewave). Come new converter time I will take all the appropriate measures but I have fighting this stupid thing for going on two months now and at this point just want the ECM to see a MPH reading again.
Which brings me to be the bearer of bad news. I built the converter a couple hours ago and hooked everything up. With the cluster on the test bench the 4017 is outputting 1 pulse for every 4 received at the clock pin however installed into the car with the laptop scanner hooked up the ECM is still seeing 0MPH.
I am suspecting there is more to this than just dividing the magnetic VSS signal.
And BTW you needn't worry about an old alternator going haywire.
I am running a CS130 alternator. Only has about 1k miles on it.
firedemonsic 
Member - Posts: 23
Member spacespace
Joined: September 09, 2011
Location: Virginia, United States
Posted: September 05, 2013 at 8:22 PM / IP Logged  
Also if it helps, here is a picture of the optical VSS buffer that goes to the back of my stock speedometer head and supplies the ECM with a signal:
The 3 wires go to the optical pickup in the speedometer head and the connector at the other side of the PCB goes into the dash.
firedemonsic 
Member - Posts: 23
Member spacespace
Joined: September 09, 2011
Location: Virginia, United States
Posted: September 05, 2013 at 8:26 PM / IP Logged  
Not sure why the IMG codes didn't work but here are the direct links:
https://i20.photobucket.com/albums/b239/PhotoBucketIsGhey/IMG_20130723_132709_zpsefa680be.jpg
https://i20.photobucket.com/albums/b239/PhotoBucketIsGhey/IMG_20130905_211811_zpse0c9d5bc.jpg
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