924Board.org

924RACR · Posted: Fri Nov 05, 2004 10:42 pm Post subject:

Good work, Smoothie. This may explain why even Permatune warns that you may experience misfires when swapping in their CD ignition box, if any part of the ignition system is not up to snuff...

OTOH, to be able to swap in a box with a rev-limiter would be nice for the racers...
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Smoothie · Posted: Sat Nov 06, 2004 12:55 am Post subject:

Here's an adjustable shift light - no "pills" needed. Problem is it only deals with one rpm setting at a time. Then again, you could get 3 for ~$110, but then again by the time all 3 lit up, you'd be blinded. - http://store.summitracing.com/default.asp?target=esearch.asp&N=100&Ntk=PartSearch&Ntt=SUM-G2945+&x=18&y=12

Or how about a light that uses pills - figure out the resistances needed to replace the pills and give you the shift points you need, then wire the 3 resistors into a 3-position switch and mount the switch to your shifter. -Or [this one's off-the-wall and would be tricky] if there's a way to use momentary switches - mounted so they're activated in turn by the position of the shifter. - http://store.summitracing.com/default.asp?target=partdetail.asp&part=ATM%2D5330&N=110&Ntk=KeywordSearch&Ntt=shift+light
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

CMXXXI · Joined: 05 Nov 2002 Posts: 1939 Location: Vicksburg, MS

Just so happens I re-set my timing and idle this afternoon. My hand-held tachometer has been acting up (bad tach-dwell switch) and on a fluke I measured the voltage on the (-) side of the coil, and it was something like 5.6v, regardless of RPM. As JB says, the tach just measures the pulses, on/off/on/off/on...

On a side note, does anyone have informal records of the difference between a shop tachometer and the one that is in the dash? My hand-held says 1000rpm, but the in-dash one reads about 1200. At 2Krmp on the hand-held, the dash says about 2300. I set my idle based on the hand-held to 900 rpm, but the in-dash reads about 1050-1100. Just curious if there is any documented history as to the OEM tach's accuracy. (Is there any way to "positively" calibrate a hand-held unit?)
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'79 Eurospec 931

augidog · Joined: 03 Mar 2003 Posts: 1360 Location: New Jersey

Smoothie · Posted: Sat Nov 06, 2004 11:05 am Post subject:

I think a better choice of light would be the o2 sensor light if you have one. That thing really gets your attention - it's downright annoying in fact while the highbeam light isn't all that bright. -And if what you're pondering works out for you, I'd take the voltage you get out of it (assuming it'll be something lower than 12V), use that lower voltage to switch on a relay that's in turn wired to supply 12V to light your light.

Here's something on high-pass filters - http://en.wikipedia.org/wiki/High-pass_filter
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

augidog · Joined: 03 Mar 2003 Posts: 1360 Location: New Jersey

John Brown · Joined: 07 Nov 2002 Posts: 903 Location: Leesburg VA

Thanks for the confirmation. Thought maybe I was going senile.

I'm no expert but CD ignition doesn't make much sense to me. Not with today's technology.

Transistor ignition was a big deal around 1965 or so I think. Remember my Dad putting 'transistor ignition' on a TR3 about then. 'Course, to the extent that transistor ignition is mainly about how the primary is switched, it would apply to either inductive or capacitive systems.
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John
80 931 - #931 44Cup
99 Escalade - tows track cars
gone but not forgotten: original 924.org car - 82

Smoothie · Posted: Sat Nov 06, 2004 2:35 pm Post subject:

Sleykin · Posted: Sat Nov 06, 2004 4:38 pm Post subject:

The digital voltmeter is measuring the peak (or nearly the peak) voltage of the pulses. It is concivable to make a simple R/C time constant that would output a voltage proportianal to the frequency of the pulses. Pulse duration would also come into play here. You could then use a diferential amplifier with a pre-set voltage on the proportioning input and wire it as a schmdit trigger to fire a lamp or a relay. It would not be very precise though. A better method would be to use a DAC and set a trigger point from that. You could use micro switches on the shift lever to insert the appropriate bias for the shift points you want to set. That way one light would tell you when to shift...but it seems like a lot of bother to me

Several other ways would work but all would be more involved.
Sensors on the tach to fire a light when the needle goes by?
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Glenn Neff
Medford, OR
87' 924S

augidog · Joined: 03 Mar 2003 Posts: 1360 Location: New Jersey

I just found this-Will it work?
Quote is taken from the below source:

You could adjust the value of the 100k resistor to change the RPM setting. This should work, as when the engine is running at too low an RPM the 555 will time out before the next input pulse.

http://www.electronic-circuits-diagrams.com/forum/viewtopic.php?t=66&sid=8d7012bacb798ec1f4e327a62660b092

http://www.electronics123.com/amazon/catalogue/c3-3-11.htm
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1978 924 95 mile daily driver.
Audi TB/POR174M/High Flow Cat/2.25" exhaust
I knew that positive thinking thing wouldn't work.

Sleykin · Posted: Mon Nov 08, 2004 3:23 pm Post subject:

Unless I am mistaken this circuit will give you a blinking light below a certain input frequency and a steady light if the frequency goes above the set point. Let me dig out some of my old "cookbooks" and see if I don't have a circuit for you.
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Glenn Neff
Medford, OR
87' 924S

Lizard · Posted: Mon Nov 08, 2004 3:41 pm Post subject:

leadfoot · Posted: Wed Nov 10, 2004 7:38 pm Post subject:

You will have a steady stream of incoming pulses with no break, there may be better ways of doing this but you could run the square wave signal from the tach into an opto isolator(so that only signal passes, run this signal through a bandpass filter and use this to trigger a flip flop arrangement(simple 555 timer circuit) driving an led or globe. This with pulse the led, also this timing can be altered by running it through a logic gate to slow down the pulse counting...
For the bandpass filter you can use the example of a parametric equaliser. This circuit will also give you an adjustable Q and centre frequency shift, allowing you to tune into the desired trigger frequency by narrowing the bandwidth, you can just ignore the amplification of this circuit by not running it through the feedback resistor arrangment.
Leadfoot

MunkPuppy · Posted: Thu Nov 11, 2004 10:11 am Post subject:

if you really want a simple alternative for a tach shift light, you could always put in a little wire that contacts teh tach needle at a certain RPM. Of course, doing that would require disassembly of your tach, and teh possibility of it never working again

It is my understanding that you need a circuit that will trigger a light when it recieves pulses that are less than x milliseconds apart. Obviously, the circuit must know to keep the light off when the pulses are greater than x ms apart. I'm no electronics guy, but my brother is an electronics tech and designer. I'm sure he'll have an idea or 2 on how this can be done. Seems to me like it would be a simple timing circuit probably requiring minimal components.
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'80 931 FOR SALE

"It's a shame that stupidity isn't painful"
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