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Old 10-16-2003, 01:00 AM   #1
SJwrx
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Default How Shiflights Work?

I was pondering this in the local electronics store... does a shiftlight's (lets say an Autometer w/ pills) light go off due to the pills being different resistance?

If so, how could you find out how much the resistance is for lets say...6,000 RPMS?

Rich
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Old 10-16-2003, 09:21 PM   #2
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not oe person knows how they work?

Rich
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Old 10-18-2003, 07:01 PM   #3
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What's a "pill?"
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Old 10-19-2003, 01:11 AM   #4
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it has something to do with the frequecy of noise on the electrical system.

Noise is generated by the alternator spinning, when the engine spins the alternator faster and faster the frequency of noise is altered on a known path. This is read some how by the shift light. I don't know if its resistance or what.

www.ask.com?
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Old 10-19-2003, 02:26 PM   #5
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Wait, are you saying that a shift light doesn't hook up to the ECU's "tachometer output" signal?
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Old 10-19-2003, 04:33 PM   #6
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Yeah I don't think noise has anything to do with it. The voltage on the ECU RPM wire varies as RPM changes. There's something in the shiftlight that when it sees a certain voltage, switches the light on. I'm not sure exactly what electrical component that is, but it uses the voltage somehow.
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Old 10-19-2003, 04:42 PM   #7
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Close.

The signal on the ECU's engine speed wire is a square wave whose frequency varies with engine speed...

So if you have a shift light that you hook up to that wire, it activates when the frequency exceeds a certain preset threshold.

I still don't understand what "pills" are.
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Old 10-19-2003, 09:02 PM   #8
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The "pills" are different settings for the shift light. They equate to a different resistance that allows the light to go off at different RPM's. There is a pill for 6000, 6200, 6400, etc, for example.

So, you just plug the appropriate "pill" into the back of the light and it will light up at the correct RPM.

AJ
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Old 10-19-2003, 09:51 PM   #9
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Quote:
Originally posted by midnx03
The "pills" are different settings for the shift light. They equate to a different resistance that allows the light to go off at different RPM's. There is a pill for 6000, 6200, 6400, etc, for example.

So, you just plug the appropriate "pill" into the back of the light and it will light up at the correct RPM.

AJ
Exactly... but my question is how to make a shiftlite instead of buying the whole unit (had one b4, and didn't like how mulky it was). I wanna run a progessive stream of LCD's in the intrument bezel when approaching reline. Now the question is how to do that?

Rich
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Old 10-19-2003, 10:00 PM   #10
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Ah...

Get an LM2917 chip. It's a frequency-to-voltage converter. It also has a built-in comparator so you can use it to drive a shift light pretty easily.

If you want a stream of LEDs you can couple it with an LM3914, which is a bargraph driver. It is basically a voltmeter whose output controls ten LEDs. Most "Air/Fuel Ratio Gauges" contain an LM3914, a couple resistors, and 10 LEDs.

A Google search can find you the datasheets on each of them, which also include some example circuits.

I've never disassembled a shift light but I would expect that most of those contain LM2917s (or LM2907s or LM3907s which are close relatives). Since the comparator would need a reference threshold voltage to compare against, the pills probably contain resistances that become part of a voltage divider setting the threshold.
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Old 10-19-2003, 10:03 PM   #11
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Well according to the pinout of the ECU for a MY00 RS (don't have the WRX pinout handy), the RPM wire varies from 0-13V in waveform. I guess the first thing you'd want to do is tap the RPM wire and hook it up to a multimeter. Check and see what the voltage and resistence is at different RPM points and see if you can find the pattern. Then find someway to have an LED light only when the voltage/resistence is in between a certain range.
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Old 10-19-2003, 10:10 PM   #12
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The voltage switches between 0 and 12-14 rapidly, with frequency proportional to engine speed. I don't know for sure, but it probably cycles once (or four times) per engine revolution.

A multimeter may not be able to give you a good indication of what's going on, depending on how fast it updates. Though not as accessible to most people, an oscilloscope can give a clearer idea.
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Old 10-20-2003, 01:37 AM   #13
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http://www.mini.org/nmd/namd/projects/tacho/tacho.html

this help a bit to the other people?

Rich
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Old 10-20-2003, 01:53 AM   #14
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Nice find, Rich! That's exactly what I was talking about!
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Old 10-21-2003, 03:48 AM   #15
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Quote:
Originally posted by vrg3
Most "Air/Fuel Ratio Gauges" contain an LM3914, a couple resistors, and 10 LEDs.
so you care saying a a/f ratio is determined by frequency also?
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Old 10-21-2003, 11:48 AM   #16
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No, sorry if I was unclear.

The LM3914 is a bargraph LED driver; you give it a voltage and it displays it on a bar graph of 10 LEDs.

Most "Air/Fuel Ratio Gauges" are just voltmeters; each of the ten LED represents an increment of about a tenth of a volt.
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Old 10-22-2003, 12:33 AM   #17
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Quote:
Originally posted by vrg3
Nice find, Rich! That's exactly what I was talking about!

Thanks to you guys, the correct term and number, and google...first seach, first page.

Rich
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Old 12-31-2003, 03:12 PM   #18
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Quote:
Originally posted by SJwrx
http://www.mini.org/nmd/namd/projects/tacho/tacho.html

this help a bit to the other people?

Rich
This site does not work anymore, it has been moved...
http://www.niksula.cs.hut.fi/~mdobru...cho/tacho.html

Rich
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Old 01-02-2004, 03:36 AM   #19
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Quote:
Originally posted by vrg3
The voltage switches between 0 and 12-14 rapidly, with frequency proportional to engine speed. I don't know for sure, but it probably cycles once (or four times) per engine revolution.

A multimeter may not be able to give you a good indication of what's going on, depending on how fast it updates. Though not as accessible to most people, an oscilloscope can give a clearer idea.
I was worried that such a high voltage signal might kill the 2917 - but I looked up the datasheet and it will be just fine. Only problem is the 2917 only has one built in comparator - and unless you're doing something really clever - I'd expect you'd need a comparator for each shift light. An LM339 could fill in this void nicely I should think. (quad comparator)


Anyways this could also be done with most any microcontroller - such as an AVR or a PIC (though AVRs are my personal favorite )
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