$1.49 LM2917 IC does tachometer, MFI Speed Switch, rev limiter, or shift light ...
 

For all of you electronic experimenter types out there ...

I was looking at the data for this IC normally used and recommended for tachometers ... and, lo and behold, it has other capabilities that come to mind for 911 applications ... only a few additional components are needed to have a complete, working 'black box' ...

Here is the site to download the datasheet and application note AN-162:

http://www.national.com/pf/LM/LM2917.html

Different versions are available for use with points [single-ended input] or magnetic reluctor inputs [SC and Turbo distributors] ... so if you have a non-functioning tach or speedo that is already apart, here is an easy way to revive it with some modern IC technology and just a bit of fabrication on a universal breadboard -- if you aren't inclined to make a circuit board from scratch! There is also a LM2907 version, but the LM2917 version is preferred because of a built-in Zener to regulate Voltage to the circuit.

Just a few 911 applications:

1. Tach repair -- replace old VDO board needing unobtanium parts ...

2. MFI Speed Switch/RPM Transducer/Speed Relay ... replace an ancient Bosch board that has seen better days ... at very low cost!!!

3. Adjustable rev-limiter/shift lamp ...

4. Speedometer repair -- for electronic versions, only -- easy calibration for new/different tire sizes ...

Sounds very interesting Warren, since my speedometer has been being flaky lately. I'm not very electronically inclined, but not a total clown either. Will you be ordering some of these and perhaps performing a procedure or two you might care to document?

ianc

Hmm. . . . all sorts of possibilities here. By converting ground pulses from the tach into a voltage, you could drive a meter movement, also known as the tach needle! By converting the same pulse to a voltage, as soon as it was exceeded, this could trigger a transistor, hooked to a relay to actuate the stop solenoid. . . .

Pretty cool Warren. Thanks.

Someone (I'm not an EE) figure the circuit and create directions to use a common low cost digital speedo with this...and a relay (voltage or frequency based) that will close a switch...like a 964 decklid on an earlier car for automatic raising of the spoiler...or something else that might be too graphic for the general Pelican Board :) ... something that makes the ear wheels spin faster :D

Thank again for the help with tranny information these last couple of days, Warren. And for the rest of the five years of advice I have cherished. It'd be cool if you'd consider performing these upgrades and enhancements and repairs for a fee. Let the speedo shops replace odo gears and repaint gauge faces. You repair the CD boxes, the MFI speed switches and the other stuff I don't understand like what you said above. I'd like a switchable CHT gauge. And a voltmeter, but even I know that's easy.

I second that.....If you can build a box that I can tap into my speedsensor & trigger my soon to be new 964 spoiler on my 84 911, I would buy one....providing i dont have to mortgage the house to do so....

So if you build one, they (me at least) will buy.......

Let me know, Thanks
Chris
chgrec@yahoo.com

Warren, Hi there. Funny you should write about this chip for this use. I have a current lunchtime project going of just such a shift lamp for my GT4 911. We are using exactly this chip; built on a breadboard with potentiometers for range and span adjust. Found the trigger points to be ratiometric to the input voltage, so we added a 10volt regulator circuit. Some protection diodes on the lines and.......
It works great on the bench with a function generator. I am using a small LED array for the light itself. The scope indicated system voltage (13v) of 3 pulses per rev on the tach signal line. When I start the engine, the light "flickers" a few times, but that is it.
Any ideas? I'd really like to get it going. Peter

Peter,

What kind of distributor signal are you using, i.e., points or electronic conversion ... or SC/Turbo magnetic reluctor signal???

Which exact chip designation [LM2917N-8 or LM2917N] are you using?

Also, do you have a sketchof the current breadboarded circuit you can post?

This is good stuff guys, i am going to lurk here and see how it turns out.
Not sure if I can add much, but I will if I see an opportunity to do so...;-)

Hi Warren, I am trying to use the circuit on a 2.8 RSR clone. It has a bone stock ignition system i.e. points in the distributor with Bosch CD box. The chip we used is the LM2907N. I guess we missed the fact that the other has the zener built in, so we added this externally. I am using the tach line as my trigger (Black/Purple if memory serves). I will clean up the beer napkin sketch so I can post it. I leave for Mosport tomorrow, so I will get it posted as soon as possible (it may not be until next week). The circuit is based off the drawing #26 in the National Semiconductor book (2-95). By the way, I am the mechanical Porsche guy and my colleague here at work is the EE.....Thus we! Anyone into this stuff, please feel free to write directly. Peter

Look into a Freescale 68c908qt4 8 bit micro controller and you could go nutz with these for auto hobby applications.

Built in A/D, timer, FLASH, Oscillator, Reset and the C compiler/emulator is free from Metroworks. I have played with several of these for applications like logging temerature, counting pulses etc and I have the cost of 5 parts invested (less than $1 each).

No idea what you guys are talking about (zener, ...) but I'd like to come up with a shift light for my electromotive crankfired / autometer setup that is not the real huge flashlight type that autometer makes.

Can you help ???

Great info! Anybody get it working yet? I just replaced my '71 E tach today that has been woking intermittently with a spare T tach... works fine except for the wrong rev-limit redline. Would like to fix the old one if possible. Evren. ps: Warren thank you for the CDI help!.. :)

Guys, Sorry for the delays. I'm still working on posting a copy of the schematic. Peter

Greetings, Don't know if anyone is still into this thread, but I finally got drawings of our circuitry. I have it in .jpg and a couple of other formats. Can anyone help me on how to attach or upload to the thread? Otherwise I can private email. The drawing includes the connector, light, and car side as well as the main circuitry. As I had said earlier, it worked on the bench, but only flickers on cranking in the car. I feel we are close, but maybe some E.E. can tell me where I went wrong.
Peter

Peter,

Click the 'post reply' button lower right.

Click 'Click here to upload a photo'. Browse to where your .jpg is.

That is all.

ianc

I,m sure many people think this is very intereresting ! but i had nothing to add to this thread so i diden,t reply. Please keep the info coming this is a great thread :)


Cheers,

yep... still here, lurking

yes, very interested.... would appreciate it if you could email schemetic.

Hi Folks, Well it's taken me forever, but I finally have it. Here is a link to the schematic.

http://innoves.com/pmaehling/20050817_ShiftLight.pdf

The left 2/3 is the actual shift light circuitry; on the right is a voltage regulator and the 4 pin connector showing the integration into the vehicle. I can try to explain bits of it as needed. Remember this took a while because I am the Porsche mechanical guy and my friend here at work is the Electrical Engineer. He quickly made the drawing, so I hope he got it correct. We used potentiometers so I could adjust the RPM trigger point and a second to adjust the hysterisis for turning the lamp back off.

Now the bad news. It works on the bench, but only flickers on cranking when in the car. I am running a totally stock ignition system in my 72 RSR clone. Any thoughts are welcome. Feel free to steal as desired!

The trigger is the tach line, which is 3 pulses per revolution at system voltage (12.5 or so).

"Only flickers on cranking."

Do you mean your light flickers on cranking, but at the selected RPM is off? On? Why should it flicker on cranking anyway? Or did you set the pots for a very low RPM so you could test it at cranking speed?

I think Zeners have car poltergeists. I built an LCD panel meter to read my O2 sensor (before I realized this was pretty much useless information with the inexpensive sensors anyway) and powered it with 9V. I built a Zener regulator to get the 9V I needed in a package which would fit on the top of a 9V battery connector (so I could also use a battery to run it if using car power introduced fuzziness). It worked great from my bench power supply. In the car it didn't work, even from the car battery with the engine off. So on that anecdotal evidence I curse zeners. [I ended up using 9v batteries, but kept forgetting to turn themoff when not using the car. And in hot weather the front mounted oil tank heated the display to where it went all black - LEDs would have been better. So I haven't installed an ignition powered relay to connect the 9v battery to combat forgetfullness.]

But what if you use an external power source in the car?

I breadboarded a Tach D/A converter and a separate four comparator chip and four pots to see if I could get a speed signal to lock out gears successively as speed increased and avoid missed shifts. Problematic was the low frequency involved even at fairly high speeds (800 wheel revs/mile x 6 CV bolts, you do the frequency math at any speed you might want to pick) and the hysteresis needed to make sure things like solenoids didn't chatter away. I suppose that at engine speeds x 3 the frequencies of interest (redline or close) are less of a problem?

Walt Fricke (with yet another half done project)

Thanks Peter ! sounds good Walt will try that. Warren any idea,s ?





http://forums.pelicanparts.com/uploa...1124833776.jpg

I'm not understanding the voltage regulator shown in the diagram. Where is the regulated output to the LM2907? Is it from pin 3 of the LM 2940 to the other places marked "12 v"? If so, the regulator needs more than 12 volts input to get a regulated 12 volts out. Here is a possible explanation for the flicker: When the starter is cranked, every time there is a compression stroke, the starter sucks in more current, dropping the battery voltage a slight amount. If this dip in voltage goes below the 12 volt level, the regulator won't regulate during this interval. This could upset the LM2907 during the cranking interval.

But I could be wrong...

Tacho circuit replacement
 

Warren,

I have used this chip to repair my tacho. I could not find replacement parts for VDO circuit so I decided to build my own using breadboard. I downloaded the data sheets and also found a useful article at http://www.rgp.nl/bird/BritishWheels.htm
(BTW there was one typo in the circuit dig, where PIN12 should be grounded also).

To check that it worked on the bench, I had to use a frequency generator, and then adjusted the Pot Resistor to match signal revs with dial reading.

Here are pics of how I attached the breadboard to the rear of the dial.

http://forums.pelicanparts.com/uploa...1124891922.jpg

http://forums.pelicanparts.com/uploa...1124891936.jpg

Greetings all, I am very excited at the prospect of potentially finding the problem with my shift light! To answer Walt; I have the revs set to about 6000 RPM on the bench with a frequency generator. It is literally at the start of cranking that it flickers a few times and that is all. While out on track, it does not illuminate ever (my rev limiter is 7300). The circuit is wired into the car power, so Walt's idea of an independant power supply is interesting. As for Jim's question of the regulated output, we don't explicitly show the lines. The regulator output is shown as 12v on pin 3. At the top of the circuit, you will see 3 other points that are the recipients of the regulated power, also marked as a 12v pad. By the way, as a result of Jim's question, we noted that 12 volt is an error. It is actually 10 volt, so as to have some headroom. Hopefully soon we can update a revised drawing (point out any other discrepancies). Keep firing questions cause I think we can have a nice little device for about 5 to 6 dollar total investment! Peter

Peter,

I see one major, and a couple of minor problems ...

The datasheet for the LM2940 regulator specifies that a minimum value of 22 mF must be used on the output terminal ... or instability will occur. You can change the value of the capacitor, or just add a 22 mF tantalum cap to that output terminal.

Your '72 CDI uses nothing but a 33 Ohm pullup resistor as a switched load for the points, so minimal transients [no inductive spikes at all] will be present on the input ... so the two diodes on the input line at pin 1 of the IC are really not needed, or you could put a single SA15A Transzorb (rated at 15 Volts) there. The 12 Volt Zener could actually introduce switching noise into the circuit rather than eliminate it from the signal!

The transient protection and hysteresis adjustment circuitry are probably superfluous because of the Voltage regulator IC, and I suggest eliminating those components until you have the 'basic' speed switch circuit (fig. 9) shown on page 8 of the application note below.

Likewise, I suggest driving the LED or lamp from the output terminal 8, so you can eliminate the PNP transistor and its' bias resistors.

Once you have a working prototype when connected to the ignition system in the car ... some probing with an oscilloscope should be able to tell if any noise or transients are present that need to be taken care of.

http://forums.pelicanparts.com/uploa...1125088012.jpg

Warren, This is GREAT stuff. I have only a hazy understanding of your advice. As such on Monday I'll relay this info for further guidance and testing. Kinda' goes along with Walt's mentioning zener voodoo. I'll have one more opportunity to test on the track next month.