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HKZ Bob's Avatar
 
Join Date: Nov 2006
Location: North Carolina
Posts: 676
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Dear all
in order to get a nice square signal I am going to build a schmitt trigger.

This is an educational thing. The 3 pin cdi trigger is so full of noise of the DC/Dc convertor. I hope this will prevent the noise to couple in.

Here my proposal.





and here the function behind.


It should produce a a small defined square signal. I hope the pulse is not to fast.


Let me know if I am on the wrong path.

bob

Old 06-07-2009, 09:23 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #41 (permalink)
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Location: Eindhoven, Netherlands
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Max,
Great thread, extremely useful! Experienced exactly the same problem, opened up the RPM Transducer and also in my case D1 was missing (and one of the legs of T1 broken). With what part exactly did you replace D1 (zener 8.4v?). How exactly should I put the diode back in?
Thanks,
Ed
1972E Targa (back to life after 18 years of sleep)
Old 01-17-2010, 12:30 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #42 (permalink)
Max Sluiter
 
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Join Date: Mar 2006
Location: So Cal
Posts: 19,574
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In the schematic, what is shown as "D1" is actually a transistor on the board. The transistor functions as a diode and one leg is not soldered to anything.

We replaced that "D1" transistor with an 8.2 volt Zener diode. It was either a 400 milliWatt or a 1 Watt diode.

The cathode (marked with a band on the body of the diode) of the diode goes into the hole in the board labeled "E" for the emitter of the transistor.

You will need to adjust the values of R5 and R6 to tune the relay operating range to the desired RPM values. I did this with a potentiometer soldered in for R5 and R6 during testing. I found the resistance that worked for the RPM I wanted and then got the two resistors that combined for the closest value to that potentiometer setting. These became the permanent components.

For your reference, I wound up using a 3.3K resistor for R5 and a 1K resistor for R6.

Also, I removed C6 from the circuit as it was causing the relay to "chatter". I never figured out why, but it seems to work fine without it.

Let me know if you want more help.

Tschüss (if the Dutch also use this)
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1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/

Last edited by Flieger; 01-17-2010 at 01:17 PM..
Old 01-17-2010, 12:58 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #43 (permalink)
Max Sluiter
 
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Join Date: Mar 2006
Location: So Cal
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Do you have acess to a signal generator to make a 12V DC offset square wave? If not, you will need the distributor points simulator posted above. You will need to hook up the board to 12V power and the distributor simulator. Adjust the signal and use an occiliscope to see when the realy operates. You can correlate the frequency of the square wave pulses to the RPM that the swtich will operate.
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911S
1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/
Old 01-17-2010, 01:21 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #44 (permalink)
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Join Date: Jul 2017
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Reviving an old but great thread... Thanks to all information in here I was able to easily repair my broken RPM transducer. One transistor broke right off, but as I replaced it, and carefully looked at the others, it turned out that three needed replacing. One of them was the D1 zener diode, but I've just replaced it with a regular NPN transistor. I used BC549-C for all of them, which seems to be the "default" low-power NPN transistor in Europe.

Because I don't have a signal generator, I've just tested the repaired board in the car, and it worked perfectly right away, outputting 12 V to the switch when > 1500 RPM, and going back to 0 V below 1300 RPM. It was quite hard to manually get the engine to an exact RPM level slowly, so these are approximate values, but it seems good enough to me. Connected everything and if you manually press the switch when the engine makes around 2000 RPM, it starts going up and down in a cycle just as described in CMA. A short test drive confirmed that everything is OK. I'm happy! Thanks everyone for the contributions in this thread (and others), it really helped.

Anyway, it looks like the voltage set by D1 is not that critical (unless I was just lucky) and it's the C4/R5+R6 combination that matters. I didn't need to touch these at all. Maybe this will help others.

Old 06-12-2019, 11:36 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #45 (permalink)
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