Ignition-Tach interface for Wideband Data Logger - Alfa Romeo Bulletin Board & Forums
 
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post #1 of 5 (permalink) Old 11-12-2018, 06:26 PM Thread Starter
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Ignition-Tach interface for Wideband Data Logger

Some Wideband AFR instruments will connect to additional signal sources and log them with AFR. RPM is the most useful parameter to have with AFR so that you can do WOT runs and see how the AFR varies with rpm and that enables you to make intelligent changes to Weber jetting. I have had a Zeitronix ZT2 on my spider for a few years and it enabled me to adjust the Webers to compensate for changes in cams, exhaust, etc. I did all of that with an MSD 6AL ignition which has a nice clean tach out signal that is well matched to the Zeitronix. Now I have removed the MSD and returned to inductive ignition and the coil -ve terminal is the only source of a tach signal. Unfortunately this signal is not compatible with the Zeitronix. The scope pictures show this signal with high voltage spikes and ringing but the Zeitronix wants a fairly clean square wave. So I set about building an interface circuit to modify the coil waveform. I wanted to use a simple low pass filter but I was unsuccessful and I reluctantly decided that it needed a more sophisticated circuit. I used a 555 timer chip configured as a monostable and after a little development it is working well. It should also work well with any device that monitors or logs RPM and needs a clean signal.

The pictures show scope traces of the coil voltage and the clean signal from the interface. The circuit diagram shows the components used and the pictures of the circuit board show the physical implementation. The potentiometer adjusts the sensitivity so that the signal is strong enough to trigger the timer but not so strong as to swamp it. If Alfar7 is reading this he should be pleased that it has a screwdriver adjustment. The board is mounted on a slope so that the surplus electrons drain from the empty corner

EDIT: I omitted a component from the circuit that is needed. It is a 3.3K resistor connected between the base of the transistor and ground.
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Ed Prytherch
79 Spider
76 Suzuki GT500
2011 Jaguar XKR

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke

Last edited by alfaparticle; 11-12-2018 at 06:46 PM.
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post #2 of 5 (permalink) Old 11-12-2018, 06:37 PM
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Thanks for posting this, Ed. Very helpful for those of us not using spark boxes with a tach output.

Rich Hanning
'65 Sprint GT, '78 Spider, '88 Quad
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post #3 of 5 (permalink) Old 11-19-2018, 07:44 AM Thread Starter
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Here is a screen shot of a portion of the Zeitronix log of RPM and AFR showing a 3rd gear WOT pull this am. The rpm trace is quite smooth and similar to the ones from the tach output of the old MSD box.
I made one modification to the circuit, adding a 470 mFd capacitor between pins 1 and 8 of the 555 chip.
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Ed Prytherch
79 Spider
76 Suzuki GT500
2011 Jaguar XKR

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #4 of 5 (permalink) Old 11-21-2018, 09:30 AM
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I wonder if a circuit like this would help our regular ignition driven tachs. They may be more robust, but a clean signal might be nicer. My Alfetta's tack is not so accurate when I compare it to a strobe. Or maybe it's just old...

Stefano
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post #5 of 5 (permalink) Old 11-21-2018, 10:48 AM Thread Starter
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I run my tach straight from the coil -ve terminal. It reads 10% high. I had problems when I ran both the tach and Zeitronix from the same tach output connection on the old MSD6AL. The tach would flop to zero above about 5000 rpm so I ran the Zeitronix from the MSD and the tach from the 123 distributor black wire. I will try running both the tach and the Zeitronix from the new circuit and report back.

Ed Prytherch
79 Spider
76 Suzuki GT500
2011 Jaguar XKR

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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