Tach Output GPIO | AEM
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andyftp
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Tach Output GPIO

I wired originally to use GPIO 1 for tach output, but I can't seem to get it to work.  It's pulled up to 12v w/ 2.2k resistor.

Can the GPIO pin1 generate the square wave needed or do i need to move to pin 5+ for that capability?

andyftp
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I read the tach FAQ.  But I'm

Maybe I need to try a lower value after re-reading faq

andyftp
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Tried a few different

Tried a few different resistors to pull up. Got nothing. Put the tach on my bench and tapped +12v to the trigger input and the gauge needle moves.

So something is odd to say the least. I'm going put a 10k pots instead of the resistor and run leads out from behind the dash to try and see if I can't find some value that will work. Failing that, maybe use a mosfet as a switch for the 12v

andyftp
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Put it on a scope and I am

Put it on a scope and I am seeing a nice square wave.  

 

Replaced the 2.2k with a 10k pots.  Down at 1k with care off I get 12v to the tach.  As soon as the car is started and GPIO is in the picture, highest voltage I am seeing is 3v to the tach.  

Would it be safe to drop below 1k?

AEM_NS
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Sorry but we don't have any

And what kind of tach are you trying to use?

andyftp
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It's a Nippondenso from a

It's a Nippondenso from a Mitsubishi Starion/Colt...

AEM_NS
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It probably needs a signal

It probably needs a signal directly from the ignition system. Check appropriate wiring schematic to verify. Probably won't work with a square wave unless that's what it triggered off of in stock form.

andyftp
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I do know that others with

I do know that others with same car and factory tach say it fires off a megasquirt tach output with a 2.2k pull-up to 12v. Way old posts on the forum from 12+ years ago I see people using old haltechs saying they needed to use a coil relay to make it fire.

I can get the tach to read by tapping 12v quickly to the signal input on a bench.

I'll have to monkey around a bit. 1k on my 10k potentiameter will get me 3v, anything of higher resistence the voltage is lower.

So I'll have to try lower resistor value or maybe a relay or mosfet. The mosfet might be better as protection to the ems-4 than too low resistor value?

AEM_NS
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Well a GPIO is good for a 1

Well a GPIO is good for a 1 amp continuous (limit will be resistors power rating) but it's not going to have an output signal like you'd get with an inductor. I don't know what a megasquirt tach output has in its circuit - perhaps special hardware to work with tachs that need a higher voltage signal. Just because it works with megasquirt doesn't then mean it'll work on an EMS-4. I've personally never been able to get tachs that expect a coil primary signal to work off of a square wave. You might save yourself some trouble and try implementing the relay coil inductor.

andyftp
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Worked! I wired a n channel

Worked! I wired a n channel mosfet.

gate: gpio 1
drain: tach to 2.2k resistor to +12
Source: ground

Revs match exactly to the readout in aemtuner.

andyftp
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Just to note, it fires from

Just to note, it fires from coil negative. Thought I mentioned that.