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Tachometer adaptor circuit


Vertigo

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Sup guys. Ive built the following circuit to translate what I thought was a 5v pull-high square wave tach signal into the signal that the AW11 tachometer requires, originally derived from the -ve of the ignition coil - 12v pull-low square wave + voltage spike.

FBCQj3e.png

The idea is that the tach sees 12v most of the time, then when the transistor is closed briefly, it sees 0v and the inductor is charged. When it opens again, the tach sees 12v again, the field in the inductor collapses from lack of current, and spikes a voltage. This gives the correct signal to the tach.

It doesnt work, because the output from my coils IGF line is actually 5v pull-high square wave instead - I needed to have 5v supplied to the IGF line, where the coils switch it to ground. I can make this change easily enough, but that means that the output signal will be inverted to pull-high instead, so I attempted to modify the above circuit. See below:

0Z9cUu6.png

Ill admit, this is a guess. I have adjusted the transistor trigger appropriately, but Im unsure about the 12v side. Need to connect the tach output to the inductor, but seems to me that the inductor will still need a fairly free flowing path to charge it, which is now missing. Any ideas here?

The other option I guess I could do is to invert the inversion with a second transistor stage, but that seems over the top.

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As drawn, you'll only get 5V - V(be) of the xststor (4.4v) over the inductor. When it turns off, there's no path for the inductor current to 'kick' voltage to the tach. It'll just try to maintain the steady current (if any).

Probably easiest to put in the 2nd transistor as you're suggesting. Level translations can be surprisingly tricky.

Good diagram BTW, nice n clear. (Hand sketch is good, too).

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I have redesigned the circuit with a second stage, though I am not sure exactly how to separate the two 5v lines - Ive put resistors in as a guess, but this isnt really my forte. Can you advise?

The rest of the circuit should be sound. Added some waveforms and helper indicators in red and blue, and wires with grounds in grey overlay.

Here is going high:

EbUZOMk.png

And going low...

ia71sca.png

 

Edit: Actually, the spike is triggered on voltage going high, not low, because its there that current flow drops to near zero. But thats just the diagram indicated incorrectly. The circuit remains valid.

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Ive decided that dealing with four IGF lines and a 5v supply wire and a second transistor stage is just too finicky to bother with. Im sure it would work, but Ive decided to go the route of the Megasquirts own tach output function - just need to solder a wire into the board.

Last night I tested the original circuit in the first post with a square wave function generator output of my oscilloscope, and it works flawlessly - tach responded to frequency tweaks from 10hz to 200hz @ 10% duty cycle (actually, up to 50% duty cycle was fine also, and Im sure 90% would work too, but I prefer pulses to be as short as possible). Above 200hz, the tach wanted to go way out of bounds.

Tonight I finish the board modification and test with the car running.

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