Non-CAN Tach input

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bravo685
Posts: 13
Joined: Mon Nov 28, 2011 12:41 pm

Non-CAN Tach input

Post by bravo685 » Sat Apr 07, 2012 10:38 am

Lance,

VR3 is not in the build guide for the 4L60E. Where will I find this information for assembly?

Also will this circuit be assembled differently for a Square Wave (Hall Switch) input vs. a Sine wave (Variable Reluctor) sensor?

Rob

Bernard Fife
Posts: 1696
Joined: Fri Apr 04, 2008 1:28 pm

Re: Non-CAN Tach input

Post by Bernard Fife » Sat Apr 07, 2012 3:52 pm

Rob,

The circuit would be built the same as the VR1 VSS input circuit for a VR sensor. For the equivalent component numbers, see: http://www.msgpio.com/manuals/vr.htm or see the 4L80E guide by Dave Hjort here: http://www.msgpio.com/manuals/mshift/4L80.html

This is meant to convert a variable amplitude AC signal to a 0-5V signal suitable for the processor. It will also work with a hall sensor, but it is a bi9t of over-kill. If you have a Hall sensor, you don't need such an elaborate circuit.

If the Hall output is nominally 12V (but is actually the car's charging system voltage), you can use a simple voltage divider and a Zener diode to cut it to less than 5V. (For the 9S12 processor, 'low' is 0 to 1.75 volts, 'high' is 3.25 to 5 volts ). You need to make sure the signal never exceeds 5.6V, so you likely need a voltage divider circuit. To do this, put a resistor (about 1K ought to do it) in between the signal and the processor, and a 1K Ohm resistor between that resistor and ground. These will cut the voltage in half. Then to get the voltage down to safe, stable level, use a 5.1V Zener diode (such as 1N5231BFSCT-ND) connected to ground on the non-banded end, and connected between that resistor and the processor on the banded end.

If the Hall output is 5V, you can feed it to the processor through a 1K Ohm resistor (any wattage is fine since the processor pin is high impedance - little current flows).

If the Hall sensor is 'open collector', then you need a pull-up circuit (voltage supply and current limiting resistor) and you can use the 5Vref for a 5 volt signal that needs little conditioning.

Lance.
"Never wrestle with pigs. You both get dirty and the pig likes it." - George Bernard Shaw

bravo685
Posts: 13
Joined: Mon Nov 28, 2011 12:41 pm

Re: Non-CAN Tach input

Post by bravo685 » Sat Apr 07, 2012 4:09 pm

Lance,

Using the VR1 circuit, will I still require a pull res. if a open collector hall is used?

Bernard Fife
Posts: 1696
Joined: Fri Apr 04, 2008 1:28 pm

Re: Non-CAN Tach input

Post by Bernard Fife » Sat Apr 07, 2012 4:32 pm

Rob,

Yes. You will need a pull up on the Hall input if used with a VR input. The VR circuit(s) take any voltage greater than about 0.05v and turn it into 5V (i.e. it will turn 1.5 Volts into 5V, and it will turn 75V into 5V). It also turns any voltage lower than about 0.05V into 0V.

The pull-up can be either 5V or 12V, it doesn't matter. Put a ~100-120 Ohm resistor (if using a 5V pull-up source) or a 220-330 Ohm resistor (if using a 12V pull-up source) in the path from the pull-up voltage to the collector to limit the current.

The circuit the way it is set for the VSS VR sensor is very sensitive to low voltages (to accurately capture the 'zero-crossing'). For a Hall sensor, you might want to reduce it's sensitivity a bit. To do that, you replace R39 (680K) with a 220K Ohm resistor and R53 (680K) with a 300K Ohm resistor. The other components are the same. This will make anything below 0.7 Voltage be 5V, and anything above that be 5V, so small voltage fluctuations near ground won't trigger the circuit.

Lance.
"Never wrestle with pigs. You both get dirty and the pig likes it." - George Bernard Shaw

bravo685
Posts: 13
Joined: Mon Nov 28, 2011 12:41 pm

Re: Non-CAN Tach input

Post by bravo685 » Sat Apr 07, 2012 5:01 pm

What I'm wanting to do is use the Tach output from an HEI distributor to run the GPIO.

Have you tried this or know anyone who has? Using this method the tach output is the Neg. side of the ign. coil.
My other option is to use the pick up coil in the distributor which is a VR output. I'm just not sure if it will cause any issues with the OEM ignition module tapping into it.

Once the trans is complete, I do plan on adding fuel injection. With this I can send a tach signal from the ecu, I think I would prefer to use the VR1 circuit to have the option of both square wave and sine wave inputs.

Your thoughts or experiences?

Bernard Fife
Posts: 1696
Joined: Fri Apr 04, 2008 1:28 pm

Re: Non-CAN Tach input

Post by Bernard Fife » Sat Apr 07, 2012 5:58 pm

Rob,

The tach output from the HEI module is the negative side of the coil, as you say. The coil causes a quite large spike in the voltage when the coil is shut off (which is when the spark is produced). This shouldn't hurt the VR circuit, though I haven't tried it. If this was mine, I would probably use the VR circuit with the R39 and R53 changes (above). There is plenty of resistance in the input math (10K plus 220K) to limit the current.

You don't need a pull-up with the HEI tach output - the coil voltage is effectively a pull-up.

I would be wary of trying to share the VR sensor - it might cause more trouble than it is worth.

Lance.
"Never wrestle with pigs. You both get dirty and the pig likes it." - George Bernard Shaw

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