Date: Tue, 3 Dec 2013 19:41:03 -0500
Reply-To: James <jk_eaton@HOTMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: James <jk_eaton@HOTMAIL.COM>
Subject: Re: Arduino and Vanagons
In-Reply-To: <CAPAEXFcj+_N6SfdGswTidzOKBidp9YPW0wEcOJ0HFm+imX2D9A@mail.gmail.com>
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I'm not surprised that your homemade voltage divider grounded out the Hall Effect sensor signal - typical sensor signal is under 10 milliamps at 5 volts (though maybe VW used 12), and feeds an op-amp (think, very high input input impedance, maybe 1 Mohm). Putting 13.3 kohm in parallel with 1 Mohm diverted nearly all the signal away from the ECU. The existing ISV control box probably uses an op-amp as an input stage, and our replacement will need to as well. (The actual Hall Effect signal is around 30 microvolts, typically, and has already been amplified inside the Hall Effect sensor's circuitry up to a 'useful' level.)
The 10 kohm resistor should have been more than enough to protect the Arduino's inputs from harm. (Remember, resistors are current limiting devices, and it's too high a current that damages a microcontroller, not just the voltage.)
It's fascinating to read your progress reports. I predict that your oscilloscope check on the output of the Hall sensor will produce a square wave of about 5V, with an on period of about 50 microseconds, in a 2 millisecond period. (Duty cycle of about 3% on a 400-450 Hz square wave signal.)
James Eaton
Ottawa, ON
> Date: Tue, 3 Dec 2013 14:18:10 -0800
> From: brettn777@GMAIL.COM
> Subject: Re: Arduino and Vanagons
> To: vanagon@GERRY.VANAGON.COM
>
> Just a quick update.
>
> I bludgeoned together an Arduino program to calculate and display the
> engine speed based on the Hall sensor output(my programming "style" makes
> hackers' code look elegant). To deal with the presupposed high voltage(to
> a microcontroller, anything over 5v is high voltage) from the Hall sensor ,
> I put a 10k and a 3.3k resistor in series with one end connected to the
> Hall output and the other to ground. The Arduino input pin was connected
> between the two resistors, which should give a voltage output of 1/4
> whatever the voltage is coming from the Hall sensor. I tried hooking it up
> a couple of times with the engine running and it stopped it cold
> immediately each time. That is one *weak* signal! A 13.3kohm path to
> ground shuts it down. Just to live life on the edge, I removed the 3.3k
> resistor going to ground and tried again. This means that the full voltage
> coming from the Hall sensor will be going to the Arduino input pin(it's
> only $2-3 for a new chip if I burn out the pin). This time, the engine
> continued running without complaint and I was getting rpm readings on the
> LCD. WooHoo! The readout was too high and very unstable, so there is
> still much work to be done in this first step. I'll have to recheck my
> code; my formula was based on timing 8 pulses, but I may have set the
> counter wrong and it's counting only to 7 pulses, giving a higher than true
> reading. I think the the instability has to do with the handling of a very
> weak signal.
>
> Sometime in the next day or two I'll drag my laptop out and hook up my USB
> oscilloscope to it and get a better idea of what this Hall sensor output
> looks like. Because the Hall signal is so easily disrupted, It looks like
> we'll need to add an op-amp voltage buffer in order to be able to use this
> signal without affecting the ignition system.
>
> This gives yet another option for those thinking of installing a hidden
> vehicle disabling system for security. Tie in a wire to the Hall output,
> run it to a hidden toggle switch that switches it to ground via a 5k
> resistor. Less obvious than a fuel relay disabling system. It'd take a
> good mechanic at least a half hour to find it. An average mechanic would
> take several hours and would have replaced the engine computer, coil, cap,
> rotor...
>
> I've started looking into the circuitry required, but am not to a stage yet
> where I think it would be fruitful to report.
>
>
> Brett
>
>
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