Hall Effect VSS Setup

[shaodome]

So I have this hall effect (3 wire) sensor for the VSS on this toyota box. The signal it puts out is battery voltage when active, but I believe it floats when inactive.

Initially I build the VSS input on the GPIO as a voltage divider with a 5.6v zener, but the circuit doesn't appear to trigger at first glance. If I tap the input for the VSS (amp pin 2) on a 12v from the power supply it works. I'm not sure if this means I need to put some sort of pulldown resistor on the VSS input?

[shaodome]

OK definitely doesn't need a pullup.

When I connect the hall effect sensor to the input on the GPIO the measured output voltage drops from around 13ish to 9v, which equates to a pretty small voltage at the processor with my current divider circuit (same as brake sense per BOM). I'm using a 10k/15k/5.6v Zener combo. The measured voltage at the processor pin is only 3.5ish volts. I'm not sure if that is enough to trigger the VSS? I'm not sure what's causing the voltage to drop though as soon as I connect it to the ampseal VSS pin?

[Bernard Fife]

shaodome,

I am not sure I understand completely, so please bear with me. The processor pin needs a voltage below no more than 1.75V to be 'low', and a voltage above no less than 3.25V to be considered 'high'. It is probably more reliable to get the low voltage below 1.00V, and the high above 4.00V (your measured 3.25V is just within spec assuming the voltmeter is 100% accurate, but not ideal - I would want it higher on my installations).

So if you are using a voltage divider and your voltage range is from 9V low to 13V high, then you need a ~9x divider factor and the 13V won't be high enough (13/9 = 1.44V). But I don't know if you were saying the range is from 9V to 13V or the amplitude varied between 9V and 13V. If you are saying the later, then you might want to measure the 'low' voltage as well. Ideally, a scope shot from the processor pin would help.

The voltage drop from connecting the sensor will depend on the sensor itself, as well as the resistance values used in the voltage divider. Higher resistances will reduce the voltage drop (i.e. 100K/150K instead of 10K/15K). That assumes there are no problem with the circuit (excess residual flux, bridging, etc.).

Lance.