MoTeC Global Forum

Hi all!

I have an "old skool" M8 ecu that's done sterling service in my rally car, and although it's getting on a bit now, it works well

However, i'd like to "Tee off" the crank VR sensor signal into another device. A quick check with a scopemeter shows that the M8 pulls up the VR sensor pins to 2.5v (half 5v rail) with what appears to be 10k impedance.
Unfortunately, the second device that i would like to use the sensor signal for uses a ground referenced VR sensor (ie sensor -ve is held at 0v).

So, to the question, why does the M8 use a 2.5v VR Sensor offset, and what happens if i "externally pull down" the VR sensor to 0v ref??

i guess the positive voltage offset used by the M8 could be for various reasons i can think of, such as diagnostics, or noise / spurious trigger reduction etc??


Thanks in advance for any help / information

The M8 has dirrerential inputs on Ref & Sync to help noise immunity.

You could simply wire the REF- pin to sensor 0V this will have less noise imunity however it will still have a voltage offset on the REF+ pin

People with a feeble disposition, look away now!

Popped open my M8 for a look, and it just uses a std analogue comparitor (LM2903N) as a "zero crossing" detector. Hence the sensor bias to 2.5V (so no -ve supply is required). So rather than possibly mess up the input filtering for the crank VR sensor, i've just taken the output of the comparitor, fed it to a line driver IC, and sent it out a "spare" IO pin on the main connector. That way, i now have a TTL representation of the crank signal i can use for my external device!








(yup, i know, warranty void etc etc!!! Think you'll find the warranty ran out about 20 years ago

You can't leave anything alone !! lol