UK Speed limits are driving us mad - pun intended! They prosecute at 33 MPH
in a 30 limit.
And my TDCi Focus Ghia is so high geared and quiet, and the speed looks and
feels so slow, that I find that I creep up to 35-38 without realising it.
So what I want is a 33 mph warning. A quiet peep would do nicely. I'm an
ex-electronics engineer so designing something isn't a problem. But what I
need is a speed signal. What's the best tapping point to get a speed
reading. It should be easy as I have a trip computer, but where are the
cables and what is the signal - analogue, digital, pulse?
The radio gets louder as road speed increases, can this input to the radio
be used, what's the signal format?
As I recall, a hall effect sensor, mechanically connected to the
transmission is used. I'd imagine it's squared up and appears at the engine
control computer. As the cops get greedier and greedier, there might be a
market for such an indicator...
It's a piece of cake for anyone who has a rudimentary understanding of
electronics to safely tap into a VSS signal without loading the circuit.
The source impedance isn't particularly high, nor is the signal difficult
to trigger off, since it is a nice clean square wave. So unless you
put a fairly low impedance load on the signal, you're OK.
I once designed and built my own electronic speedometer for an '83 Olds
that had a defective mechanical speedometer. I tapped into the VSS
signal at the speedometer head, and used it to gate a counter that drove
the address lines of an EPROM, the EPROM data was two BCD nybbles (?)
to drive some TIL311 latched LED display units. It worked very well and
had no effect on the ECM.
Nowadays, I would lean toward tapping the OBD-II bus instead, although
it's a lot more work than just hooking up to a VSS signal, there's a
lot more interesting data there that would come along for free, to be
displayed on a multifunction LCD screen.
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