Speed monitoring tap wanted

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?
Richard
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

MPH
and
an
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...
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Or maybe mount a second ABS sensor by the ABS gear on one of the hubs. Then, there wouldn't be any chance of computer problems due to tapped sensor signals.

it.
I
the
radio
engine
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@prodigy.net wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

MPH
and
an
In my 1976 VW Rabbit, I put in an aftermarket "Rev Counter" ;) and it tapped of the + side of the Ignition Coil. Definately and analogue input.
Dan
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.