How to best isolate a sensor signal from a speedometer

Discussion in 'General Electronics Chat' started by klauzja, Aug 25, 2010.

  1. klauzja

    Thread Starter New Member

    Aug 25, 2010
    Hi Everyone,
    My first post here. I have visited/used many different forums, and I must say this is one of the most feature reach and advanced I’ve used. And the TEX editor rocks!

    I studied electronics many years ago, but have not actively used it for designing circuits for many years, since I changed professions…

    I have tried searching the forum looking for an answer to my particular question but have not had much luck, and there’s a tone of info here!

    Here’s my problem:
    A friend is installing an Audiovox Car Cruise Control on a Motorbike with a digital LCD speedometer output signal. The cruise control has a wire that needs to be connected to the speedo wire to pickup the speed signals/pulses. On most bikes you can simply splice into the speedo wire and all is well. On this bike however, splicing into the speedo wire causes the speedometer to stop functioning. I guess that the cruise controller’s circuitry “drains” the signal too much…

    My suggestion was to simply splice into the speedo wire with some form of Optocoupler device or a solid-state relay. One of the posts here mentions a MOSFET relay, which sounds like a good idea. I’m looking for something off the shelf rather than trying to build a custom circuit.

    So do you think this kind of MOSFET relay would work (out off the box) for this sort of an application? If not can anyone suggest something more robust that could simply be used in series with the cruise control’s speed sensor input wire that would not be taxing the speedo signal enough to cause it to stop working, whilst being fast enough to cope with the signal frequency?

    Is there a more simple/elegant solution for a layman to use for this? Simply plugging in different resistor values or a POT and hope to find a sweat spot setting seems like a long shot...

    Thanks for your help guys.
  2. SgtWookie


    Jul 17, 2007
    Well, you need a very high-impedance input, and a low-impedance output.

    You might use a comparator or a high-speed opamp (operational amplifier) to accomplish that. You should use components that are rated for automotive use.

    But before we can proceed, I'd like to hear from the Moderators. A cruise control installation on a motorcycle can have safety implications, as a failure could possibly lead to unintended/uncontrolled acceleration, possibly resulting in injury/property damage.
  3. beenthere

    Retired Moderator

    Apr 20, 2004
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
    That Mosfet relay is simply an LED drien photocoupler that drives a MOSFET output stage. No way would you be able to drive the LED if the actual controller overloads it.

    You're going to need to build a very high input impedance buffer amplifier circuit, that or there may be a switchable load built into the control that just needs to be changed.

    Any simple op-amp circuit with unity gain should work.