Help with circuit design

Discussion in 'The Projects Forum' started by Giffer2, Dec 20, 2008.

  1. Giffer2

    Thread Starter New Member

    Dec 20, 2008

    I'm after some help designing a circuit for use in my car and need to find the best way of doing it.

    I'm trying to add a switch to allow me to switch between 2 sets of connecting wires, I'll try to describe what is currently happening -

    Common rail pressure sensor (part a) has 3 wires normally these connect directly to the connector lead (part b), however now these connect to a box (part c) which changes the resistance to increase the fueling pressure, the box then outputs via another 3 wires which goes to the connector lead (part b).

    So what I need is the ability to chose to have part a connected to either part b or part c via a switch mounted in the car.

    Now here is the tricky part I have some needed and some ideal parts for this circuit -

    Needed -

    Switch must have indicator to show me what is connected

    Ideal -

    Switch can be operated whilst the engine is running - this is the tricky part as if you just flip the connections whilst the engine is running it causes the ECU to error as the common rail isn't getting a voltage, even if it's for a second.

    Having not designed a circuit in over 10 years I don't know where to start - Building it isn't a problem, it's just the design and parts I need help with.

    Thanks in advance.
  2. Søren

    Senior Member

    Sep 2, 2006

    If you make a resistance chain (perhaps followed by a buffer/amplifier) and let one resistor be shunted by a transistor or a relay, it would keep a voltage, although with a step in amplitude of course.

    An dual color LED could be used for positive indication of either/or.

    If you provide the details of the resistance box and the resistance range of the sensor, a more detailed solution and/or a schematic could be made.
  3. Giffer2

    Thread Starter New Member

    Dec 20, 2008
    That information might be a bit tricky as the manufacturer doesn't really want to give that information up and I don't want to sit under the car measuring the resistance range! (Whoever designed my engine thought it would be fun to hide things in odd places!)

    I'm thinking that the easiest option is to make a circuit that will allow me to switch it over before starting the car.