Relay control by adjustable Voltage input

Discussion in 'The Projects Forum' started by Graphite SRT, Dec 10, 2008.

  1. Graphite SRT

    Thread Starter Member

    Jan 11, 2008
    23
    0
    I am in need to build a circuit

    Heres what it needs to do.

    First i need something to bring 5 volts down to 3.8 volts

    I need a 12 volt relay to be activated at and adjustable voltage (0-5 volts) that i can show that voltage on a little display.

    What this does it take input from a cars map sensor ( the adjustable input that is displayed) and activate a relay that takes and isolates the TPS signal wire and sends 3.8 Volts down it instead of the Throttle Positions output.

    I hope i explained this well enough.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    This could be a 10k pot, depending upon how tolerant the source is to a load. If the output signal will drive something, an opamp could be used as a buffer amp/voltage follower; output connected to the inverting input, and the pot's signal connected to the noninverting input.

    That could be accomplished using a voltage comparator like an LM2903. One input gets a reference voltage level, the other gets the input signal. The comparator's output could control the base of a TIP120 to switch the relay's coil. You could use a cheap digital multimeter from Harbor Freight Tools as the display, or spend $20-$25 on a dedicated DMM display from someplace like Digikey or Mouser.

    Why do you want to do this? What do you hope to gain?
     
  3. Graphite SRT

    Thread Starter Member

    Jan 11, 2008
    23
    0
    The multimeter is a good idea as they can be had for cheap

    The purpose of this is to send 3.8v Wide open throttle to the computer through the tps wire when any boost if detected. SRT-4s that have part throttle boost become a problem as anything other then WOT the pcm tries to command a 14.388 Air fuel ratio which is too lean for boost. With this mod it will put the pcm into the power enrichment table( WOT ) when their is any boost.

    And now that i think about it the 5v to 3.8v is not going to be a problem as i can just use another TPS and glue it at 3.8 volts.

    The only thing i need is a way to activate a 12 volt relay with a 0-5v adjustable signal and then i will display it with the volt meter that i take apart.
     
    Last edited: Dec 10, 2008
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Every couple of months they go on sale for $2.99. It's cheaper to buy a new one than replace the batteries or test leads.

    That makes sense if you added a turbocharger or supercharger.

    A trimmer pot would be much smaller, lighter, cheaper, and perform the same function - output a fixed voltage. Power it from the existing TPS connections.

    An LM2903 dual comparator is likely your best option, as they are very cheap and are rated for automotive temperatures.

    Where are you getting the boost signal from? What voltage is it?
     
  5. Graphite SRT

    Thread Starter Member

    Jan 11, 2008
    23
    0
    if a trimmer pot is cheaper then i will use that.

    I am getting the voltage signal from the map sensors output. Here is a table of the SRT-4 sensors

    I need to be able to adjust between 1-8 psi on either sensor. I have the stage 0/1 now but will be upgrading to the stage 2/3 later. So the trigger input will have to be adjustable between 1.65 Volts and 3.15 volts

    [​IMG]
     
  6. Graphite SRT

    Thread Starter Member

    Jan 11, 2008
    23
    0
    can anybody help me design a circuit with the trimmer pot to bring 5v to 3.8v and the LM2903 dual comparator to activate the relay.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I'm still with you. I have a number of things going on.

    Rather than using a relay (which is power hungry, relatively large, slow, expensive and electrically noisy) I'm considering using an analog switch, such as a CD4066B. This would require an extended temp range IC, and also an opamp used as a voltage follower.
     
  8. Graphite SRT

    Thread Starter Member

    Jan 11, 2008
    23
    0
    you would know better then me
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Ok, here's a (somewhat ugly) schematic.

    See the attached.

    The upper left corner is your basic voltage regulator. I suggest an LM317AT, for greater temperature range. The LM317xT is maximum overkill for the current being used, but this will minimize your thermal rise. R1/R2 should be metal film resistors for lowest noise and best reliability (actually, all of the fixed resistors should be metal film). Nominal output voltage will be 6.8v to 7.4v, depending upon the individual IC's Vref; the voltage regulation is more important than the absolute voltage. I don't recommend the 78xx series fixed regulators for this reason; they aren't as stable.
    As long as your battery voltage stays above about 8.7v when starting your engine, you'll get decent regulation.

    I simulated your MAP sensor output using a simple 0v to 4v ramp.

    For simplicity, your TPS input was represented by a fixed 2v source.

    R3 sets the threshold level for the comparator U2A.
    C4 helps reduce the resistor "noise".
    R8 provides hysteresis feedback, to keep the comparator from oscillating at the trip point.
    R4 provides the source current for U2A's open-collector output.
    VcSW1 and VcSW2 represent a couple of channels on an MC14066B (ONsemi/Motorola) or CD4066BM (National Semiconductor) quad analog switch; my simulator didn't have that particular IC. Unused inputs must be tied to Vdd or GND.
    If the output of U2A is off, VcSW1 will be turned on, and Q1 will conduct to turn VcSW2 off.
    If the output of U2A is on, VcSW1 will be turned off, Q1 will be turned off, and VcSW2 will be turned on due to R5.
    R7 is adjusted to output 3.6v.
    U3, an LM2902, is an automotive temperature equivalent of an LM124/LM324 quad opamp.
    It's used as a voltage follower/buffer amplifier. There will be a small error in the TPS signal level introduced by the offset voltage of the opamp, likely somewhere between 3mv to 7mV. You may need to adjust your TPS rotation slightly to compensate. The LM2902's output is not rail-to-rail, but it can get within 20mV of the ground rail (some may go within 5mV under light load) and with Vcc=7, can go as high as 5.5v with a light load.

    U2, U3 and the 4066 should have a 100nF cap directly across their Vdd/GND pins.
    The unused opamps/comparators need to be connected similar to what's shown; opamps output to inverting input, noninverting input to ground; comparator inputs to Vcc and GND. Otherwise, they may oscillate at high frequency.
     
    Last edited: Dec 12, 2008
  10. Graphite SRT

    Thread Starter Member

    Jan 11, 2008
    23
    0
    looks good
    I don't know if im reading it right but it lokks like you have the MAP sensor output as negative. I am almost sure that it is positive like the TPS.

    Do you think you could make a pcb layout with the traces for me as i don't understand some of the symbols and what not.
    as for the tps voltage offset, it should not be a problem as just going to a bigger throttle body would change the tps voltage to amount of air consummed more then that.
    I wish i could be as good at this as you.

    thanks for your help, hope im not asking for too much.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The simulated MAP sensor range is from 0v to 4v. At no time does it fall below 0v.

    No. I won't do it for you, as it's your project. I've already made a schematic for you, which is more than I should've done.
    You misunderstand the TPS. TPS is an indication to the ECU as to the absolute position of the throttle, and where to map the fuel mixture in open-loop mode until closed-loop mode can be re-established. If you changed the throttle body, the ECU map would need to be reprogrammed, as otherwise the fuel map would be all wrong. Say hello to burned/broken pistons.
    Give yourself 20-30 years. ;)

    You're welcome. However, I would not be doing you much of a favor if I completed the project for you.

    If you don't understand some of the symbols in the schematic, then edit it using MS Paint, and make circles around samples of the symbols in question in a contrasting color. Either myself or someone else will explain them to you.

    Vcc7 may be confusing; that simply indicates a connection to the output of the LM317 regulator, which is about 7v. The symbol under that is a circle, with a line to the nearest connection point.

    VcSW1 and VcSW2 may be confusing; they're merely voltage controlled switches. The + side is the gates for the 4066 channels.

    If you want to re-create the schematic, and try to lay it out in Eagle v4.16r2, I'll help you - but won't do it for you.

    You can download the free version here:
    ftp://ftp.cadsoft.de/eagle/program/4.16r2/
    Get the appropriate version for your OS, and the manual.

    A really good Eagle tutorial is here:
    http://www.sparkfun.com/commerce/tutorial_info.php?tutorials_id=57&sipp=1&page=8

    It should show you the ropes enough to get started on this project.

    If you install a later version of Eagle Layout Editor, you will be on your own.
     
Loading...