This should be very simple but you tell me

Discussion in 'The Projects Forum' started by mdweiss, Feb 11, 2010.

  1. mdweiss

    Thread Starter New Member

    Jan 28, 2010
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    Hi guys,

    Im looking to make a circuit that when it sees a 0-5 volt dc input it grounds an output that will in turn trigger a relay.


    This is being used in an automotive application but inside the car, i need to have it adjustable with a pot so i can have it ground the wire at say 3.8 volts and above. Then when the voltage drops below 3.8 volts it undergrounds the wire.

    This is going to be referencing a drive by wire pedal that outputs 0-5 volts to the computer. i need to dial in with the pot at what throttle position (voltage) the relay gets grounded. This will allow me to activate the relay at say 70-100% throttle and have it unground when it drops below 70%.


    thanks in advance guys
     
    Last edited: Feb 11, 2010
  2. Wendy

    Moderator

    Mar 24, 2008
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    A simple 555 would do that...

    [​IMG]

    Remove C1/R1 and use pin 2 and 6 as inputs, maybe with a pot.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Won't this have a hysteresis of 1/3 of Vcc? In other words, it the OP powers the 555 with 6V, the turn-on would be 4V, and the turn-off would be 2V. There is a way to get a smaller hysteresis using a 555, but it gets quite complicated.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    This could be a problem. It met the specs given. If the OP needs a specific voltage then a comparator would work, but it would still need some hysteresis to prevent relay chatter.

    He didn't mention how it had to recover, it triggers for 4V and 8V, which seems to fit the criteria.
     
  5. whatsthatsmell

    Active Member

    Oct 9, 2009
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    Is this for a nitrous oxide system?
     
  6. mdweiss

    Thread Starter New Member

    Jan 28, 2010
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    its not for nitrous but a supercharger solenoid, but it needs to be just like one made for nitrous
     
  7. whatsthatsmell

    Active Member

    Oct 9, 2009
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    Take a look at these two links:

    http://www.holley.com/data/Products/Technical/199R10406-1.pdf
    http://www.holley.com/HolleyNews/article.asp?ID=100

    The problem with a home made solution is that you have to wire part of the circuit in parallel with the existing circuit that goes from the pedal to the computer (unless you were planning to read the feedback from the throttle position sensor). Any problems with the home made circuit can result in throttle control problems, ranging from no go to "Toyota Throttle Syndrome".

    Manufacturers spend a lot of money on engineers, R&D, and lawyers. A homemade circuit is going to be hard to test, and if it doesn't work right and you get a lean condition if your solenoid does not activate, it is going to be an expensive testing process replacing melted pistons. :(
     
  8. mdweiss

    Thread Starter New Member

    Jan 28, 2010
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    yes i have looked at the off the self solutions.. holly, nitrous express, and zex all make the units. Im not going to be cutting the factory wire. im just going to be tapping into it for a reference.

    what would i need to do to add hysteresis control? and how would i go about adjusting its threshold.

    i could tap off the tps sensor but the problem is that the computer has a throttle map, where the throttle body does not do exactly what the pedal does. there are many factors that go into what the throttle body ends up really doing.

    for my application i want the pedal to directly control what the solenoid does.

    thanks for the help so far guys
     
  9. mdweiss

    Thread Starter New Member

    Jan 28, 2010
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    also to make this easier. i could have say 4v trigger the circuit and it would need to drop below say 3.5v to turn off. Would this .5 volt gap help reduce the need for hysteresis and reduce possible relay chatter?

    also are there any cheap and easy off the shelf products that i can use?
     
  10. mdweiss

    Thread Starter New Member

    Jan 28, 2010
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    [​IMG]

    this guy made a dual trigger system that gets a 0-5 volt input, can i use the LM339N that he has in the top left, and have it simply ground the wire, i would also need the 5 volt regulator
     
  11. mdweiss

    Thread Starter New Member

    Jan 28, 2010
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    so would this work? or am I way off.

    sorry for posting so often, I'm excited to find a solution
    [​IMG]
     
  12. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    This circuit has no hysteresis; it will chatter. You should have at least a few mV of hysteresis; say turn on a 4V; turn off at 3.98V.

    There should be some conditioning of the 12V line to prevent big spikes from screwing up either the regulator or the comparator chip

    The LM339 cannot switch a relay directly, so you will need a power transistor.

    I will work on this tomorrow, if you still would like some help.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    The LM339 is not rated for an automotive temperature environment. Further, it's only rated for sinking up to around 6mA current.

    There is no hysteresis, so the solenoids/relays would be clicking on and off.

    There are no reverse-EMF protection diodes on the coils of the solenoids/relays.

    If the output of a comparator failed open, the MOSFET gate would be pulled to 12v, turning them on, turning on the solenoids/relays and you would have boost 100% of the time.

    I don't know why you think the TPS does not follow the foot pedal? The throttle is connected directly to the pedal, unless your throttle is completely controlled by the ECU.
     
  14. williamj

    Active Member

    Sep 3, 2009
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    MDWIESS,

    If your solutions doesn't have to be solid state perhaps all you need is a relay, a limit switch and a limit switch trip, as illustrated in the attached.

    williamj
     
  15. mdweiss

    Thread Starter New Member

    Jan 28, 2010
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    The pedal has no cable, it connects to the computer and the computer connects to the throttle body. You would be amazed to see a data log of a car driving and the difference between the drivers foot position and the actual throttle position.

    Im trying to stay away from a microswich style activation, but yes thank you i do know there is that option.
     
  16. whatsthatsmell

    Active Member

    Oct 9, 2009
    102
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    To me, this makes the argument to go with the voltage off the tps. That would be the true position of the butterfly in the throttle body as opposed to the distance your foot has moved the pedal. There is probably some smoothing built into the software of the ecu to prevent throttle modulation.

    So, technically, it is more the position of the butterfly in the throttle body that dictates the performance of the vehicle than it is the position of your foot.
     
  17. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The attached will pull in the relay when the pedal sensor outputs a voltage adjustable from about 2V to 5V.

    There is a single adjustment pot. The simulation shows at what input voltage the relay pulls in at various setting of the pot wiper.

    The circuit uses a 555 wired as a (possibly novel) Schmitt Trigger, where one resistor sets the hysteresis. The voltage to 555 is regulated so make the trip points immune from variation in battery voltage.

    The relay driver will pull-in a typical 12Vdc automotive relay with a coil resistance of 85Ω or higher.

    The plot of current through the relay shows the effect of moving the pot wiper from 10% (from the bottom) to 90% in steps of 20%. The progression is red, blue, green, lt blue, violet, respectively
     
  18. SgtWookie

    Expert

    Jul 17, 2007
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    Interesting circuit, Mike ML.

    But, are you using the Linear-supplied NE555 model? I'm pretty sure you're aware that it's an idealized model, but just wanted to remind you about that. Also, there will be a brief short as the output changes states; and I'm concerned that 1uF will be inadequate to handle the transient. National Semiconductor recommends a 0.1uF in parallel with a 1uF electrolytic as minimums.

    Also, it might not be a bad idea to put a 1nF-10nF cap to ground on the trigger input; the engine compartment is a mighty noisy place.
     
  19. MikeML

    AAC Fanatic!

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    Yeah, I'm using the idealized model, but the only part of it that matters is the trip points of one of the two comparators and the bias resistors, which I happen to know is modeled very well. I'm not relying on almost any other parameter of the 555. Even the output is hardly loaded, minimizing the need for a big bypass...

    As to making a LPF on the input, probably not a bad idea. As designed the circuit has about 100mV of hysteresis, which ought to be enough.
     
  20. mdweiss

    Thread Starter New Member

    Jan 28, 2010
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    thanks for the diagram, im going to build it and see how it works, any tips when i build it? is there a good online source that i can order everything from?
     
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