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Need a simple adjustable device to lower signal voltage?

Discussion in 'General Electronics Discussion' started by supak111, Mar 13, 2019.

  1. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

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    Hey everyone I'm looking for a very simple device which can be used to LOWER the voltage, instead of amplifying it. PS my circuit is very low current, under 50ma and I need to keep it about the same.

    Basically I have a voltage signal which varies between 0.3v to 1v at about 1-2Hz. I just need to be able to adjust this voltage DOWN by 0v to -0.3v.

    Would a simple adjustable microphone or audio module amp be able to do this? I know it would be able to adjust the voltage UP, but not down. But what if I where to hook up that amp backwards (switch around the output and input)? Would that lower the voltage instead?

    For example this LM386 amp module:
    [​IMG]
     
    Last edited: Mar 13, 2019
  2. davenn

    davenn Moderator

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    amps amplify

    no, IC's don't work that way

    why not just use a variable resistor .... it's the way it's been done for 100 years
     
    Cannonball likes this.
  3. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

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    Wouldn't I need a low ohm variable resistor which are expensive and kinda big?
     
  4. davenn

    davenn Moderator

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    around 5mm square too big ?

    [​IMG]
     
  5. Bluejets

    Bluejets

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    Did you post that correctly?
    You need to change a positive signal to negative...??

    Don't know what you mean by "down by 0V........."

    Perhaps if you post more about what you are trying to do, I'd say many of the experienced ( as above) would be able to steer you in the right direction.
     
  6. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

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    Oh no, I have way more room than that. I can't seem to find those in like 1-10ohm range, its always 1kohm+++ range
     
  7. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

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    No I'm simply trying to lower it by up to 0.3volts. My signal wire just keeps fluctuating between 0.3v and 0.9v, i need a device which can lower it from 0mv to up to 300mv by say tuning a Potentiometer so that the end signal is not .3v to .9v but that I can have it fluctuate say at .1v and .7v or 0v and .6v or .2v and .8v....
     
  8. Bluejets

    Bluejets

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    Show your circuit and application like I said above.
     
  9. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

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    Well I'm making this device to offset the voltage coming out the oxygen sensor on my motorcycle. Signal goes into the engine computer.

    What I do know is that I need the current coming off the sensor to stay pretty close to what it is now so I don't get a checkengine light. Only thing I want to be able to change is the voltage

    I was thinking making a simple voltage divider 1 low ohm resistors, and 1 low ohm potentiometer but I can't seem to find a cheap low ohm potentiometer which is why I was hoping to find some cheap mass produced module which might be able to do this without me having to build it
     
    Last edited: Mar 13, 2019
  10. Harald Kapp

    Harald Kapp Moderator Moderator

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    A voltage divider can divide (as its name says) the voltage from 0.9 V to 0.3 V. But that will decrease the low end from 0.3 V to 0.1 V only.
    To get what you want you need two operations:
    1. offset the input voltage by 0.3 V to change the range from 0.3 V...0.9 V to 0 V...0.6 V
    2. divide the new range by 2 to change from 0 V...0.6 V to 0 V...0.3 V
    Depending on the precision required several circuits are possible.
    • A highly accurate circuit would use an operational amplifier as difference amplifier to subtract the 0.3 V offset, then possibly a resistive divider to change the scale. (or change scale first, then subtract the offset which has the advantage of a rather low impedance output.
    • A less accurate circuit could use a Schottky diode (e.g. this one) to subtract the offset (UF= 0.32 V for the linked type), then divide using a resistve divider (see picture)
    • Other circuits are possible, too...
    upload_2019-3-13_7-39-19.png
     
    Tha fios agaibh likes this.
  11. BobK

    BobK

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    Depending on what you mean by this it might be very difficult unless the loading of the signal is a simple resistor.

    If what you really mean is the current going into the load, which I expect is the case, then it is impossible. No current can flow to the load when the voltage is 0.

    You have also said nothing about the signal. DC? AC? If the latter, frequency and waveform?

    Bob
     
  12. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

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    I've mentioned its a DC sign wave which fluctuates between 300mv and 900mv at 1Hz. So its basically AC but offset into the DC range. I just need something which will offset it back down by 0-300mv where I can adjust the offset back DOWN by anywhere from 0mv to as much as 300mv depending on what I need. Peak and the valley of the signal should both go down by same among adjustable by me anywhere from 0-300mv.

    If you look at this image for example, just imagine moving the whole waveform down on the scale by either 5mv or 300mv or anywhere in between 0-300mv.

    [​IMG]
     
  13. BobK

    BobK

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    So, is it the signal or the DC offset you are trying to reduce?

    Bob
     
  14. hevans1944

    hevans1944 Hop - AC8NS

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    So, what's wrong with the original oxygen sensor signal going into the engine computer? Why do you want to mess with this signal?

    You cannot change the oxygen sensor voltage without also changing the oxygen sensor current. Why are you trying to do this? Please tell us WTF you are trying to DO!

    So, where did you come up with the idea that you needed to do that?

    A typical Bosch zirconia lambda oxygen sensor (see this page for info) is a voltage-producing thermo-chemical device that must be heated to exhaust temperatures before it will produce any output. After it warms up, it produces an output voltage that increases with increasing fuel richness, "flat-lining" at about 0.8 V DC at maximum richness or minimum oxygen-to-fuel ratio. As the fuel is throttled back (reduced) to the engine, the oxygen-to-fuel ratio increases, causing the sensor output to decrease until it "bottoms-out" at about 0.2 V DC.

    In between those two limits there is supposed to be the "ideal" stoichiometric air-to-fuel ratio that will result in complete combustion of the fuel and therefore minimum emissions of un-burned fuel exiting the exhaust pipe. This is just one part of the engine emission controls, mandated by current laws intended to preserve and maintain air quality. It is the responsibility of the Engine Control Module (ECM) to adjust the air-to-fuel ratio to maintain a stoichiometric mixture of air and fuel, a difficult task considering how often throttle demand and acceleration vary during a typical motorcycle ride. There are basically only two ways to do this: modulate the air intake or modify the fuel intake to the engine. Most ECMs modulate the fuel intake, which may affect engine performance.

    Clearly, unless the motorcycle is motionless, running its engine at a constant speed with a constant torque load (dynamometer test), at a fixed throttle position, the oxygen-to-fuel ratio is NOT going to be constant. It will vary up and down constantly according to the engine load and its environment. On the road, the ECM "sees" these variations in air-to-fuel ratio and issues corrections intended to restore a stoichiometric ratio, more or less in real time. This is a negative feedback loop. The oxygen sensor output oscillates as the ECM attempts to discover the stoichiometric mixture. The feedback loop has a time constant associated with the response time of the oxygen sensor to changes in the air-to-fuel ratio, and to the update rate of the ECM control algorithm, which together create the one or two hertz frequency of oscillation. Note this oscillation does not occur unless the negative feedback loop is closed and the ECM is controlling the air-to-fuel ratio. Nothing you can do about that, other than attempt to defeat it with an illegitimate signal, as its a "hardware feature" engineered into the emission controls system.

    So, again, WHAT ARE YOU TRYING TO DO?
     
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  15. Tha fios agaibh

    Tha fios agaibh

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    @hevans1944
    Seems you answered your own question with that well written post.
    He wants to offset the O2 sensor feedback to make the engine run more rich. Presumably to attempt to increase horsepower.
     
  16. Alec_t

    Alec_t

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    Offsetting the O2 sensor output will change the average value from the 'normal' one, so it wouldn't be surprising if the ECU flagged the difference as being due to a faulty sensor and set an error code. This would be stored and detectable at the next official emissions test, even if it didn't cause the vehicle to go into 'limp mode'.
     
  17. BobK

    BobK

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    Okay, so the peak-to-peak voltage of the sine wave minus the offset should remain the same, only the DC offset should change.

    Use an op amp difference amplifier with the - input at an adjustable 0 to 300 mV and a gain of one.

    Bob
     
    supak111 likes this.
  18. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

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    I know how oxygen sensors work guys, I drive a 580 whp Evo 8 tune on e85 by me. I also have a Dyno at a friends shop I've used on this motorcycle. Reason for the simple device is to be used on this motorcycle when a few very common modification are done to it, (intake and exhaust) after which the motorcycle runs VERY LEAN throughout the RPM range. This is more of a safety device than more horsepower device, but I suspect it will do both. Running lean is terrible for any engine. Its without a doubt that a simple device will be very useful in this situation and I suspect it would be on many other bikes with these simple modifications which also utilize an oxygen sensor (not a lot of bike do). Other people have tested such device who also have a wideband O2 on there motorcycle and it did help, but the device they used are way expensive. I was just hoping the same task can be accomplished with something under $10 if not way less seeing how cheap electronic components are today but I didn't know how to go about it so I figured I ask here.

    Though a simple inline Schottky diode might work but the voltage drop isn't optimal nor adjustable for every exhaust or every intake combination, or if someone only installed one of the 2 modifications.
     
  19. hevans1944

    hevans1944 Hop - AC8NS

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    Okay, so now we are getting somewhere and we know what you want to do. I know diddly about modifying or tuning motorcycle engines, but it seems that most "improvements" fall into one or both of these categories: improve the aspiration of the fuel-air mixture during the intake cycle or improve the removal of combustion products during the exhaust cycle. On automobile hot rods both of these methods are well understood and work well if exhaust emission controls are not a consideration. For example, "she's ported and relieved and stroked and bored" refers to mechanically smoothing and polishing the path taken by the fuel-air mixture into the intake valves, and increasing the size of the piston cylinders and the the displacement of the crankshaft to increase the volume of air-fuel mixture taken in during the intake cycle. Both approaches will significantly increase the performance of an internal combustion engine.

    Going further, one could add an engine-driven compressor (supercharger or turbocharger) to increase the effective volume of air admitted to the cylinders during the intake cycle, modify the exhaust path to reduce back pressure during the exhaust cycle, and introduce nitrous oxide into the cylinders during the intake cycle to increase and accelerate fuel combustion during the power cycle. The latter is an especially simple and effective way to boost motorcycle crankshaft power.

    The downside to all of the common "improvements" is the defeat of mandated exhaust emission controls. Which is okay for off-road racing and closed-track racing because the "bad" emissions do not occur often, or in large enough quantities, to significantly affect the local environment. Imagine an Indianapolis 500 race occurring every day though: that's Los Angeles traffic on a bad smog day without the advantage of a high-speed commute.

    I could be wrong about this, but it appears that the engine oxygen sensors are designed to produce their median output when a stoichiometric mixture has been achieved. If an engine runs too lean, the sensor would "bottom out" while the ECU module would continuously call for a richer mixture, either by increasing fuel flow or restricting air flow or perhaps both. In any case, if the correction is effective, the air-fuel mixture will become richer in fuel and the sensor output will then increase and "peak out" at some level of fuel richness, overshooting the desired stoichiometric fuel-air mixture ratio. The ECU module would then call for a leaner mixture, either by decreasing fuel flow or increasing air flow or perhaps both. If the correction is effective, the air-fuel mixture will become leaner in fuel and the sensor output will then decrease and "bottom out" at an overly lean air-fuel mixture ratio.

    What I fail to understand is how the sensor output can be "bottomed out" on the lean side if the ECU module is doing its job. If that does indeed happen, there can be no oscillation in the oxygen sensor output. It will remain stuck in a limit cycle, continuously calling on the ECU to increase the richness of the fuel-air mixture, but with the ECU apparently unable to comply. I don't see how changing the sensor output is going to do anything to correct this.

    Since you have access to a dyno, it might pay to temporarily disconnect the oxygen sensor from the ECU module and substitute a DC signal derived from a low-voltage power supply. Adjust this substitute DC signal to a level that avoids the "check engine" condition. Using an exhaust gas analyzer (if one is available) tune the engine for a stoichiometric air-fuel mixture at various levels of engine loading. Measure the output of the oxygen sensor as you do this. Vary the air-fuel ratio from lean to rich and observe how the oxygen sensor output varies. The oxygen sensor output should produce an output voltage somewhere between its minimum and maximum output voltage when a stoichiometric air-fuel ratio is achieved. If this does not occur, or if you desire to run the engine rich, determine what setting of the substitute DC signal will allow proper engine operation to occur. It is likely that the oxygen sensor will "top out" before reaching that level, in which case you simply cannot use the stock oxygen sensor at all, even if you offset and scale its output. Perhaps the "wideband" oxygen sensor you mentioned will allow operation at a richer mixture than the "stock" setting the ECU expects. If that is so, then we can advise you on how to build a simple, low source-impedance, device that will offset and scale its output to place its output in the "stock" range, even though your engine is running richer than a stoichiometric mixture ratio.

    It is possible (programmers are sneaky) that the programming of the ECU module requires the sensor output to limit-cycle between the rich and lean extremes, producing the roughly sinusoidal waveform, as the ECU attempts to control the air-fuel ratio. The only reason I can think of to program it this way is to verify that the oxygen sensor is actually working: if oscillations are observed it must be working; otherwise turn on "check engine" light and go into "limp home" mode. If that is the case, then you can "fake out" the oxygen sensor by providing just such an oscillation to the ECU and disconnecting whatever the ECU is controlling that varies the fuel-air mixture ratio. You probably want to do this anyway if tuning the engine for performance. Let the ECU handle the "tough stuff" like rpm-sensitive ignition timing and spark advance while the rider controls the fuel flow with the throttle.

    An after-market thingy that the oxygen sensor plugs into, that has adjustable "offset" and "gain" controls, could be a viable product. If it really does need low-valued variable resistors to be practical, these can be manufactured to spec. Thick-film potentiometers, or variable (rheostat) resistors, deposited on ceramic substrates would be inexpensive to make in values ranging from about one ohm to ten ohms or so.
     
    Last edited: Mar 16, 2019
    Harald Kapp likes this.
  20. Alec_t

    Alec_t

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    Does it need to be a low source impedance? From my limited reading on O2 sensors they seem to have a ~10k resistance when the mixture is rich and a ~20k resistance when the mixture is lean, albeit their heaters are ~10Ω.
     
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