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Design an adjustable voltage switch

Discussion in 'General Electronics Chat' started by stuhagen, Sep 27, 2008.

  1. stuhagen

    stuhagen Member

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    Well.......go figure~! The reason why I used this 5V reg was because my last circuit I built using the car's alternator voltage, didnt work. Although, that circuit was using a LM311. So maybe the LM311 operates at a lower voltage. Again, my stupid attempt to add something without sufficient knowledge ha..ha....

    So...If I switch to a 12V regulator, will this work? The reason I am thinking a regulator is better is becuase of the type of supply. Auto voltages swing from 12-14.5 depending on battery condition/charging. Plus it isnt all that clean of a voltage.

    Again, I want to thank you guys for helping me. This means a lot.

    Stu
     
  2. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    As well as 7805 and 7812 Vregs there are 7808 and 7809.

    If you have either a 7808 or 7809 I could modify the circuit.
    The 7812 will require about 2 to 3 volts higher than 12V on the input in order for it to give a regulated 12V out.!

    EDIT:
    Look at this image.
     

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    Last edited: Nov 21, 2008
  3. stuhagen

    stuhagen Member

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    Well, maybe the 1st thing to do is see how this works without the regulator. Maybe I won't get the relay chatter like I did with the other circuit. If this LM393 can handle crappy supply voltage, then I guess I don't need a reg. Otherwise I will go buy one of those 8v/9v reg's and have you revamp the diagram.

    BTW...is a 1n0 cap a .001 uf?
     
  4. dave

    Dave New Member

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  5. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    Yes.
    Also a 1000pF

    Lets know it goes.:)
     
    Last edited: Nov 21, 2008
  6. stuhagen

    stuhagen Member

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    It works...........Now I need to burn another board since I destroyed this one~!

    Stu
     
  7. 427mustang

    427mustang New Member

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    Stu

    Can you post a schematic of the final design?
     
  8. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    Do you want exactly the same action from the circuit as 'Stu's'.??

    I modified his circuit to de-energise the relay when the Vsource reached about +4.7V, with a hysteresis 'deadzone' of about +/-0.1V.
     
  9. 427mustang

    427mustang New Member

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    Eric

    Actually, I would be looking to turn the relay on and off around 2 volts. A .1 to .2 volt deadband would be more than acceptable. The relay would turn "on" on a falling voltage and "off" on a rising voltage.

    Thanks for your help.

    Tom
     
  10. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Tom,
    This circuit will turn on the relay below the Vset and off above Vset.

    The Vset has a range of about 0.5V thru 5V, so set it about 2V for you application. I would suggest a multi turn trim pot for Vset if you require setting accuracy.
     

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  11. Roff

    Roff Well-Known Member

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    Eric, the Thevenin resistance looking into the pot wiper varies from 0 to 2.5k, depending on the setting. The hysteresis will therefore vary a lot. If you add a resistor (≈10k) between the pot and pin 3, selecting the feedback resistor for the desired hysteresis, the hysteresis will be relatively independent of the pot setting.
     
  12. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Hi Ron,
    In the general purpose version of the circuit I do have a 10K, same as the Vsource input.

    The last two OP's have asked for a 'fixed' Vset level, so I dont think the 10K is required for these two examples.

    Once 'Vset' is set to 2V for Tom's application it should work as expected.

    The 270R in the transistor base is there to increase the Vout +feedback range, raises it from about 0.6V to 1.5V.

    As always, your comments are appreciated, thanks for your input.

    Regards
     
  13. 427mustang

    427mustang New Member

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    Eric

    The 2 volt setpoint I gave was an approximation, and can quite possibly vary from app to app, in a range from say 1 volt to 5 volts, so a single design to accomodate this would be ideal. Once it is set for a particular app, it won't change, however.

    Tom
     
  14. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Hi Tom,
    The 5V6 zener in the circuit posted will allow a 0.5V to 5V setting.

    If you wish to add the 10K resistor [posted by Roff] from the Vset pot wiper to pin3 of the LM393, this will ease the setting of the required level.

    Lets know how it goes, is it for a car mod.??:)
     
  15. 427mustang

    427mustang New Member

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    Eric
    Yes, it is. In fact, I think what I will do is use the output of the transistor to trigger my timing circuit. It is a dual delay timer (On delay, then interval) which will control a relay, which in turn will replace a signal to the car's ECM with a variable signal that the user will control. I have had the circuit working on a breadboard, but my crude soldering skills have led to a couple of prototyping failures.:eek:

    Although when complete there may be a small market for my design, I don't expect a huge return, and I've not been impressed with the quotes
    I've gotten for pcb manuf'g and assembly.
     
  16. stuhagen

    stuhagen Member

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    Eric,

    What is the difference between the 5V1 and 5V6 Zener?
    Would adding a 10K to the wiper add versatility to this overall design?

    When Eric posted this latest circuit, it was intended to supply a 12V signal to a Electric Switch. My circuit was also designed so the Switch was activated up until the Vsrc reached a desired voltage. (set by vset). Unless I am mistaken, this circuit can be universal. If I switched the the relay to the N/O configuration, I am understanding that the operation would be the opposite. In other words, the "switch" will be deactivated, and activated when the Vsrc reached the same set Vset. Just a matter of how you used the SPDT pin outputs. Eric could correct me though. ALso, this same circuit could be used as a regular "on-off" switch, by disconnected my 12v to the center pin of the relay, and use the N/C and N/O pins as a "turn-on" or "turn-off" switching.

    My circuit works great without the 10K resistor into pin 3. I have made voltage checks, and I am getting the desired levels on the Vset.

    I will eventually use this same circuit as just a "on"-"off" switch for another application. I have already completed the PCB123 layout, just havent done the pricing thing yet.

    I also plan on giving a few away to other freinds when all said and done. Might send Eric a donation for his help.

    Stu
     
  17. 427mustang

    427mustang New Member

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    Stu

    If you take the circuit in it's entirety, including the relay,what you're saying is true. Where I caused the confusion is when I referred to turning the relay itself on and off, rather than it's output. There are sometimes advantages to either turning it off or turning it on (the relay, that is).

    If I had to guess, I'd say you're using the output from your MAP to control NOS or something similarly sinister.
     
    Last edited: Dec 1, 2008
  18. stuhagen

    stuhagen Member

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    This circuit will do just that....energise or de-energise a relay. You can do many things with this same set up as I had mentioned. I am not using this for NOS. I actually have received a few PM's from people that want to use this circuit for that. I am using for boost pressure activation via on board MAP out put. MAP's are very linear. As an example, 0 psi = 0V 20 psi = 5v on a straight line. So If you want to activate, or de-activate something at any given boost pressure, then this circuit will do this. The RD16 SPDT Relay is only 1/4"x3/4" and can handle 16amps. All of this fits into a 1-3/4x3" board. My only concern is how universal I can make this board so it can in fact be multi-functional. There are a couple of additional pinouts on this circuit now for this reason. Even including "foolers" so the on board ECU is not aware that a specific harness is unplugged. People who have stand alones probably don't need this since they already have inputs/outputs for just these needs.

    Stu
     
  19. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Stu,
    The difference between the 5.1Vz and the 5.6Vz is only 0.5V.

    If you are considering making this a general purpose Tacho circuit for third party use,
    I would change the 5VZ to a 6V2 zener [the 6V2 has an almost flat temperature versus Vreg curve]

    Also in order to make the Vset easier,[when changing the 'deadband/hysteresis resistor]
    add the 10K reistor from Vset wiper to pin3 of the LM393.

    You can make the 'deadband/hysteresis' adjustable with another pot if required

    Connect a 1N4001 diode in series with the incoming +V12 line, this will protect the circuit
    should a user reverse connect the power leads.

    If you consider a donation is necessary , please make a small donation to your favourite charity.:)
     
    Last edited: Dec 2, 2008
  20. egil

    egil New Member

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    Hi, I've just begun learning about electronics with a project in mind. I want to add a low fuel warning indicator light to my 70's era truck. I've been reading the various links in the FAQ but also found this thread. I've still got a lot to learn, but a few newbie questions, if you don't mind helping me:

    1) Am I correct that one of these circuit iterations could likely be used to take the signal from the fuel sender (which is a variable resistor 73 ohms at empty/10 ohms at full) as the voltage input signal?

    2) The gauge and sender are on a circuit with a 5v constant voltage regulator. However, it is not a solid state one that produces a "true" constant, but rather a "pulsating" one that produces a square wave that averages to 5v. Would these circuits here work with a "pulsating" regulator? Is it even possible to get a useable signal from this?

    Many thanks.

    Charles
     
  21. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Charles and Welcome,

    I dont quite follow your description :
    Circuits can be made to do most tasks, but the actual design depends upon the input and output signals.
    If you can give more info, maybe we can help.
     

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