Circuit to switch LEDs on 12V but not 7V

Discussion in 'The Projects Forum' started by losinj, Oct 26, 2010.

  1. losinj

    Thread Starter New Member

    Oct 25, 2010
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    Hello Bertus (et al),

    I appreciate your pointer to the read-me. Having now reviewed the Forum's position on automotive discussion I would request a 2nd chance for my previously closed topic (44710).

    To clarify, the brake light in question is an optional accessory kit which Givi (motorcycle luggage manufacturers) supplies in the U.K., for fitting into their m/cycle topcases. As such, it becomes an additional, high-level brake light on the m/cycle.

    Prior to self-assembly and fitting I chose to replace the four pea bulbs with high-brightness white LEDs, with suitable current-limiting, and parallel diodes to give a degree of reverse voltage protection. When installed into the topcase the results are comparable with the original bulbs, but with faster response, and less chance (I hope) of needing to be replaced in time due to broken filaments.

    There is still the option to refit the original bulbs. However, regardless of this, the unit is looking for a solution which will allow it to function as intended on my m/cycle, i.e to light up on 12V (approx. 80mA required), but not on the constant 7V present (with the ignition on) when the brakes are not applied.

    I would emphasize that am not especially interested in where the 7V is coming from - apparently back from the main brake light itself, or in attempting to block it using maybe a diode, as any direct interference could introduce a point of failure into the main circuit.

    I hope this revised topic now falls within the recognised scope of the Forum ...?

    Thanks.
     
    Last edited: Oct 26, 2010
  2. SgtWookie

    Expert

    Jul 17, 2007
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    The 7v you're measuring is to keep the filaments of incandescent bulbs somewhat warm, so that they turn on fully more quickly, to keep their resistance moderately high, and are not overly stressed when going to full brightness. This also increases the service life of the incandescent bulbs.

    Further discussion of this topic will require the approval of one or more of the Moderators.
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Agreed.

    losinj, we would like to help, but with all of the DOT regulations, and different rules in different localities, we don't want to inadvertently instruct people to do something that would result in a ticket or worse.

    Example is that in South Dakota, exterior Neon/LED underbody lights were recently banned. Other areas have some really strict rules on how bright or dim lights can be, and their viewing angles so they do not blind other drivers.

    As a rider, I think having as many attention getting brake lights possible on a motorcycle is best (without going to obnoxious white strobe lights). The local authorities disagree.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Given the location is on the other side of the Atlantic and that it involves an aftermarket kit, we can allow this to be discussed.
     
  5. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    The OP stated
    The Moderator stated
    I disagree. Modifying an aftermarket kit (which more than likely meets the countries DOT standard) is no different than modifying the OEM equipment.

    Sounds like a double standard to me. It's up to the OP to know the rules in their own country. I'm sure a more knowledgeble person in the UK can find the British Version of DOT to advise the OP on these points.

    If one's ethics prevents them from modifying OEM equipment citing safety, it should prevent it from modifying aftermarket kits. It's all automotive safety.
     
    Last edited: Oct 26, 2010
  6. losinj

    Thread Starter New Member

    Oct 25, 2010
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    Hi Guys,

    I can accept this is a borderline case in your view, but I wouldn't want you to get too hung up on the fact that I have modified the extra brake light. The main issue surely is the bike's existing circuit, which would interfere with the way any extra light is meant to work. 7 Volts would still be enough to cause the original pea bulbs to glow, but please see below.

    Imagine for the moment that the extra brake light is a standard, unmodified unit. Is there a type of circuit that could go between the bike and the light, that will ignore 7V but pass 12V?

    I would add that the bike's main brake light is an LED unit, so there are no filaments to warm up. There are three leads into the unit, including ground, the other one being for the tail light. The tail light and brake light use the same LEDs but at different brightnesses, so I suspect the 7V I'm seeing is being derived within the unit itself for tail light purposes.

    It is probably safer to assume that the main light is not designed to have an extra light connected directly in parallel, as I couldn't be certain that sinking extra current from the 7V source would not harm the source. Thus it seems that my particular bike demands a separate 12V supply for the extra light, and a driver circuit with a low current input to sense the voltage on the bike's circuit. All I need now is to understand what kind of driver.

    If you really don't wish to discuss this further I can respect that; but in the meantime I'm trying a suggestion from Dave's sticky: "be persistent" :)

    Thanks.
     
    Last edited: Oct 27, 2010
  7. tom66

    Senior Member

    May 9, 2009
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    You could try a voltage comparator (like an LM339) but I'd be hesitant to say it would be okay given the possibility of voltage spikes on the brake light line.

    Or, you could try a 12V relay, which might not switch on at 7V but on at 12V. You could try adding a resistor in series to fine tune the turn on point.
     
  8. DigitalReaper

    Member

    Aug 7, 2010
    70
    2
    How about a zenner diode that has a breakdown voltage somewhere between 7 and 12v connected to a transistor? The transistor would switch the full 12v to the LEDs.

    edit: something like this:
    [​IMG]
     
    Last edited: Oct 27, 2010
  9. SgtWookie

    Expert

    Jul 17, 2007
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    I'm thinking that you aren't really getting 7v; it's more likely a PWM signal that switches between 0v and ~14v.

    If you don't have an O-scope, you'd have to test that theory using a diode, cap, and resistor or two.

    See the attached.

    TP1 represents what you're measuring now.

    TP2 detects the peak voltage out, less the Vf of D1.
     
  10. losinj

    Thread Starter New Member

    Oct 25, 2010
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    Many thanks all for useful ideas.

    Taking DigitalReaper's suggestion for the moment, would this make for a sensible adaptation? Please comment as you see fit, as I have no real basis for believing that any of my mods are needed, or will work - such is my knowledge on this subject.
    Circuit.gif
    (Everything above Q1's collector is in the extra brake light.)

    • R1 reduced to 470Ω, for about 5mA to flow through D1, as I read this is needed for efficient regulation.
      > Would this base current cause Q1 any problems?
      > Am I correct to include a B/E voltage drop across Q1 of about 1V (worst case) in my calculation?
    • Is my choice of Q1 up to the job?
    • R2 added so that Q1 is never left floating
    • D2 added for reverse protection of Q1
    • Is there enough protection against spikes?
    • Would SgtWookie's idea of a PWM signal prevent the circuit from doing its job? I don't possess a scope, or gash components, to test his theory.
    Thanks again.
     
  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    What do you have for a multimeter?

    If yours has a Frequency/Duty Cycle measurement, try to see if you can get a frequency or a duty cycle. If it shows both frequency as zero and OL for duty cycle, it is straight DC, not PWM. I know a Fluke 87 has this function, I have several and use it often. You can get them for relatively cheap on ebay, and if you intend to do a lot of electronics work, you'll never regret it.

    If it is a basic digital multimeter, try switching it to AC V, and see if it shows a value in the Volt range (not millivolt). PWM will be read as AC by most meters, but the actual voltage will be off as most are calibrated only for sine wave AC, not square wave.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    If the input is actually a 7VDC when the brake light is off, then it should work.

    If it's PWM or 1/2 wave rectification of the alternator output, then it will work as it does now (ie: still dimly on when the brake light is supposed to be off).
     
  13. losinj

    Thread Starter New Member

    Oct 25, 2010
    5
    0
    Mines only a cheap, basic voltage/current/resistance/continuity sort of meter (digital), but I'll try your suggestion, thatoneguy.

    Thanks again, it may be a couple of days before I am able to post back with my findings.
     
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