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555 problem with magneto

Discussion in 'Electronic Basics' started by Jordan, Dec 20, 2005.

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  1. Jordan

    Jordan Guest

    My old vehicle has been fitted with blinkers for the first time.
    As it's a 6 volt system, and I want good bright lights, I fitted LED type.
    To drive these, I find that normal blinker cans don't work, and 6V
    electronic cans aren't available. So, I made up one using a 555 timer.
    This has proven problematic, with unreliable flashing whenever the motor
    is running. It doesn't even have to be fitted to my vehicle - just being
    nearby upsets it. I used a separate battery etc for this test.
    I'm thinking it's got to be some "unwired" interference source from the
    ignition (magneto sparks) - either RFI or some induction effect?
    I thought 555's were interference-resistant?
    Any ideas, advice appreciated!

    Here's a diagram of the existing circuit:

  2. What is the value and type of the supply bypass capacitor across the
    555? What kind of capacitor is the 1uF timing capacitor?
  3. Jordan

    Jordan Guest

    I've tried various types of bypass caps - electros, MKT, ceramic disk
    (and combinations of these), values from 0.1 to 470 microfarads.
    For timing I'm using an electrolytic cap.
  4. Jordan

    Jordan Guest

    To be clear, the circuit operation is adversely affected, even when it's
    not installed on the vehicle. It only has to be near it.

    I'd been using an 8.2V zener - too high?

    I'll try load resistors if there's no other solution, but would prefer
    to fix this. I'm afraid 6V cans will one day be like hen's teeth, and
    anyway they never seem to be as reliable as 12V.

  5. I suspect that you have an unconnected pin or something like that. Is
    the circuit soldered together or plugged into a proto board? If the
    latter, you may have a lose socket.

    You might try adding a .47 uF capacitor between pins 6,2 and pins 8,4
    to make the timing more immune to supply noise. It will also shorten
    the first long pulse.

    I would use a mylar for the timing cap, also, just for leakage
    reasons. The 470 k timing resistor is a bit high for the 555, since
    the bias current for pins 2 and 6 can be as much as a microamp. So a
    bit larger cap and lower value resistor would make the timing more
    temperature stable. Probably not involved in the present symptom, but
    a good idea.
  6. ehsjr

    ehsjr Guest

    Try changing the base drive to the output transistors by
    eliminating the LED between the 100 ohm resistor and the
    transistors. Are you using te CMOS version of the 555?

  7. Jamie

    Jamie Guest

    maybe your generator is not stable enough to maintain the voltage?
    maybe a simple zener to ground and a series load R for the Vcc of the
    timer along with a coupling cap may help.
    select a 5.1 zener. and maybe a 100 Ohm R in series from your battery

    also, the bi-metal flasher may have worked if you placed a load
    resistor on the output to double up on the current.
  8. Each transistor has an open emitter when not being used.
  9. Jamie

    Jamie Guest

    i would put a Resistor from the Emitter of Each Transistor back
    to its base.
    it looks like he is leaving the base wide open when the 555 is in
    high state., the LED is thus picking up the EMI noise like a
    detector because he obviously has a long run to the dash for an
  10. Jamie

    Jamie Guest

    yes that is true, but from what i get his problem is when
    he using it.
    the 555 goes in hi-state thus leaving the base path in
    now since there is no current flowing in the LED that he
    is using for his Dash display but still in the circuit! i think
    that he is getting EMF/EMI , this only applies when the
    555 output is in the High state! when it goes to low then the
    LED's in the tail lights should be constant.
    some where in the back of my head i seem to remember that an
    LED's PN junction is prone to reacting to EMF/EMI when no current is
    passing through it thus generating noise.
  11. kell

    kell Guest

    Good advice all, but why use a chip?
    Build a flasher circuit yourself, it is probably just a simple
    RC circuit driving a little relay to make the clicking sound.
    You are probably smart enough to locate the information
    on the net and build one.
    You can optimize the component values to make it operate
    at the low current draw of led's.
    Flashers are better than 555 circuits because if the light
    ever fails (admittedly unlikely with led's, but you could have
    a broken wire), they stop flashing, letting you know there's a
    problem. The 555 circuit doesn't have that feature.
  12. Jordan

    Jordan Guest

    I've tried most suggestions, but the interference from the ignition
    system seems too great - can't get the flash rate stable enough.
    I have to give up with the 555 for now, and just use a standard 6V
    flasher can with loading resistors in parallel with the LEDs - works OK
    for now.
    Thanks for all the help, folks.

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