Flasher circuit - two flashing/alternating light pairs?

Discussion in 'The Projects Forum' started by PureWhiteWings, Apr 23, 2011.

  1. PureWhiteWings

    Thread Starter New Member

    Jan 2, 2011
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    I tried to sum it up as best I could in the title, and I've yet to find anything quite like what I need.

    Basically, I'm trying to figure out how to get two pairs of lights to flash alternately, which wouldn't be too bad on its own, but the key is that I need one pair to flash two or three times, then the OTHER to flash two or three times, alternating back and forth in a steady pattern.

    Simple version: Left flash three times, right flash three times, repeat.

    My only stipulation is that I'd like to keep ICs out of it if at all reasonable, since I have access to most any other components I would need without having to purchase anything, but if it's unavoidable (or SIGNIFICANTLY easier), then that's fine.

    The power source is 12V DC, and the lights are 27W@12V each, pairs wired in series (54W per side).

    (If anyone's curious, yes, this is for emergency lighting, and yes, it is a legal implementation. ;))
     
  2. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    I think the easiest way to do that would be with a 4017 decade counter IC. I know you said you don't want to use ICs, but you will thank yourself if you get one. They only cost a dollar or two, and it is very simple to use. Just connect your first LED via diodes to the first three outputs of the decade counter, the second LED to the next three, and the 7th to the reset. Very simple. Of course, you will also need a clock signal, best created by a 555 timer chip (again, very cheap). I strongly recommend you go ahead and buy the chips. You may even be able to find them on old circuit boards, but it would be easier to just buy them.
    Good luck!
    Der Strom
     
  3. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Actually to get a "double blink" on each side you need to use every other output on the 4017 and diode-OR them. (attached) A 555 as a clock input and a couple of power N-channel MOSFETs to drive the lamps meets the OP's minimum requirement. For three blinks per side you would need to cascade another 4017.

    Ken
     
    Last edited: Apr 23, 2011
  4. designnut

    Member

    Apr 21, 2011
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    You cannot power12 vplt lights in series from 12 volts, they will not be visible. I've seen some circuits using SCR flashers usually a series of lamps that flash. where the one fired puts out the previous one through a capacitor. a ring around the rosy sequence. The starting surrge of a cold lamp is up to 10 times the current of a lit one.
     
  5. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Ah, yes, thank you Ken. That is correct. Flashing twice for each LED would be achievable with a single 4017, but for three flashes, you would need a second one. Thank you for pointing that out, and I apologize for my mistake.
    Der Strom
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    With a 4017, one could use the Q0, Q2 and Q4 outputs for 3 flashes on one side, and Q5, Q7 and Q9 for 3 flashes on the other side.

    It would be better to use a 3-input OR gate like a 4075 per side than diode OR; as otherwise the MOSFETs will turn off very slowly.
     
  7. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    I had thought of that as well, but it depends on whether the OP would like to have a delay between the last flash of the first LED and the first flash of the second LED. Your suggestion would make it so there is not a delay between "sets" of flashes. This part is completely up to the OP.
    Der Strom
     
  8. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    I was thinking I would want an equal space between each flash, even when it switched sides. But, that would work too.

    The resistors on the output of the diode OR's would also be from the gate-to-source on the N-Ch MOSFETS and provide a discharge path for Ciss. I don't think the OP needs MHz switching speeds. ;)

    Ken
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Well, they're planning on using 54 Watts' worth of incandescent lamps per side. That's a pretty good justification for wanting the MOSFETs to switch on and off quickly, or power dissipation in the MOSFETs will go up very significantly. Don't forget that when power is first applied to an incandescent lamp, the filament resistance is practically a dead short.
     
  10. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    OK, then I'd add an 8-pin 4427 dual gate driver. Even the direct turn-on drive from the 4017 would have been a "liiiiiiittle weak".

    Ken
     
  11. PureWhiteWings

    Thread Starter New Member

    Jan 2, 2011
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    Another possibility is to just wait until I can replace the incandescents with 12v-configured LED boards, which would remove the ridiculous initial power requirements. Another possibility is to simply change to one flash per side, which would be a considerably simpler circuit.

    I just don't have much in the way of money or patience, I guess - the latter probably being a real problem with EE!

    I'm pretty sure with a 555 and an SPDT relay (have plenty) I could just get wig-wag lights of sorts, which works just as well I suppose.

    I guess my path of action will be to simply drop a 555 and a relay into the existing circuit until I can get the LED boards I'd really like to get, then I'll get my hands on a 4017. Since I'm fine with two flashes per side, and keeping even space between flashes, would using Q0+Q2 for one side, Q4+Q6 for the other, and Q7 wired to reset give even spacing? When voltage is applied to the reset pin, does Q0 activate in the same or next clock cycle?
     
    Last edited: Apr 23, 2011
  12. SgtWookie

    Expert

    Jul 17, 2007
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    When the 4017 is reset, Q0 goes high immediately.

    Only one output is high at a time.
     
  13. PureWhiteWings

    Thread Starter New Member

    Jan 2, 2011
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    Well, I've still got my grab bag of ICs that I never really took inventory on (and keep forgetting about), and I'm starting to wonder if I made a stupid move dismissing ICs in general for projects, since god knows I've probably got something useful in this box. Guess it's time to start taking numbers and getting datasheets.

    Barring finding anything interesting in there, looks like 4017 + LEDs is my way to go, thanks everyone!
     
  14. Wendy

    Moderator

    Mar 24, 2008
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    I would have responded, but you nixed my input when you said no chips. I've designed several, one with 4 555's (3?). Have you tried a search for police lights?
     
  15. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    I moved the reset to output Q8 on the schematic to keep the even spacing. That's what happens sometimes when you reuse old schematics. :(
    Q0 goes high the instant Q8 tries to go high.

    Ken
     
  16. PureWhiteWings

    Thread Starter New Member

    Jan 2, 2011
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    I'm actually curious about the one with several 555s, since they're cheap, easy, and quick to come by (whereas 4017s and the like are a slow process to get). Police lights are similar in so many ways to what I'm after, and now... I feel stupid because I didn't think to Google "police lights schematic" and now I find something like this...

    That was just the inspiration I needed to get out of my stupid-mode that I get into whenever I start killing my brain with electronics, THANK YOU.
     
  17. Wendy

    Moderator

    Mar 24, 2008
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