New project: Need help setting up sequential leds

Discussion in 'The Projects Forum' started by cg8798, Oct 28, 2008.

  1. cg8798

    Thread Starter Member

    Oct 18, 2007
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    Ok guys this is my situation. I want to make some new led tail lights. My plan was to make probbaly 6 lines of maybe 11 leds. The top four lines would act as the running lights and the brake lights, but the bottom two lines would act as running, brake, and sequential turn signals. I wanted them to light up normally with the all the leds, but when the turn signal is activated to go into a sequential mode that is the same brightness as the brakes. I figure it is possible but I'm not sure how. I'm in a few forums trying to figure this out with people.

    So first, I know my setup will contain a pwm for each side of the car to run the running and brake lights. I plan also to use a sharp v.regulator, and a 555 timer hooked up to a 4017 decade counter to make the sequential stuff. But my question is if it's possible to make the signals light up like I want. I figured there was a way to run the power from the pwm for the running and brakes, but then do reroute the power through the 555/4017 chip to make the sequential part. Possible?

    Here are some diagrams I've been going over to setup the pwm and sequential setup.
    [​IMG]
    PWM schematic^^

    [​IMG]
    Sequential Signal Schematic^^

    I believe there is a sharp vregulator before the 555 timer to have constant power so the sequential lights will all stay lit till the sequence is reset.

    So is my goal achievable?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    The schematic looks mostly reasonable, since the SCRs will stay conducting once triggered and current is flowing through them. You may have a problem using PWM though, bacause the SCR's will need current to stay conducting. How much current are you wanting through the LED's max?
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Yep, Bill nailed it. ;)
    You're also going to need something to turn off the SCRs by removing the current path. You might use a P-ch MOSFET with the gate controlled by IC2 Q7. Also, use IC2 Q8 to control it's RST pin. That way, the sequence will repeat immediately rather than staying dark for an extra couple of clock pulses.
    [eta]
    Note that the 1N4004 diodes will have a significant Vf with current flowing through them. Typical Vf is 1.1 @ 1A. This may cause you problems.
    The 1N5404 will have a similar Vf, but at a higher current level.
     
    Last edited: Oct 29, 2008
  4. Wendy

    Moderator

    Mar 24, 2008
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    OP didn't say it, but I assumed he's thinking the circuit would reset when the lights went out instead of cycling. The problem I'm thinking of if he varies the intensity via PWM then the SCRs will turn off during the off part of the PWM signal.

    Didn't we see a similar query using a ramp function and an LED bargraph not too long ago?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Well, as you've already explained, the SCRs will remain turned on until the current through them falls near zero.

    That's true. If he wants to use PWM and the SCRs, I suppose he could use a diode array to keep the current and all previous SCRs turned on via the 4017 outputs. Would get klunky in a big fat hurry though. Might be easier to use D-type flip-flops and just drive the set/reset inputs.

    Yes. An LM3914N driven by a ramp generator makes it pretty easy to do this sort of thing.
     
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