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Simple way for LED to indicate at 2 different brightness's

Discussion in 'LEDs and Optoelectronics' started by AFex54, Aug 14, 2015.

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  1. AFex54

    AFex54

    144
    2
    Apr 10, 2015
    Im building a simple 10 step analog sequencer with a 4017 decade counter
    below is an extracted circuit of a step indicating LED for a single step of the sequencer,
    I need to indicate 3 things with the LED
    off -step inactive
    on, dim -step active, gate off
    on, bright - step active, gate on

    what i need help with is getting the 2 different brightnesses.
    my schematic shows a simplfied version of an active step,
    a transistor before the LED triggered by the 4017 will dictate whether a step is active in the full circuit.
    the pulse from the 555 is the gate, as you can see it has less resistive pathway to LED so it will give the brighter glow,
    when the pulse is over the lower current signal directly from the supply would need to 'take over' to leave a dimmer LED.
    I feel this circuit in its current form wouldnt work properly due to low current signal being connected at the same time as the higher current signal from the 555, how would they effect each other when connected simultaneously?
    I have briefly tested this with strange results but i want to inquire here to actual find out what will be going on electronically
    seqled.png
     
    Last edited: Aug 14, 2015
  2. Alec_t

    Alec_t

    2,927
    797
    Jul 7, 2015
    When the 555 output is low you will be wasting energy by driving current into it via the left-hand resistor. Connect a diode in series with the 555 output (pin 3) to prevent that.
     
    AFex54 likes this.
  3. AFex54

    AFex54

    144
    2
    Apr 10, 2015
    ok , so other than that it should behave as I described ?
     
    Last edited: Aug 14, 2015
  4. Alec_t

    Alec_t

    2,927
    797
    Jul 7, 2015
    As you show only a part of your circuit it's impossible to say. How's the 4017 couple up to what you show?
     
  5. AFex54

    AFex54

    144
    2
    Apr 10, 2015
    the 4017 is just and extension of the circuit, it wouldnt disturb the circuit above.
    really what im wondering is what happens when the lower current signal and higher current signal from the 555 combine/meet ?
    my thought was maybe I needed to use a JFET to pinch closed the low current signal with the pulse from the 555
     
  6. Alec_t

    Alec_t

    2,927
    797
    Jul 7, 2015
    If I've understood what you're trying to do correctly, here's a sim of how I'd do it (only 3 diodes shown, but extendable as required).
    GatedCurrent.png
    The HC4017 was chosen so as to provide enough drive current for the LEDs without the need for one transistor per LED.
    When Vgate is high the transistor turns on and shorts R2 to give a higher LED current.
     
  7. AFex54

    AFex54

    144
    2
    Apr 10, 2015
    very clever, the transistors for each LED are actually part of the gate system for the sequencer which has now made me realise illuminating the LEDs during the ''step active,gate off'' stage is not as simple as thought.
    I will just forget about this trivial feature ,
    it doesnt effect the functionality of the sequencer itself so no need to stress over it.
    thanks for the help:)
     
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