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Help with IR sensor controlled off-delay dimmer.

Discussion in 'Sensors and Actuators' started by dlevsApiJ, May 19, 2016.

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  1. dlevsApiJ

    dlevsApiJ

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    May 19, 2016
    Hello,

    While I am doing a course in electronics, which is quite theoretical, I decided to start a side project.

    Based on the LED Off Delay with dimming effect of this link http://pcbheaven.com/circuitpages/LED_Off_Delay/ I wanted to create a circuit where an IR sensor turns on a LED light, which then slowly dims. Here is my enhanced circuit. For the elco I am currently using a 47000uF version which lets the LED stay on for quite some time, which gives a nice effect.
    LEDdimming_off_withIR.png
    My question is whether this circuit seems legitimate? Are there components that should be replaced, or that can be left away? Maybe something is missing? I am mainly hoping for tips and tricks to extent my knowledge in a more practical way than the course offers.

    Besides, what is a suitable programm for drawing electric circuits...?

    Thanks in advance!
     
  2. hevans1944

    hevans1944 Hop - AC8NS

    4,560
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    Jun 21, 2012
    Looks "legitimate" to me. If "magic smoke" isn't pouring out of something, then I would consider any circuit "legitimate" whether it worked as intended or not. Your 47,000 μF electrolytic capacitor is gonna stress that 47 Ω resistor when the BC327 comes on, so make sure it can take the heat. There may be "better" ways to use the PIR detector to trigger a quick turn-on of the LEDs, followed by a slow dimming of their brightness when the PIR goes off, but if it works "gud enuf" for your purposes, then it is good enough. No more bells and whistles needed.

    Oh, and welcome to Electronics Point. It is good to see you supplementing your studies with "hands on" experimentation.
     
  3. dlevsApiJ

    dlevsApiJ

    5
    0
    May 19, 2016
    Thanks for your reply.

    So it cannot be the case that components get too much current / have too much voltage over them, which will not directly break the component but what will result in a shorter lifespan? Or that a significant amount of current goes lost in the circuit, which can be seen as a big loss? I ask this because the reason I am learning electronics is because I want to realize circuits that will be used by others (in this case it would be a lamp). A "better" circuit is therefore what I strive for, or can this "better" be seen as insignificant?
     
  4. dlevsApiJ

    dlevsApiJ

    5
    0
    May 19, 2016
    @hevans1944
    I do not want to be too demanding but it would be really helpful if you explain more in detail how you mean "better". See my previous post
     
  5. hevans1944

    hevans1944 Hop - AC8NS

    4,560
    2,132
    Jun 21, 2012
    "Better" depends on your design criteria. Simple, inexpensive, and efficient are some you might consider. Form factor (how big it is and where will it fit) is usually a desirable consideration. A 47,000 μF capacitor is likely to take up more space than, say, a Microchip PIC which can perform the same timing function.

    Using a transistor as a "variable resistor" to control the load current to a string of LEDs is pretty simple but not very efficient. What if someone asked you to apply your circuit principle for use in a large auditorium to slowly dim the "house lights" prior to the beginning of a performance? Sure, this is not the original purpose for your circuit, but why not think of other applications where a controlled dimming effect is desirable? A new circuit could be "better" in the sense that it is more versatile if the circuit can be applied to more than just dimming a few LEDs.

    When controlling large amounts of power, a transistor operating in its linear mode is very inefficient. That does not mean you should not do it. The popular three-terminal series regulator uses a transistor in this manner. However, a circuit that uses a transistor in switch mode (off and saturated on) is more efficient, because the only time significant power is dissipated in the transistor is when it switches from "off" to "on" and from "on" to "off". This switching action can be extremely fast (microseconds) and the ratio of "on" time to "off" time (duty cycle) is easily controlled to deliver a variable amount of power to the load. This is called pulse-width modulation (PWM) of the power source.

    A disadvantage of PWM to control power applied to a load is added complexity, but with modern integrated circuits complexity is a given that is built-in at no additional cost. Amazingly enough, complexity at the level of integrated circuits often costs less than a discrete analog component design, of which your circuit is an example. Does this mean one circuit approach is "better" than the other? Not necessarily. It all depends on your design objectives, your criteria.

    For example, consider an animated advertising display operated by a small permanent-magnet (PM) DC motor: potential customer presses a button, or just walks by the display, triggering a PIR sensor, and the motor starts. The mechanical animation runs for awhile and then slows down to a stop, awaiting the next activation. It could have been designed to just abruptly stop, but market research has shown that slowing down the display is a more effective "attention getter" to passers by. (I am making this part up, but it sounds plausible, right?)

    As you learn electronics you will accumulate a rich set of tools (previously designed circuits) that you will use in future designs. The circuit you have shown us is just one of those tools. There may be "better" tools to use for different or similar applications, and it will be your job to select the "best" tool for the job. I would compare this to my woodworking hobby, where I have many saws that bear this similarity: the all cut wood. The saw I choose to use depends on how I want to cut the wood. I would not use my portable contractor's saw to cut pins and tails for a precision Dovetail joint, for example. And I would not use the saw that I use to build Dovetail joints to build a house.

    If you want to "improve" your basic circuit consisting of a PIR sensor, a string of LEDs, and something in between to create the slow dimming action, I would investigate PWM controlled by a small PIC microprocessor. You can leave the 47,000 μF capacitor behind for use in filtering rectified power in another project.

    Hop
     
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