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Author Topic: LED PWM Dimmer with LM555 - Question  (Read 3574 times)

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modaristis

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LED PWM Dimmer with LM555 - Question
« on: December 08, 2010, 23:23:14 PM »
Hey guys,
I'm new to electronics and i'm trying to learn some things. I have studied a lot from online resources and i came across this site. I studied most of the wiki pages and i had many of my questions answered. However i dont understand much of the information (located here) about the LM555 IC. In fact, in this project i cannot understand why the led does not flash because of the 555, which produces pulses. As i think, the 555 produces pulses for example a series of 0 and 1 which control whether current go through led or not. Please correct me if i do a mistake.
Furthermore i would like somenone to explain me the general use of a transistor and what the base, the emitter and the collector do in general.
Thnx,
Modaristis

(I would be grateful if anyone could explain this to me in Greek, because i dont understand that much of English :/ )

modaristis

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Re: LED PWM Dimmer with LM555 - Question
« Reply #1 on: December 09, 2010, 00:37:24 AM »
I had an idea. Why do we have to use the 555?
Could we use just a potentiometer, a led and power supply to achive the dimmer effect for the led?
I though that the potentiometer is a variable resistor, so if we keep a resistance value above a certain lvl (to not destroy the led) and we gradually increase the resistance value we can achive the LED dimmer effect...

kam

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Re: LED PWM Dimmer with LM555 - Question
« Reply #2 on: December 09, 2010, 08:04:46 AM »
hi modaristis.
Quote
As i think, the 555 produces pulses for example a series of 0 and 1
You are absolutely correct

Quote
i cannot understand why the led does not flash because of the 555, which produces pulses
The LEDs does indeed flash, but the frequency is very high (around 1.5KHz) and the human eye cannot see this flash.

Quote
Furthermore i would like somenone to explain me the general use of a transistor and what the base, the emitter and the collector do in general.
Have you read the transistor theory?
I will explain the transistor, as if it was a hydraulic component. First, the collector-emitter contact. Imagine this as a Ball Valve, a normal ball valve like the ones we turn on the water. Our power source, is a huge water tank. We connect the power source (tank) to the collector (first end of the ball valve). Now, the more you open the valve, the more water you get right? right!

And now comes the base of the transistor. The base of the transistor is a separated circuit with a hydraulic actuator. This actuator is connected to the handle of the ball valve, and it can be controlled with a very small amount of water (small power supply), like 10 or 100 times less than the tank of the collector. Suppose now that you put water in the base (actuator). It will turn on the ball valve, and a great amount of water will flow within. Actually, the more water in the base, the more the valve is open, the more the water you get from the emitter (other end of the ball valve).

But the thing is, that with just a few amount of water that you provided in the base, you got a huge (and proportional) amount  of water through the collector-emitter.

I hope i helped



modaristis

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Re: LED PWM Dimmer with LM555 - Question
« Reply #3 on: December 09, 2010, 16:58:48 PM »
Thnx kam!! You helped me a lot! At least now I have a "visualisation" of the transistor. Actually I did read the Transistor Theory before and then i posted here because i didn't understand a lot.
What do u think about post 2?

kam

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Re: LED PWM Dimmer with LM555 - Question
« Reply #4 on: December 10, 2010, 11:42:49 AM »
post 2: you can actually use the pot to control an LED. But here is what happens: Using the pot, the LED dims because the power is dissipated on the pot itself (in the form of heat). For example, if the LED needs 80mW for full power and you dissipate 40mWatts in the pot, then it will receive half the power so it will dim.

With PWM, you do not dissipate any power, you just never deliver it (because you turn it off for half the period).

The difference? Efficiency. PWM is much much more efficient, because it loses no power on dissipation. It becomes more and more efficient as the power is increased (for example if you have to dim 10 LEDs).
Also, you cannot dim lot of LEDs with a pot, because they do have a power limit to dissipate, which is rather low. PWM components such as the IRF540 can control nearly 8 amperes of load... that is HUGE if you consider that an LED draws 30mA .

modaristis

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Re: LED PWM Dimmer with LM555 - Question
« Reply #5 on: December 10, 2010, 17:50:13 PM »
so it's just a waste of energy with a little number of leds and with more leds it's a efficiency problem..i see! thnx for ur time replying me!