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struggling to learn basics - 555 timer led flasher

Discussion in 'LEDs and Optoelectronics' started by messy, Jan 12, 2017.

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  1. messy


    Jan 12, 2017
    Not sure if this is the correct board but here we go....

    I've been trying to teach myself electronics as a hobby from various books and online tutorials however Im having a hard time almost as soon as i start. I have a fair understanding of components and what they do but when I try to use that info to construct basic circuits it seems to fall apart - the guides and schematics that I try to learn from just slap loads of components in with no explanation of why they are there or how the current is supposed to be flowing.

    Example - I thought id try using a 555 timer to make a continuous led flasher. I found lots of diagrams for this kind of circuit but they all keep showing the pins in weird configurations. - Are you meant to just ignore where they have physically drawn them and just trust the numbers?

    Also - doesnt current flow from negative to positive? Lots of diagrams i see like this one seem to suggest the opposite

    AND - in that diagram, im sure its supposed to flash the two leds alternately but i can only see how they could both be on at the same time - if pin 3 provides an additional current to allow it to push past the resistance of the second resistor and led then surely both would turn on at the same time?

    When pin 3 is not providing an output voltage can current pass in the opposite direction? Allowing a circuit to be completed for just the 1st led and not the second?

    Using these explanations of what each pin does I tried to follow what was going on on the left side of the diagram, bascially every pin except 3 and i just get lost each time..

    Pin 8 is taking in the voltage from the supply (but also sometimes 3 is taking a lesser voltage in as well?)

    Pin 4 - the reset, no idea

    Pin 7 - discharge, "may discharge a capacitor between intervals" i can see a capacitor but i cant see how that pin discharges the capacitor or what defines when it does so.

    Pin 6 - threshold, "timing interval ends when the voltage is greater than 2 thirds the ctrl" (ctrl pin is 5 - doesnt even seem to be in use) - also, even if 5 was in use - i assume its the capacitor discharging that would change the voltage going into pin 6? but in this diagram it suggests that the current is flowing in the other direction (away from 6 toward 1)

    Pin 2 - trigger "timing interval starts when voltage drops below half the ctrl (again, there is no ctrl?!?!)

    Pin 1 - ground reference voltage - is this acting as the ctrl? If so whats the point in pin 5?

    So - as far as i can tell, the capacitor discharges and puts the voltage of pins 2 and 6 up which starts a timing interval which sends a voltage out of pin 3? And then when the capacitor is empty the voltage over 2 and 6 falls back down it stops at which point pin 3's output stops?

    What are the 22 and 220 resistors for?

    Sorry for the wall of ignorance :( I think this would be so much easier if i had someone in person to talk to : /
  2. chopnhack


    Apr 28, 2014
    Yes, and its called electron flow theory. It is predominantly used in physics and as far as electronic's are concerned, you can choose to ignore it and use conventional flow so that passive components will make sense, such as the arrow of a diode.

    Indeed, when you first start out its a vertical wall to climb. We were just discussing how a chat feature would be useful on this forum, LOL.

    A 555 timer chip is probably not the best place to start as a newbie. I would seriously consider some more elementary circuits and a lot of reading to get you up to speed. Look at the Elenco 130 in 1 circuits.
  3. Bluejets


    Oct 5, 2014
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Yes. They are drawn this way on circuit diagrams to make them easier to understand, tidier, and to follow important conventions.

    Following on from the comments earlier... When polarity was assigned it was an arbitrary decision. When the direction of flow of current was first assumed, it was a coin toss. Unfortunately the direction was wrong. By the time the middle was realised, the direction of current flow was so ingrained it wasn't changed. It really matters, and the devices where it does ( vacuum tubes) are rarely used today.

    Find a datasheet for the 555. There will be several and they may go into different levels of detail about the 555. They will probably have a block diagram of what is logically inside the device. Whilst you may not initially understand much of what is on the datasheet, you should aim to understand most of it because this is the way you get information about most semiconductors, and understanding datasheets is very important.

    Having said that, even though a 555 is a simple IC, it's probably a lot more complex than anything you've built, so you shouldn't expect to understand everything straight away.

    There are whole books written about the 555, so even of it is a simple device, it can be used so many ways...

    Think of the output as a switch which can be connected to either one of the supply rails. Does that make the flashing LED circuit easier to understand? Yes, that means current can flow in both directions, it in the words you'll find in the datasheet, it can both sink and source current.

    You've asked a lot of questions. I think we've answered most of them, but I'm sure we've not done a perfect job, and or answers have probably raised even more questions. But yes, you've found the right place to ask them. Please feel free to ask more :)
  5. AnalogKid


    Jun 10, 2015
    Sorta kinda in the order of post #1..
    1. The primary function of a schematic is to show the signal flows and signal flow interactions, not to act as a physical wiring diagram. A schematic is a language unto itself. Generally speaking for both an overall schematic and the individual component symbols, inputs are on the left, outputs on the right, positive power sources toward the top, negative power sources toward the bottom, ground symbols *always* point down, and for digital logic ships the power and ground pins often are not shown to leave more room for signal clarity.

    2. Without the 555 powered up and operating, you are correct - both LEDs would be on continuously. But the 555 output can source or sink more current than either LED requires. When the 555 output is high, that effectively "shorts out" the upper LED, and current from the 555 output pin flows down through the lower LED to ground. When the 555 output is low, that "shorts out" the lower LED, and current flows from the positive power source through the upper LED through the 555 output (now acting as an input) to the 555 ground pin to ground.

    3. A 555 output stage is called a totem pole or push-pull stage. It can both source and sink current depending on its state and the external load.

    4.When pin 3 is high and sourcing current out to the load, that current comes from the circuit power source through pin 8. The transistors between 8 and 3 are saturated or nearly so, so there is very little voltage drop between 8 and 3, typically 1 V or so. So if the power is 9 V, the most that can appear at pin 3 is 8 V -ish. Same for when pin 3 is low. It tries to pull the load all the way down to ground (pin 1) but can't quite make it. However, it gets closer to GND than to Vcc, usually around 0.5 V.

    5. When grounded, the Reset input forces the output (pin 3) low no matter what is going on with any other pins.

    6. Pin 5 (control input) - ignore it. That is there for more complex applications, such as making the 555 output pulse width vary according to an analog control voltage.

    7. Pins 2, 6, and 7 are the pins that determine how the 555 operates, at what frequency or pulse width, and when it leaps into action. There are hundreds of 555 help pages on the web, so surf around until you find one that speaks to your level.

    messy likes this.
  6. BobK


    Jan 5, 2010
    This is a misconception. Pin 3 always supplies an output voltage. It might be 0, but that is not the same as no voltage. This should help you understand how the two LEDs alternate. When pin 3 is 0 which LED(s) light? When it is at 3V the which LED(s) light?

    By the way, 3V is too low, that circuit will not work.

  7. Audioguru


    Sep 24, 2016
    The datasheet for an ordinary 555 says that its minimum supply voltage is 4.5V when it barely works.
    The circuit does not have 22 ohms or 220 ohms resistors, they are 22k ohms and 220k ohms which are 22000 and 220000 ohms because "k" means thousands.
    bushtech likes this.
  8. messy


    Jan 12, 2017
    thanks everyone - very helpful, and i think anaolgkids post has cleared a lot up for me :D

    one last thing id like to ask before i go and do some more reading - so if the electron flow doesnt really matter, and in fact can flow both ways - how do you know in what order to place your components? For example if the current hits a led full on because the resistor thats supposed to protect it is on the other side of the diode from the direction of current wont it burn out?

    EDIT - also shout out to bluejets - that 555 timer page is great ! :D
  9. AnalogKid


    Jun 10, 2015
    1. In a series circuit, all of the current goes through all of the components. The current is determined by the voltage source and the sum of the impedances of the components in series. So if you have a 10 V battery connected to a 2 ohm and 8 ohm resistor in series, the total resistance is 10 ohms, the total current is 1 A, and that 1 A goes through both resistors. The voltage drop across each resistor is determined by Ohm's Law.

    2. You didn't ask about this, but it is the companion - In a parallel circuit, all of the voltage is across all of the components. The current in each component is determined by the voltage source and the individual impedances of the components. If you have a 10 V battery connected to a 2 ohm and 8 ohm resistor in parallel, the current in each resistor is determined by Ohm's Law, and the total current is the sum of the individual currents.

    messy likes this.
  10. chopnhack


    Apr 28, 2014
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