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Help required with 555 timer project

Discussion in 'General Electronics Discussion' started by Steve333, Aug 24, 2013.

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  1. Steve333


    Aug 24, 2013

    I'm new to electronics and am struggling to understand a circuit I've just made. I'm probably being really thick, so apologies if this seems really obvious or dumb.

    I've attached a circuit diagram of the 555 timer circuit, which I've assembled. It works as it should, however I don't understand which path the electrons are taking, as there seems to be so many possible different paths.

    My guess is that when I touch the fly leads together the electrons go across the fly leads, onto row H on the breadboard and then into the 'trigger' PIN on the 555 timer. They then come out of the output PIN on the 555 timer and onto the LED? But wouldn't that mean the LED would be the wrong way round? Or do they go through the LED, then onto the capacitor, and then once the fly leads are disconnected, then go back through the LED in the correct direction? If that's the case, however, wouldn't it mean the electrons would be going back to the 555 timer via the output pin? Would that be a problem?

    As I say, I've got it working and have fitted different value resistors and capacitors to change the timed interval, I just need help understanding the different routes the electrons take so I can visualise the current flow better. I'm particularly interested in the capacitors role, as I was under the impression current doesn't actually flow through it, as the electrons can't pass between the two plates.

    If anyone could give me any help it would be much appreciated.





  2. KJ6EAD


    Aug 13, 2011
    You seem to be laboring under the impression that all circuits consist of a single path for all current flow. This would be true only for the simplest series circuits. If you'll download a block diagram of the 555 usually included with the datasheet but available from many other sources, you'll see that the current flowing through the load is controlled by an output transistor which is isolated from the rest of the circuitry in the 555 by the high impedance of it's base.

    I've used some terminology that may be unfamiliar to you so I encourage you to research any terms that you need to and ask more questions.
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    As KJ6EAD says, it's not just a case of electrons flowing through your wire, into the IC on one pin, and out on another.

    The 555 connects to both sides of the battery, and there are LOTS of current paths within the 555. It uses current from the battery for its own internal operation, and it can supply current (which ultimately comes from the battery) to the LED connected to its output (pin 3). This is the case generally with ICs and other circuit modules.

    BTW you should have a resistor connected between pin 3 and the LED. Something like 470 ohms is fine. With the LED connected straight from pin 3 to ground, you will get quite a high current flow in the LED and the battery voltage will drop when the LED turns ON, which is not the way that circuit should work.

    Please feel free to ask any other questions here.
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