Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Help interpreting a simple circuit?

  1. Dec 2, 2006 #1

    For an electronics project, my friend and I created this circuit that we found online: http://www.redcircuits.com/Page72.htm" [Broken]

    We had no problem putting it together, but we’re not really sure how it works. Our professor wants us to have at least a basic understanding when we present it to him.

    The diagram is attached and here is the accompanying description:

    R1______________1K5 1/4W Resistor
    R2____________680R 1/4W Resistor (Optional, see text)
    C1_____________47µF 25V Electrolytic Capacitor
    D1_____________LED any type
    Q1___________BC337 45V 800mA NPN Transistor
    P1_____________SPST Pushbutton
    LP1____________Filament Lamp 12 or 24V (See text)

    The purpose of this circuit was to create a ring in which LEDs or Lamps illuminate sequentially. Its main feature is a high versatility: you can build a loop containing any number of LEDs or Lamps, as each illuminating device has its own small circuit.
    The diagrams show three-stage circuits for simplicity: you can add an unlimited number of stages (shown in dashed boxes), provided the last stage output was returned to the first stage input, as shown.
    P1 pushbutton purpose is to allow a sure start of the sequence at power-on but, when a high number of stages is used, it also allows illumination of more than one LED or Lamp at a time, e.g. one device illuminated and three out and so on.
    After power-on, P1 should be held closed until only the LED or Lamp related to the module to which the pushbutton is connected remains steady illuminated. When P1 is released the sequencer starts: if P1 is pushed briefly after the sequence is started, several types of sequence can be obtained, depending from the total number of stages.

    Could someone explain the roles of the components for the basic stage (in the dashed boxes)?
    The LEDs light gradually, so we think the pattern must be caused by the charge and discharge of the capacitor. But how does the charge flow through the circuit? And in particular, what does the transistor do?



    Attached Files:

    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Dec 2, 2006 #2
    Alright dusty,

    I'm not the best electronic engineer in the world but I'll have a go mate. I don't know how the push button works but I'll try and explain the rest of it. Looking at the far left arrow entering C1, when current flows to here from the positive source it charges the capacitor which then discharges and causes a voltage to occur across the resistor R1. This voltage will rise until it overcomes the breakdown voltage of the transistor which is normally around 0.6V. This then turns on the transistor allowing current to flow between the two vertcially opposite terminals which are termed 'collector' (the top one) and 'emitter' (the bottom one) to be precise.

    So now you have a voltage across your LED and a means for current to flow through it via R2 and the terminals of the transistor to the negative, therefore the LED lights up. Part of the current flowing through the LED goes into the next module and into that capacitor which activates the same process in that module and so on and so forth.

    The values of the capacitor and R1 and R2 are arranged I presume so you don't dump all your current to ground/negative through the first module. They need to be arranged so that there is an incentive for it to flow through the other modules too. The transistors's gain value will also I think, affect this.

    Just one more note also, I think that having the push-button switched on will only guarantee that the last lamp/LED in the chain will be on during start-up. I can't at the moment see how it could turn all of them on.

    I hope that helps mate, it's the best I can do for now :wink:


    Hmm.. had another look and I'm close but not quite right there. From what I can see the applied voltage can work on overcoming the breakdown voltage of any one of the transistors in the chain so exactly how they operate in sequence I don't know. Lol I'm an electronic novice though. A very good guy to ask would be one called NoTime. Drop him a pm. Anyways the advice I have given above is mostly right and enough to allow you to convince your professor you at least did some work on it :tongue:
    Last edited: Dec 2, 2006
  4. Dec 3, 2006 #3
    Thanks so much Adder_Noir ! That's a lot more than I could come up with, so I don't think we'll look like complete idiots when we present it... :)
  5. Dec 10, 2006 #4
    I do believe this is how it works, keep in mind i am just starting in electronics.
    and am not university schooled.

    References will be made in Left(first) to right(last) ordering.

    When the button is pressed it causes a Circuit in the last Led to ground through the first Cap resistor pairing. Charging the cap, untill the capacitor is charged which then stops the Last led lighting, and Discharges the first Cap through the transistor to ground because it will be the path of least resistance.

    While it discharges it cause a transition effect, from the Positive rail to ground out also. Which lights the first Led. Also charging the second Cap off of the Positve rail. which starts the effect all over again.

    The switch acts as a catalyst (sp?), you must press the switch long enough for the cap to charge high enough to trigger the transistor. But after that it maintains its own charges.

    Each resistor keeps the Cap from Discharging through ground too quickly.

    If you Change the Capacitors to a higher rating the lights will stay lit longer.
    But you will have to hold the button longer to start it, probably.

    And if you change the type of transistor you can create a different effect. Possibly a ramping(Fading) Effect. Using amplifier transistors instead of gating.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook