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Need help with 555 timer circuit

Discussion in 'General Electronics Discussion' started by robertgzzzt, Jun 26, 2013.

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  1. robertgzzzt

    robertgzzzt

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    Jun 26, 2013
    I'm obviously new here, so let me start off by saying I'm a total rookie at electronics. I just got into this a few weeks ago, when I decided to take on the task of building a very detailed 1/350 scale model of the star ship Enterprise from Star Trek. This model is almost 3 feet long when completed, and is designed to be lit, so I decided that I would build my own LED circuits instead of buying an expensive pre-built kit, which has now introduced me to an equally interesting hobby in electronics.

    I'm going to need four circuits. One of them is a simple blinker circuit, a strobe circuit, a chaser circuit (10 LEDs lighting in sequence), and a fader circuit. I've already got the chaser, it's good to go. I've built it on the breadboard and tested it thoroughly. Only thing to do now is transfer it to a circuit board.

    It's this simple little blinker that I'm having trouble with. I can build it, no problem. It's a simple circuit to build with the 555 timer, but here's my problem. When I wire in more than one LED in parallel (anode to anode thru a resistor and cathode to ground), the LEDs get progressively dimmer the more I put in. After wiring in just three LEDs, the third one is just barely visible compared to the 1st one. I'm powering the circuit with a 9v 1amp wall wart. I was under the impression the 555 timer could output up to 200 milliamps. These LED's current rating is only 20mA, so what's the deal?

    My circuit is basically the same as the schematic in the link below, minus "C2", and with different values for C1, R1 and R2, and I'm using 220ohm resistors for the LED(s). What do I need to do to get 8, nice bright LEDs, in parallel, going on for 1 second and off for 1 second with my 9v wall wart?

    http://startingelectronics.com/beginners/start-electronics-now/tut5-555-LED-flasher/
     
  2. BobK

    BobK

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    Each LED needs a separate resistor.

    Bob
     
  3. robertgzzzt

    robertgzzzt

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    Jun 26, 2013
    Hi, Bob. Thank you for responding. Sorry if I didn't make it clear, but each LED has it's own resistor. I place a resistor from the anode of one LED to the anode of the next, then place the cathode to the ground rail on the BB.

    One other thing, if I use two different colored LEDs, sometimes one of them won't work. In the finished circuit, there will have to be different colored LEDs. two reds, two yellows, two green and two blue if I remember correctly. I'm I gonna have to use all white LEDs and paint them with a magic marker if I go the 555 timer route??
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Hey, Robertgzzzt, there have been a number of people outfitting their starships in the time I have been here. Use the search to find them and see what they've done. Some of the threads have gone on for a long time describing exactly what you want to do in quite some detail.

    I guess "starship" "Enterprise" and "Star trek" would be good words th start your search with. They're not common in everyday talk here :D

    edit: Here is one of them: https://www.electronicspoint.com/project-star-trek-room-t232479.html
     
    Last edited: Jun 27, 2013
  5. davenn

    davenn Moderator

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    That's not the definition of a separate resistor for each diode. You have wired all the resistors in series. Each of your anode resistors needs to go individually to +V

    look in the tutorials tab above for a section on using LED's

    cheers
    Dave
     
  6. robertgzzzt

    robertgzzzt

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    Jun 26, 2013
    Thank you for the link, Steve. I'll take a look at those. I wasn't aware of all the trek modelers on this site.


    Aww crap. OF COURSE. That 3rd LED was operating (or trying it's best) behind 660 ohms of resistance. No wonder it was dim. Now I see. Thank you so much, Dave.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Actually all of them were "behind 660 ohms of resistance" asuming you connected them like this:

    +V --^v^v^v^--|>|---^v^v^v^--|>|---^v^v^v^--|>|--- -V

    It's the total resistance that matters, not where it's placed.

    All of these are effectively identical

    +V --|>|--^v^v^v^---^v^v^v^---^v^v^v^--|>|---|>|--- -V

    +V --|>|---|>|---|>|--^v^v^v^---^v^v^v^---^v^v^v^-- -V

    +V --^v^v^v^---^v^v^v^---^v^v^v^--|>|---|>|---|>|--- -V

    The normal solution is

    +V --^v^v^v^--|>|---|>|---|>|--- -V

    But, again, it doesn't matter where you put the resistor and long as it's in series with the LEDs somewhere.

    If you look at the tutorial on driving LEDs that you've been pointed to, you'll see that if you want more LEDs, you create many strings like this and connect them all in parallel.

    edit: Oh, we didn't point you to it: https://www.electronicspoint.com/got-question-driving-leds-t256849.html
     
  8. robertgzzzt

    robertgzzzt

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    Jun 26, 2013
    I understand :)

    That's wiring the LEDs in series, where three LEDs (or any number) would only require one resistor, no? My LEDs are wired in parallel (but I had the three resistors in series).

    I don't think all of the LEDs were behind 660ohms of resistance. The first LED was very bright, the next was noticeably dimmer, and the third was barely visible. Rather than try to draw you a diagram with keyboard characters, I'll just try to explain it. I will refer to the current limiting resistors as R #1, R #2, and R #3 for the purpose of this discussion, not to be confused with other resistors in the circuit. :)

    On the breadboard, I had one end of R #1 plugged into the output of the 555 timer (port 3), and the other end in an empty space on the breadboard. The anode of LED #1 was connected in line with this resistor and the cathode to the ground rail on the breadboard.

    Then I connected one end of R #2 to that same row, where R #1 ended and the anode of LED #1 is connected, and the other end of this resistor to another empty row on the breadboard. Then I connected the anode of LED #2 in line with resistor #2, and the cathode to ground

    Then I connected one end of R #3 to the row where R #2 ended and the anode of LED #2 is connected, and the other end (you guessed it), to yet another empty row on the breadboard, which is where I connected the anode of LED #3, and the cathode to the ground rail.

    I'm pretty sure LED #1 was behind 220 ohms, LED #2 440 ohms and LED #3 660ohms.

    Anyway, after reading Dave's post and realizing what the problem was, I went back and re-configured it, and after doing so, I had.....not 3, but 5 LEDs of all different colors (each behind it's appropriate level of resistance as per the specs) and all of them were shining beautiful and bright. (thanks again, Dave)
     
    Last edited: Jun 28, 2013
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Ah, OK, I see what you did (and drawing it in ascii would be beyond me :)).

    Sounds like you're OK now.
     
  10. robertgzzzt

    robertgzzzt

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    Jun 26, 2013
    Yes, the problem for which I started this is solved, but I also have an unrelated question. If I should start a new thread, just let me know, or you can move it, whatever.

    In perusing the internet looking for tutorials on the best, easiest way, to build an LED strobe circuit, I stumbled across a method that was.....maybe too easy?? The reason I say that, is because, being as easy as it is, there must be something inherently wrong with it, even though I tested the circuit by letting it run, non-stop, for about 48 hours, and at the end of that time, it was still running fine, and nothing was overheating.

    I just built an astable circuit with the 555 timer, then wired the LED, with it's resistor, between the input and the output of the 555 timer, connecting the anode to pin 8 and the cathode to pin 3, then swapped R1 and R2, so that R1 is much larger than R2...and that's all there was to it. Perfect strobe circuit, but something tells me, if it was this easy, everyone would be doing it. If I remember correctly, all the tutorials around the net call for a diode in the circuit (or was it a transistor), and I certainly have no objection to using one, but I just don't have any diodes on me at the moment (transistors either).

    Is this thing gonna blow up in my face or what?
     
    Last edited: Jun 28, 2013
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    No, the circuit you have described sounds fine to me.

    There is a limitation of the 555 in the normal astable configuration that the ON time must be longer than the OFF time.

    For a strobe you probably want the LED to be ON for a short period and OFF for a longer period.

    The answer is to connect the LED and the resistor between the +V and the output rather than between the output and ground.

    If this is for your model, then it is perfectly reasonable to continue the conversation in this thread.
     
  12. robertgzzzt

    robertgzzzt

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    Jun 26, 2013
    Right, that's what I've read everywhere, that the duty cycle must be at least 50%.

    Right again, essentially with a duty cycle of only maybe 10-20%

    I just thought there must be some catch to it, since it was just blind luck that I found this solution while reading an article on the 555 timer. Maybe I should post a tutorial video on Youtube. :)

    EDIT: Thanks again, Steve. I really appreciate all of your help
     
    Last edited: Jun 28, 2013
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    No catch!

    Connect your load between +V and output instead of output and ground and you get the opposite behaviour -- your load will then be ON for less than 50% of the time.

    Essentially your load in ON when the output is OFF (OFF = low -- means it's sorta connected to ground).
     
  14. robertgzzzt

    robertgzzzt

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    Jun 26, 2013
    ....another issue

    Ok, I've got another problem. I need help with either one of two things. I need to know how I can drive 14 20mA LEDs (in parallel) off of a single 555 timer (which has a max output of 200mA), OR, I need to know how I can link two 555 timers in astable mode, so that all of my LEDs flash in perfect synchronization. It seems to me, the former would probably be easier, if there's a way to boost the output current of the 555 timer. PLEASE help :)
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    You use the 555 to switch a transistor capable of handling the current.

    Remember that if you place LEDs in parallel, then each one should have its own current limiting resistor.
     
  16. robertgzzzt

    robertgzzzt

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    Jun 26, 2013
    Yes, each LED will have it's own current limiting resistor soldered right to it's - leg.

    So, how would I wire that transistor? You gotta remember Steve, I am a complete rookie at this. Would I wire the transistor "Base" to the 555 output (# 3 pin), then the Emitter to LEDs + and the Collector to LEDs - ?

    I found out a little while ago that I'm only going to need 10 LEDs running off of this 555 timer, but I want there to be plenty of current to spare, as I don't want to run the 555 timer right to the raggedy edge. So based on, lets say 12 LEDs @ 20mA each, I suppose I need a transistor that can easily handle 240mA @ 9V. What would you suggest?

    I'm also going to need to wire in a fuse between the 555 timer circuitry and the LEDs, just in case something were to happen, like a catastrophic failure of a component, causing a short circuit, I don't want to lose all of my LEDs as well, as they will be sealed up inside the model. I can put all of the circuitry controlling the LEDs inside the base that the model is displayed on, for easy access to repair if need be. I want to build all of the circuit for this model to last a very long time. Any help and advice you can give me, in that regard, would be very much appreciated.
     
    Last edited: Jul 13, 2013
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    If you use a bipolar NPN transistor, you would connect the output of the 555 via a 220 ohm resistor to the base. The emitter would be grounded and the collector would be connected to the cathode end of the LEDs.

    You need to select a transistor capable of handling the current, a 2N3055 (in a TO-220 package, don't go overboard with a TO-3 package) would likely work. There are many other transistors you could use, but that one just comes to mind..

    An N channel mosfet could also be used. In this case the gate would be connected to the 555's output (via a resistor if you wish -- use a 220 ohm resistor). The source is grounded and the load is between the drain and +v.

    A mosfet is static sensitive but has far lower losses.

    Oh, only 12 at 20mA... You could use a much smaller transistor. Do you have any? Tell us what you have and we may be able to puck one out for you.
     
  18. robertgzzzt

    robertgzzzt

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    Jun 26, 2013
    All I have is capacitors and resistors at the moment. I don't have any transistors at all. I had planned on increasing my inventory of components to have on hand for other projects that may interest me, but right now my only concern is lighting this model.


    12 is just a number I threw out there. In actuality there will only be 10 LEDs, but I want there to be plenty of headroom, so that the 555 timer is not stressed, so make your recommendation based on that.

    I thought about installing an "in-line" fuse to my ground wire, just as it leaves the perfboard (for easy access), is that a sound configuration, and if so, what rating of fuse should I use to protect the LEDs?

    Thanks again, Steve :)
     
  19. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    He needs the LEDs to be ON when the 555's output is LOW, so he'll need a PNP, or a P-channel MOSFET.

    To the OP. You can probably do what you want with just the 555 if you put two or more LEDs in series for each string.

    You need to find out the approximate forward voltages of the LEDs you're using, when running at the current you want. For example, a standard red LED has a forward voltage of about 2V at 20 mA; a green LED is about 3V and blue and white LEDs are higher than that.

    If you have a 9V supply, you'll get about 8V between the +V rail and the 555's output when the 555's output goes LOW to illuminate the LEDs. (The 555's output doesn't go all the way to 0V.) Your 8V can now be split up between one, two, or more LEDs and a current limiting resistor.

    For example you can connect a red LED (~2V) and a green LED (~3V) in series to get a 5V drop, and make up the remaining 3V with a current limiting resistor, calculated according to Ohm's Law, R = V / I, where V is 3V (there will be only 3V across the resistor) and I = 0.02 (20 milliamps). R will be 150 ohms.

    Each one of these strings will draw 20 mA, and they're all connected in parallel. You should be able to roughly halve the number of strings, compared to having an individual current path and current limiting resistor for each LED, so you won't exceed the 200 mA total current specification for the 555.

    One caveat is that as the voltage across the current limiting resistor becomes a smaller proportion of the total supply voltage, the LED current becomes more dependent on the exact supply voltage, so smaller variations in the supply voltage have a larger effect on the LED current. I like to ensure that the current limiting resistor drops at least 20~30% of the total applied voltage.
     
  20. robertgzzzt

    robertgzzzt

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    Jun 26, 2013
    I don't think that is correct. I'm running the 555 timer in Astable mode, so I'm pretty sure the LEDs are ON when the output is HIGH. This is NOT the strobe circuit that I mentioned in the early post. These LEDs will blink...ON for 1 second....OFF for 1 second (or thereabouts).


    There are green and blue LEDs in the circuit, as well as white and red...and there are 10 TOTAL, all blinking at the same rate. The forward voltage for the blues and greens and whites is 3.0-3.4V (3.2 "typical"). Even wiring half of my LEDs in series will exceed the capability of my 9V PSU.
     
    Last edited: Jul 14, 2013
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