555 delay on and 555 flash, best way to connect together?

Discussion in 'The Projects Forum' started by JoshGreen, Apr 18, 2013.

  1. JoshGreen

    Thread Starter New Member

    Apr 18, 2013
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    I'm trying to create a circuit using two 555 timers, that once triggered via a switch, will stay low for a set period of time before going high and staying high.

    I've found these two seperate schematics and have altered the values in the rc network, but haven't found a successful way of using them together.

    Delay on,
    http://clarkson-uk.com/555-timer/operation/frames3.html

    Big LED Flasher,
    http://www.simple-electronics.com/2010/11/led-flasher-circuit-using-555-ic.html

    I've tried using a transistor as a switch to allow and stop the power supply to the flasher, therefore turning it off/on, however i was unsuccessful. Could this method work and i've made a mistake hooking it up? Or is there a way to do this using pin 4 of the 2nd 555 and maybe a different circuit at the start?

    Any advice would be greatly appreciated. Also this is my first post, so i hope I've done everything right?

    Thanks
     
  2. edwardholmes91

    Member

    Feb 25, 2013
    185
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    Yup, everything is fine :)

    With regards to your circuit I am a little confused... do you wan't a circuit that is off for a period of time and then flashes thereafter? Or do you want a circuit that is off for a period of time and then off for a period of time.

    • Astable configurations of the 555 timer are multivariaters, they don't have a stable state
    • Monostable configurations of the 555 timer are used for timing they can be on until you press a switch and be off for a period of time or off until you press the switch and be on for a period of time
    • Bistable configurations of the 555 timer have two stable states. They are simple memory that can be set and reset

    I would recommend this website for more information of the 555 and indeed other electronics info.
     
  3. JoshGreen

    Thread Starter New Member

    Apr 18, 2013
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    Glad to hear.

    Yeah, what I want is for when it is switched on, stay turned off until a set amount of time has passed and then start flashing. I understand both the circuits to some degree, and the different configurations, i just need help in determining the best way in connecting them together so that the signal from the first circuit triggers the second circuit.

    Thanks for the link, i'll check it out now :)
     
  4. JoshGreen

    Thread Starter New Member

    Apr 18, 2013
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    Thinking about it, would it be possible to remove pin 4 on the Led flasher from Vs and connect pin 3 from the delay on circuit to it? That way, when switched on pin 4 would be below 0.7 keeping the output low as the first circuit will remain low, and when the delay on circuit shifted to a high output, this would then change the output of the led flasher to high? right?
     
  5. edwardholmes91

    Member

    Feb 25, 2013
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    Ok, so far I am thinking a monostable and an astable. You can use a 556 instead of a 555 which is basically two 555s in one IC. Although this does mean that when turned on the LED will be flashing constantly. When triggered it will go off for set time and then start to flash constantly. Is that what you want?

    EDIT: The output of one 555 timer can be used to power the second one absolutely fine. Pin three of the monostable becomes the power rail of the second IC. Actually ignore what I said about the 556 because it shares the same power rails and I'm not sure as if would work in this case. Someone else may be able to verify.
     
  6. JoshGreen

    Thread Starter New Member

    Apr 18, 2013
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    Okay, so if i use the output from my first 555 as the power rail for the second, would the leds still be connected to the Power supply? Because the 555 will drop almost 2v right? so wouldn't it be best to connect them directly there as all the second 555 is really doing is turning on/off the transistors which the leds are using to pass through to ground?

    Can anyone else verify the 556 question?
     
  7. edwardholmes91

    Member

    Feb 25, 2013
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    I will draw up the circuit that I'm thinking of and will also try and verify the 556 thing in simulation. Will post the results when I'm done. Give me half an hour or so :)
     
  8. #12

    Expert

    Nov 30, 2010
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    The 556 has separate resets which could be used, at most with one transistor to invert a signal. The power rails are shared.
     
  9. JoshGreen

    Thread Starter New Member

    Apr 18, 2013
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    Thanks a lot! Using the 556 would be more beneficial but i've only got 555's atm anyway haha.

    Thanks again
     
  10. JoshGreen

    Thread Starter New Member

    Apr 18, 2013
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    Thanks, I may change the circuit afterwards to use a 556 after i get the simple stuff out of the way.
     
  11. edwardholmes91

    Member

    Feb 25, 2013
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    Hi Josh,

    As you said you only have 555 timers at the moment I have made it using those. Shouldn't be too hard to use a 556. I personally haven't used the 556 before and so I didn't think at first about the problem with resetting but like you say this is something that can easily be resolved later on.

    This is really messy I'm afraid and you might want to change the values a bit. If you place a variable resistor above R1 you can set a variable time delay before the LED starts flashing. Unfortunately the LED does flash very briefly at first before the circuit triggers. I think this is something to do with the R2 and C2 values but when I try changing them to smaller values it doesn't want to play :(

    The astable currently has a duty cycle (percent time on) of 52.38% and flashes at approximately 0.7Hz. Hope this helps, any questions then feel free to ask :)
     
  12. #12

    Expert

    Nov 30, 2010
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    I might be wrong because I didn't study this thoroughly, but,

    Second chip, RC delay on the reset pin so the output is forced low until the reset pin gets "high".
     
  13. JoshGreen

    Thread Starter New Member

    Apr 18, 2013
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    Thanks Edward, i'll get it breadboarded later and see how it works for me and tweak the values. As i'm using a transistor after the output hopefully the very brief flash that you've described could be ironed out, i'm looking to use 20 LEDs in total so the finished circuit will look slightly different but i'll start by making yours.
    It's been a great help and i'll report back once i've got the circuit i need! :)

    Also what program are you using to create the schematics? That ain't messy compared to the hand drawings at home haha!


    Thanks for the input, if the flash does become an issue i'll be sure to ask for more information as i'm not sure i fully understand how the RC delay affects the output. Still a complete novice here haha!
     
  14. edwardholmes91

    Member

    Feb 25, 2013
    185
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    Glad to help :) Going along the lines of a transistor you could use a transistor to switch a relay and then you could even use a separate supply for the LEDs altogether... although it does depend on how quick you want the LEDs to flash? The relay clicking on and off could become quite annoying!

    I use a program called Circuit Wizard. It's primarily an educational program and it can't simulate all of the components that some more complex packages can. Although I do find it makes nice schematics and it has a really nifty feature that shows current flow... the positive is green and it changes color gradually to red as it gets towards 0V. The speed of the little dots show the amount of current flowing. Can be quite useful when trying to work out what's happening in a circuit. I did a really silly mistake earlier on today though when making your circuit... the standard value for a resistor when you place it in a circuit is 1K unless you change it. I set it to 680K thinking I had set it to 680R and I had forgotten to change the decade drop down box and I sat scratching my head for at least 5 minutes trying to work out why it wasn't working properly! :rolleyes:

    When I said that the circuits were messy what I actually meant was that it wasn't quite as neat as I normally draw them. Normally I try and squash them up as much as I can and line everything up so it looks all nice. BTW I'm very OCD lol! :rolleyes:

    Looking forward to seeing how you get on when you breadboard it. if your breadboard has 2 voltage rails on each side I would recommend using one of the 0V rails for switched 0V... I.E. pin three of the first 555. This way you can connect all of the grounds for the astable section to this rail and all of the grounds for the monostable to the normal 0V rail.
     
    Last edited: Apr 18, 2013
  15. #12

    Expert

    Nov 30, 2010
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    I use MS Paint.
     
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