555 monostable to trigger a 555 astable

Discussion in 'General Electronics Chat' started by thedude123, Sep 9, 2011.

  1. thedude123

    Thread Starter Member

    Aug 29, 2011
    39
    0
    Hello,
    New member here and I've learned alot reading the forum.

    I am very close to figuring out my circuit but have run into a snag.

    I need to trigger the monostable timer, pause, then trigger the astable timer.

    So far I can get it to work but the astable only cycles once.

    Any help would be appreciated, thanks!
     
  2. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    Welcome to the forum!

    Can you post a schematic of your circuit? We can't be of much help until we can "see" how you've wired everything. If you don't have a way to draw it on your computer, draw it by hand, take a clear picture, and post it here.

    If your circuit is laid out well, i.e., we can see all the connections, you can take a picture of that too and we'll do our best to help you out.
     
  3. thedude123

    Thread Starter Member

    Aug 29, 2011
    39
    0
    Yes, thanks for the quick reply. I've attached a pic of the schematic and o-scope displays.

    I just cant seem to get the astable to continue...

    A bit of history,,, I am using Multisim to lay down the baseline of the circuit. The timing is WAY off, but I just set R1, R2, C1, etc,, to whatever makes the circuit work for sim purposes. I'll put the proper components in when I breadboard it.

    I made the monostable, T1 then the astable, T2. I tried a few methods of connecting them that I cobbled from the internet. Hence the inverters on the T1 output to reverse it to go High at the T2. I could go on and on,, haha....

    what do you guys think?
     
  4. thedude123

    Thread Starter Member

    Aug 29, 2011
    39
    0
    FYI...

    I originally started with this circuit, but could not make it work in Multisim...

    I made some modifications as you can see when compared to my schem.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Several points, on a 555 there is a reset on pin 4. It will shut the 555 down completely, the output will go low and stay there.

    The second point is when a 555 oscillator (astable) starts up there is a long pause while the capacitor starts up. You can see this clearly (along with an explanation) in my 555 Hysteretic Oscillator article. This applies to most 555 oscillators. There are some simple fixes if this is a problem using a transistor and 2 resistors, but most times it isn't a big deal.
     
  6. thedude123

    Thread Starter Member

    Aug 29, 2011
    39
    0
    Great reading, thanks... I briefed through it but didnt get a sense on how to properly wire the second to respond from the first......will study it more.

    I read in another forum from a search that said to trigger the second circuit that I needed to connect my monostable output (pin 3) to my Reset Input (pin 4) on my astable circuit. But for my needs, I needed to put an Inverter in this circuit. This is so the T1 output would be LOW during its time, then go HIGH for the reset of T2 and therefore activating the output to "blink". Maybe I should take a step back and retract my original concept of my learnings regarding using two 555 timers to make a circuit to the specs that I need. It might be better to ask, "What is a good starting point for the project"? "Am I on the right track"?

    I am learning, I certainly am not asking anyone to design it for me.

    Am I on the right track by using 1 monostable timer to trigger the second astable timer?

    Again,,, Basically I need to energize the circuit, have a delay, then have an LED blink. I can calculate the delay and blink rate. Just need to know how to make T1 and T2 work together.

    I also hope to learn enough regarding "daisychaining" these timers together so if I want to add another, I will know how.


    Thanks ....
     
  7. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    When a monostable is triggered the output goes high. If this high is feed to pin 4 of the second 555 (the oscillator) it will oscillate when pin 4 is high. I assume this is what you want, no inverter needed (I have had to invert the signal for other applications).

    Does the startup glitch matter on the oscillator?

    You get the same glitch when you power up the circuit.
     
  8. thedude123

    Thread Starter Member

    Aug 29, 2011
    39
    0
    The operations that I need are this.
    T1 is mono
    T2 is oscillator.

    Trigger T1 and pause, (output low) for a lenth of time.
    After that time output goes high and triggers T2 to oscillate.

    If possible, T2 should oscillate for a short time then turn off.

    Seems I need T1 to run opposite than normal. Instead of triggering to make the output high for a set time, then go low. I need it to stay low for a timeframe, then go high.. Like a simple timer.
    All the mono 555s I've seen the output goes high as soon as it is triggered.

    A startup glitch on T2 is acceptable if it is just a short blink, then it oscillates....

    I really think I'm close,,, I have breadboarded the T2 oscillator with pots on R1 and R2 for adjustability and works great. I just need a a way to trigger a pause before it starts the T2... I assumed this is a second 555 in mono?
     
  9. thedude123

    Thread Starter Member

    Aug 29, 2011
    39
    0
    I have had some more time to work on this project and I have gone back to basics. I've attached a pic of my breadboarding of the two circuits.

    The two circuits each have a 555 timer IC. The one on the right is a monostable (U1) and the one on the left is astable (U2).

    Again, the ideal circuit I need will perform the following actions.
    1.) Press and hold a momentary switch.
    2.) Turn on the circuit.
    3.) Release the momentary switch after a random time passes. (Up to the user to start the process by releasing the switch).
    4.) Delay for 20 seconds.
    5.) After 20 seconds flash a set of LEDs and a beeper for approx 10 seconds then go silent for 5 minutes.

    6.) If possible,,,,, After 5 min, flash the LEDs and "Chirp" the beeper once every 5 min until the cirquit is turned off..

    For now I have been just trying to get steps 1-5 working, with some luck, but not quite....

    Both operate as I need them to alone.
    U1 has a 3.6-4 sec delay (I changed values of R1 and C1 for quick troubleshooting results) and U2 flashes the LED as I need. (The beeber is disconnected for troubleshooting)

    __________
    -When I connect U1 pin3 to U2 pin2.
    Triggering U1 gives a 4 sec pause then the LED lights from U2, but does not flash, it stays lit instead of flashing. When I disconnect the jumper at U2 pin 3 the LED flashes as needed.

    Another problem,,, U1 will start after I quickly activate the momentary switch as it should. But I noticed than if I press and HOLD it,,, it starts anyway and when I release the switch it comes on instantly. U1 C1 starts its discharge immeadiatly, regardless of how long the switch is held.

    So,, I'm trying to as specific as I can without boring the whole community.

    Any more help is still appreciated.....
     
  10. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I'm not sure what you mean about press and hold a button for a random amount of time for step one.

    You want to turn on the circuit after you have pressed the button, then have the user start the circuit by releasing the button after it is turned on.

    Eminently doable, but over complex. Why not just have a circuit that starts when you release a button? A toggle switch might even be better, since it can be used for debounce.

    Going past the activation of the circuit (we can come back to that later).

    You activate the circuit, which starts a 20 second monostable.

    After it times out a astable that goes 10 seconds on 5 minutes off.

    Your big problem is lack of filter caps, 0.1µF per cap, 220µF for the main power supply.

    This is your current schematic, right?

    [​IMG]
     
  11. thedude123

    Thread Starter Member

    Aug 29, 2011
    39
    0

    Answers in your quote above in BLUE
     
  12. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I have had several beginners who forgot the bypass caps (the caps on the power supply). They can be critical.
     
  13. thedude123

    Thread Starter Member

    Aug 29, 2011
    39
    0
    I have noticed that in some of your circuits you would put a 100uF capacitor across the power supply. Do you think that is what is causing my problems? If so,, that is a quick and easy fix!! would be great!

    I have learned enough about the 555 to generally make it do anything I want,, but connecting them together is stumping me.....

    So I will connect the 100uF to my pwr supply and see if that helps....
    I wouldn't think that this would solve this, but I am learning,,,,,

    Thanks!
     
  14. thedude123

    Thread Starter Member

    Aug 29, 2011
    39
    0
    I connected a 100uF cap to the power supply and nothing changed,,,. URG!..
    Although, I will keep it inline to avoid any other problems as you suggest..

    What is the secret with using one 555 timer to trigger the start of another?
     
  15. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Give me your exact schematic as it now stands and I'll wire one up.
     
  16. raimadsouza

    New Member

    Sep 19, 2011
    1
    0
    Plz give a schematic diagram of the circuit so that it will be easier to solve.
     
  17. thedude123

    Thread Starter Member

    Aug 29, 2011
    39
    0
    Ok. Will make one and post tonight.
    Thanks all.
    Via droid
     
  18. thedude123

    Thread Starter Member

    Aug 29, 2011
    39
    0
    Sorry for my delay, I've been getting over some dental surgery.

    I have redone my schematic to match what I have been troubleshooting and trial and error on the breadboard.

    I am not using the reset momentary for the U1 monostable. It is not needed. Although, I just noticed that when I removed the switch, I left the 10k resistor wired on pin 4 to Vs...

    Also, the pin4 to Vs on U2 was removed in the troubleshooting process.
    Maybe It should be there,,,?

    Thanks for you'alls patience.... and look forward to figuring this out.
     
  19. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    Right off the bat, yes, connect pin 4 from both 555s to Vcc. Pin 4 is reset and must be held high for the 555 to function, unless you are resetting. Left unconnected, the 555 may work, may not, or function unpredictably.

    You don't need a 10kΩ resistor from pin 4 to Vcc unless you were also adding a switch to reset either 555. Since you're not, connect them directly to Vcc.
     
  20. thedude123

    Thread Starter Member

    Aug 29, 2011
    39
    0
    I figured, Guess this is what happens when trial and error sway to far from the original schematic.
    I will make the corrections tonight..
    Thanks

    Any thoughts regarding the monostable triggering problem I'm having?
    I posted above,, I need it to start when the momentary is released after being held.... seems simple enough, but it starts as soon as I press and hold the switch. Could the wiring of the trigger switch be wrong, or using the wrong switch....?? I chose this switch for my trigger.
    http://www.radioshack.com/product/index.jsp?productId=2049718
     
Loading...