chaining 555s to make an LED sequencer

Discussion in 'The Projects Forum' started by lowrise4, Jun 29, 2013.

  1. lowrise4

    Thread Starter Active Member

    Sep 6, 2010
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    In my Make: Electronics book, I can't get one of the experiments/projects to work (all others up till now have have worked for me). It is figure 4-29 on page 168, under the 'Chaining Chips' heading.

    You are shown how to connect the output (pin 3) of a 555 to the trigger (pin 2) of another 555. When the output pulse of the first 555 ends, this triggers the next 555 to turn on. When it ends, it triggers the next 555 downstream, and so on. The output of the final 555 is fed back to the trigger on the first 555 to restart the whole process. All 555s are in monostable mode. The output of each 555 is also connected to an LED to make the LEDs light up in sequence.

    Here is the schematic from the book. I highlighted the positive and ground 'rails' with red and blue pencil for clarity. I built this circuit on a breadboard, but in a linear rather than circular layout. When power is applied, or when the switch is moved from one position to the other, it only makes some of the LEDs turn on, but no moving sequence.

    Any ideas about what I'm doing wrong? Isn't there supposed to be a pullup resistor connecting each #2 pin to the positive rail so that each 555 remains off unless triggered by a low voltage? Thanks for any help.
     
  2. LDC3

    Active Member

    Apr 27, 2013
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    The switch is used to start the sequence. The order of operation is this:
    Power is applied to the circuit and the 555s become stable.
    The switch is momentary switched to provided 5V to pin 2 of the first 555.
    The lit LEDs progress around the circle.

    Pin 3 of the previous 555 is either a source of current or a drain. There is no need for a pull-up on pin 2.
    The schematic looks correct. How about posting a picture of the breadboard you made?
     
  3. lowrise4

    Thread Starter Active Member

    Sep 6, 2010
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    Okay, here is my circuit on breadboard which is not working as it should. I took the pic without the jumper connections between the output pins and trigger pins to reduce visual clutter...it's crowded enough already.

    Here's the component values:

    -100K between pins 7 (reset) and positive rail.
    -47uF between pins 6 (threshold) and ground
    -0.1uF between pins 5 (control) and ground
    -1K between LEDs and ground

    For the SPDT switch, I reposition a jumper wire, which should be equivalent.

    Oh - it's being powered by around 6V, not the prescribed 12V...I'll check if that makes a difference.
     
  4. LDC3

    Active Member

    Apr 27, 2013
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    Yes, it does make a difference. The current through the LEDs would be about
    (6V - 2.5V) / 1000Ω = 3.5mA
    I don't think this is enough current to light up the LEDs. Reduce the resistors to about 200Ω and see if that works.

    Let me study the photo and I tell you if there is anything else.
     
  5. lowrise4

    Thread Starter Active Member

    Sep 6, 2010
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    Hi - I tried at 12V and the LEDs are brighter, but still no sequence...just some LEDs light, others don't...depending on whether pin 2 is connected to + or to pin 3 of the last 555. I tried this order:

    1) apply power
    2) momentarily connect pin 2 of the first 555 to + (switch in fig 4-29 in down position)
    3) then instead connect pin 2 of the first 555 to the output of the last 555 (switch in up position)

    Perhaps my component values are wrong; I used the values from a previous experiment in that chapter - that one was a single 555 in monostable mode with 2 momentary-on buttons. The point of that was simply to show it making a timed pulse and canceling it with the reset pin. Fig 4-29 is unusual that values were not specified.

    :confused:
     
  6. LDC3

    Active Member

    Apr 27, 2013
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    What is the resistance of the resistor between Vcc and pin 7?
    What is the capacitance of the capacitor between pin 6 and ground?
     
  7. absf

    Senior Member

    Dec 29, 2010
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    4 wires are missing on your breadboard.

    U1/P2 ---------------U4/P3
    U2/P2---------------U1/P3
    U3/P2----------------U2/P3
    U4/P2----------------U3/P3
     
  8. KJ6EAD

    Senior Member

    Apr 30, 2011
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    They were left out of the photo intentionally.
     
  9. lowrise4

    Thread Starter Active Member

    Sep 6, 2010
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    The component values:

    -100K between pins 7 (reset) and positive rail.
    -47uF between pins 6 (threshold) and ground
    -0.1uF between pins 5 (control) and ground
    -1K between LEDs and ground
     
  10. LDC3

    Active Member

    Apr 27, 2013
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    I know this might be a pain, but could you rewire it so that the reset on all the 555s can be grounded so we can start with a known condition. Then toggle pin 2 on the first 555 and see what happens.
     
  11. lowrise4

    Thread Starter Active Member

    Sep 6, 2010
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    Yup - I will try putting a momentary switch to ground all the reset pins, and will report back.

    Incidentally, the next experiment in this chapter is using a 555 to modulate the tone of another 555 via its control pin. The tone being made by the second 555 was really weird - could best be described as a hissing noise. All connections seemed good. At a loss for an explanation, I swapped out that 555 with another fresh one, and voila! Finally makes a proper, clean tone. So even though I'm being careful to ground myself before handling the 555s, I wonder if one/some of them might've been damaged though ESD. Maybe this is why my sequencer isn't working?
     
  12. KJ6EAD

    Senior Member

    Apr 30, 2011
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    The problem is that the trigger lines are not being pulled high before the end of their respective timing periods; instead, they're being held low by the output of the previous stage. The solution according to the book's author is to link each stage through a capacitor so only the negative going trigger pulse is transmitted.

    Scroll down to the notes for page 168 in the following to see the source material:

    http://oreilly.com/catalog/errata.csp?isbn=9780596153748

    In my experience, using the Texas Instruments TLC555 devices, the capacitors alone were inadequate to prevent the trigger lines from floating so I added pullup resistors. Here's what it looks like:

    https://www.youtube.com/watch?v=fmx7E38JvE8&feature=youtube_gdata_player
     
    Last edited: Jul 7, 2013
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  13. lowrise4

    Thread Starter Active Member

    Sep 6, 2010
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    KJ6EAD, wow...thank you! I'm surprised nobody noticed that before. Will try it.
     
  14. lowrise4

    Thread Starter Active Member

    Sep 6, 2010
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    I finally got it to work! Thank you everyone and KJ6EAD for showing me the errata & correction, and your video. I guess the author lucked out that the circuit worked for him without the additional caps and pullup resistors.

    I was able to get my circuit to work by placing a 0.047uF ceramic cap between each output and subsequent trigger. But to get the circuit started I'll often need to momentarily ground the trigger pin of the starting 555. A momentary-on switch did the trick. So I wonder why the schematic shows a switch to positive? I tried that and it didn't seem to do anything.

    I also added pullup resistors on the trigger and reset pins. Before I did, the circuit didn't seem to work well. Elsewhere in the chapter it says it's good to have all pins connected either to positive or to ground to avoid problems caused by floating pins.

    I also learned that I should first make sure each of the timer circuits is working properly by itself (sending out a timed pulse) before attempting to chain them together - because if the whole circuit isn't working, it's hard to tell where it isn't working.

    Does anyone have any standard / 'good' default values to use for the pullup resistors and the caps between output and trigger?
     
  15. LDC3

    Active Member

    Apr 27, 2013
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    Since you added pull-up resistors to pin 2, then the toggle to ground, instead of to Vcc, is required to change the signal.
     
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  16. lowrise4

    Thread Starter Active Member

    Sep 6, 2010
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    Yup...that's what I figured. thank you.

    Does anyone have any suggested default component values for this circuit? (other than the cap and resistor on the pins 6 and 7 which you choose based on the timing you want).

    Here's my working circuit in prototype form:
     
  17. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Pullup 0n pin 2 10k to 100k, I usually use 9.6k because large no in stock; .1 to .001 works, I use .01 mostly. To see circuit in another use see in Projects, dthx post, " Another 555 timer question", post # 29 In this case after 6 cycles every thing is reset.
     
  18. KJ6EAD

    Senior Member

    Apr 30, 2011
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    I arbitrarily used 10k pullups and 100nF capacitors. My reset lines are tied high and I have a momentary switch to ground the first trigger input.
     
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