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coutdown 555 timer values?

Discussion in 'Electronic Basics' started by Tristar500, Jul 21, 2005.

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  1. Tristar500

    Tristar500 Guest

    I'm needing to put together a series of timers. Plan is to have6 timers
    all triggered at the same time. The first should time out and complete
    a circuit at 30 seconds, the second at 60 seconds, 60, 120, 150, 180

    Can anybody give me a basic circuit and values of the required
    resisters/capacitors here?


  2. Since all events take place at multiples of 30 seconds, you might
    consider using a single 30 second555 clock with a decoded counter like
    a CD4017 or CD4022. These counters turn on a different output for
    each clock.
  3. John Fields

    John Fields Guest

    View using a non-proportional font like Courier.

    | | | | |
    | | [RT1] +----+----+ |
    [10K] [10K] | | Vcc _| |
    | | +-----|TH R|--+
    ____ | | | |_ | |
    TRIG>--+--[0.1µF]--+-+---|----O|T | |
    | | |_ OUT|--|----->OUT1
    | +----O|D GND | |
    | | +----+----+ |
    | [CT1] | |
    | | | |
    GND>--------------+--|---+----------+ |
    | | |
    | | |
    | | |
    | | +----------+-------+
    | | | | |
    | | [RT2] +----+----+ |
    | | | | Vcc _| |
    | | +-----|TH R|--+
    | | | |_ | |
    | +---|----O|T | |
    | | | |_ OUT|--|----->OUT2
    | | +----O|D GND | |
    | | | +----+----+ |
    | | [CT2] | |
    | | | | |
    +--|---+----------+ |
    | | |
    | | |
    | | +----------+-------+
    | | | | |
    | | [RT3] +----+----+ |
    | | | | Vcc _| |
    | | +-----|TH R|--+
    | | | |_ | |
    | +---|----O|T | |
    | | | |_ OUT|--|----->OUT3
    | | +----O|D GND | |
    | | | +----+----+ |
    | | [CT3] | |
    | | | | |
    +--|---+----------+ |
    | | |
    | | |
    | | +----------+-------+
    | | | | |
    | | [RT4] +----+----+ |
    | | | | Vcc _| |
    | | +-----|TH R|--+
    | | | |_ | |
    | +---|----O|T | |
    | | | |_ OUT|--|----->OUT4
    | | +----O|D GND | |
    | | | +----+----+ |
    | | [CT4] | |
    | | | | |
    +--|---+----------+ |
    | | |
    | | |
    | | +----------+-------+
    | | | | |
    | | [RT5] +----+----+ |
    | | | | Vcc _| |
    | | +-----|TH R|--+
    | | | |_ | |
    | +---|----O|T | |
    | | | |_ OUT|--|----->OUT5
    | | +----O|D GND | |
    | | | +----+----+ |
    | | [CT5] | |
    | | | | |
    +--|---+----------+ |
    | | |
    | | |
    | | +----------+-------+
    | | | | |
    | | [RT6] +----+----+ |
    | | | | Vcc _| |
    | | +-----|TH R|--+
    | | | |_ |
    | +---|----O|T |
    | | |_ OUT|-------->OUT6
    | +----O|D GND |
    | | +----+----+
    | [CT6] |
    | | |

    Tout = 1.1 Rt Ct
  4. aman

    aman Guest

    You might want to consider a simple 8 bit microcontroller like PIC. All
    micro-controllers have timer interrupts. If not using a CD4017 as a
    counter and using 555 to generate a 30 sec pulse seems like a good idea.
  5. John Fields

    John Fields Guest

    It may seem like it is, but in reality it isn't.

    Note that the OP wanted six timeout periods all starting at once, but
    as they timed out, each one lasting 30 seconds longer than the one
    which timed out before it. Like this:

    T1___| |__________________________________________

    T2___| |_______________________________________

    T3___| |____________________________________

    T4___| |_________________________________

    T5___| |______________________________

    T6___| |___________________________

    You can't do that with a 4017.
  6. Chris

    Chris Guest

    So, the OP needs 6 ea. 100uF caps for CT1 - CT6, and the following 5%

    RT1 = 270K
    RT2 = 560K
    RT3 = 820K
    RT4 = 1.1M
    RT5 = 1.35M (1.2M + 150K)
    RT6 = 1.6M

  7. John Fields

    John Fields Guest

  8. Chris

    Chris Guest

    Right. Your component designations (RT1-6, CT1-6) for your ASCII
    diagram. The OP asked for component values, too. The six 555s is the
    best answer to the OP's question, especially considering that he's
    probably a newbie and may not need accuracy better than 20% or so. You
    did provide the equation (t = 1.1 * R * C), but I thought the OP might
    need some help with standard component values.

  9. John Fields

    John Fields Guest

  10. Byron A Jeff

    Byron A Jeff Guest

    That's the Steve Ciarcia (of Byte and Circuit Cellar INK fame) model.
    "My favorite programming language is a soldering iron."

    But it's certainly debatable, even for a newbie. The 555 method is
    going to require a minimum of 3 556s, and a handful of caps and
    resistors (some out of spec) in order to accomplish.

    The other proposal, using a microcontroller, does require the controller and
    a programmer. However, a simple programmer such as my Trivial Low Voltage
    Programmer (TLVP) for the PIC can be wired up with a single chip and a
    resistor in about the same amount of time to wire up one 555.

    The TLVP can be found on my PIC page:

    Now on to programming languages. I'm an advocate for learning
    assembly in the microcontroller world because of the prevelence of
    usage. However for a quick project an HLL may be in order. Something
    along the lines of JAL or SDCC for the PIC is open source, free to use,
    and pretty simple to get started on.

    The upshot is that wiring 6 555 circuits will complete this project and
    give some insight into wiring 555 circuits. But precisely because of
    their programmable nature, using a microcontroller can leverage the
    setup cost over a bunch of projects. A part like the PIC 16F630 or
    the 16F676 cost less than $2 and can be used for a variety of uses.

    I just don't think it's as cut and dried as you propose Chris.

  11. He would have to add some logic, like some SR flip flops and a bit
    more. I just get worried about the stability of more than a minute
    versions of 555s.
    I'll bet you could come up with something neat based on the 4015 shift
  12. Byron A Jeff

    Byron A Jeff Guest

    That's probably a better single purpose idea. The basic idea would be
    to use a shift register with a master clear input. The start of the timing
    cycle would be a master clear starting all 6 outputs. Then each 30 seconds
    the opposite bit is clocked in.

  13. Ken Moffett

    Ken Moffett Guest

    (Byron A Jeff) wrote in
    The OP might also try a PICAXE uP. These uP's are a little more expensive
    than the bare-bones PIC, and alot less expensive than Basic Stamps. The
    programming software is free and the programming is from the serial port
    directly to the uP, through a three wire cable (OK, plus 2 resistors).
    Pbasic is easy to use. The 4MHz clock is more than sufficant for his timing
    and more accurate than 555's. The PicAxe-08M only has 5 outputs, so they
    would need to go to a PicAxe-18. Lots of info, free program editor,
    software, and links at:
  14. Bob Monsen

    Bob Monsen Guest

    Yes, I like this. A '164 has 8 outputs, so his 6 output thingy could use
    a single shift register. He may even be able to drive the LEDs using it,
    through a resistor (if he doesnt' use a cmos part.) The 555 should use
    1M, 1k, and 47uF as the timing elements, so it pulses every 30 seconds
    or so.

    The '164 master reset pin is the 'start' signal, and should also be tied
    to the 555 reset pin. Bring them both up, and the thing starts counting.
    Once finished, bring them down, all the outputs clear, and it stops
    counting. If he wants it to cycle, tie these pins to the Q6 output
    through an inverter, possibly a CE transistor amplifier.

    Bob Monsen

    If a little knowledge is dangerous, where is the man who has
    so much as to be out of danger?
    Thomas Henry Huxley, 1877
  15. Tristar500

    Tristar500 Guest

    I'm still a bit unclear as to the values of the capacitors and

    one post offered

    RT1 = 270K
    RT2 = 560K
    RT3 = 820K
    RT4 = 1.1M
    RT5 = 1.35M (1.2M + 150K)
    RT6 = 1.6M

    Is this correct?
  16. Byron A Jeff

    Byron A Jeff Guest

    I think there is a concensus to abandon this approach. Better is to use
    a single 555 set for 30 seconds and a shift register.

    Review the other posts in this thread. The 555 is unstable at such long
    durations. It'll function much more accurately for 30 seconds than for
    180 or 240 seconds.

  17. Tristar500

    Tristar500 Guest

    Not sure what a shift register is? Since the 3 minutes isn't critical,
    plus or minus 20% would be OK. Is the schematic John sketched out near
    the top of this post close enough granted the instability of the 555?
  18. Byron A Jeff

    Byron A Jeff Guest

    Take a read here:
    The 555 simply isn't specified to operate with such large resistor and/or
    cap values. Caps are leaky.

    Also you'll have a lot less to go wrong with the shift register because
    there will only be two parts in the circuit instead of 6.
    Probably. You can get more detail on the 555 here:

    But the shift register will give you more repeatable results with less

  19. Chris

    Chris Guest

    Hi, Jeff. Looking at the LM555 data sheet,

    you can see that the leakage current from the threshold pin (0.1uA
    typ., 0.25uA max) is the limiting factor. If you've got a 12VDC power
    supply, and a 1.6 meg series resistor, you've got 7.5uA charging the
    100uF cap. With a 5V supply, you've got about 3uA. I can see how the
    0.4uA max. leakage current will cause inaccuracies, but it doesn't make
    the chip inoperative, even at 5V.

    Of course, the OP can use a CMOS 555, which has almost no leakage
    current at the threshold pin, making this a non-issue.

    Of more importance is the leakage current of the cap. Some
    electrolytic caps are specified as having several uA of leakage current
    maximum new out of the factory. That's more of a problem.

    The OP should use an electrolytic which is specified as having low
    leakage, such as one available from Mouser (140-LLRL16V100). These
    caps ($0.34 ea.) are specified as having 0.4uA max leakage.

    Actually, one of the major problems here that nobody is talking about
    is the power supply. The 555 changes state on 1/3Vcc and 2/3Vcc. If
    the supply is unregulated or poorly regulated, the power supply sags
    are going to change things quite a bit, too.

    I believe the OP can get within 20% of his timing requirements with the
    originally specified parts, assuming he has a well-regulated supply.
    But, if it would make anyone happier, the OP can just multiply the cap
    value by 10 (1000uF 16V, Mouser p/n 140-LLRL16V1000, $0.77 ea.), and
    divide the cap values by 10 (start with 27K for the 30 seconds, and go
    up from there to 160K). That woould make it a bit easier on the
    leakage current budget, since 10X as much current is charging up the
    cap. All legal, and according to the data sheet, permissible.

    Since we're drawing this out at length, there's also a bit of a glitch
    with the 555 in the astable configuration (oscillator) that nobody's
    mentioned here. Usually you figure pulse high (charge) time as about
    0.7 * (Ra + Rb) * C. However, on turn-on, the cap isn't charging from
    1/3Vcc, it's charging from 0V. That means the equation for the first
    output logic high is closer to the monostable constant, 1.1 * (Ra + Rb)
    * C. In other words, the first high pulse is going to be 57% too long,
    and the logic high is more than half the total period, so the first
    whole cycle of the 555 is going to be at least 23% too long, even with
    a mm value for Rb. That's out of spec here.

    In days of yore, when men were men and women were glad of it,
    4000-series CMOS used to rule the earth (mid '70s). Time delay relays
    were improved significantly by replacing the old clunky PUT-based
    timing circuits with a 4060, which is a divide-by-2^14 counter with a
    built-in oscillator circuit, which only requires two Rs and a C, kind
    of like the 555. The idea was that, even if one timing cycle went bad
    because of a power glitch or noise from the relay, well, you had over
    8,000 of them to timeout (the 2^14th output went high to drive the
    relay, so it had 2^13, or 8,192 counts. Who cares? It's still

    Anyway, even with a divide-by-2^14 counter following it, I believe the
    oscillations would be too slow for the 4060 oscillator. So I would add
    a 4020 or 4040 counter after the 4060 to allow us to get the 4060
    oscillator up to speed, and go with that. You would then have Mr.
    Popelish's 4015 shift register after, with the last output locking out
    additional clocks.

    I would guess the OP has enough information to go on. With ST CMOS
    555s in thru-hole going for $0.40 ea. at Mouser, and the 1000uF low
    leakage caps going for $0.77 ea., I would think the OP could just pull
    the trigger and put something together for about $7 in parts, and see
    if it works. If not, he can get back with another post.

    Good luck
  20. Tristar500

    Tristar500 Guest

    Hi Chris, Thanks for taking the time to reply. I hadn't thought of
    using cmos 555's . I understand they and other cmos ic's are popular
    with very low current draw is needed. I was thinking of using a
    computer power supply or a cell phone charger as a regulated power
    supply. I know the computer supplies are well regulated, not so sure
    about those little black cubes we all have at the end or our cell phone
    chargers, answering machine power supplies etc. Here is a link to a
    jpg of the schematic I put together from the earlier post from John
    Fields along with some resister values added by other members.

    Anybody care to comment? I'm ready to etch a board if all looks good.

    Thanks, Lawrence
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