timing problem

Discussion in 'The Projects Forum' started by burtS, Feb 7, 2012.

  1. burtS

    Thread Starter Member

    Mar 6, 2008
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    ]I need help in creating a circuit that will accomplish the following:

    pressing a switch (or microswitch) will initiate event 1 (relay closure) for, say 10 seconds. Five seconds after event 1 begins, event 2 (second relay) occurs and terminates when event 1 terminates. Relays need to switch 200ua at 24vdc. I think I can do this with several (3?) 555s but I can't quite get my head around it and think there may be a more simple solution. Would appreciate assistance!
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
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    One 555 timer configured as one-shot set for 10 seconds. Output feeds event #1, and diode steered RC+Schmitt trigger set for 5S delay ( event #2 ) As event #2 is diode steered, it can be made to cut off coiencident with event #1.
     
  3. panic mode

    Senior Member

    Oct 10, 2011
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    555 can't run on 24V but you can drop voltage using 7812 for example.
    then create two monostable circuits like this:
    http://freespace.virgin.net/matt.waite/resource/handy/pinouts/555/

    where trigger is button connected from pin2 (both chips) to gnd.
    add pullup resistor such as 4.7...10k.
    relays with 12V coils can be driven directly from outputs (pin 3).
    add suppression diode across relay coil.

    you can combine contacts of the two relays to get you what you ask.
    if needed you can use relays with more contacts (DPDT for example).

    but if the current is really 200uA i'd look into using transistor output.
    relays need some wiping current to keep contacts clean and 200uA may be too little.
    or maybe this will not run millions of cycles...?
    you can combine relay contacts
     
  4. burtS

    Thread Starter Member

    Mar 6, 2008
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    Brownout:
    Many thanks: sounds reasonably simple but could you suggest wiring diagram and values for components: precise timing not critical. unfortunately I'm somewhat less than an amateur, never having used a 555 before and not knowing what a "diode steered RC+Schmitt trigger' is. I plan to use relays that will work with voltage output of the 555.
    cheers and thanks again,
    Burt
     
  5. burtS

    Thread Starter Member

    Mar 6, 2008
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    Panicmode:
    sorry for not being clear: I can use what ever voltage is required by the 555: plan on having the 555 operate a relay and it's the relay contacts that will control the 24v.
    thanks,
    Burt
     
  6. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998

    I know it's not a good description. I'll try to put something down, but it won't be precise. It might take a couple days, I'm at work, and much to do right now.
     
  7. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
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    Here is a drawing of what I'm talking about. V1 is the output of your 555 timer. The output at the top of C1 is delayed by the resistor during the pulse high time, but is turned off at the same time as the pulse due to diode steering. The schmitt device cleans up the edges and makes a nice, sharp pulse. Actually, the output of the schmitt device would swing from about zero to close to VCC that powers it. I don't know why the simulation is only swinging 1V. I don't often simulate digital components.
     
  8. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    There are two easy options with the 555.
    I didn't draw the load as relays because I'm not really sure that you need/want relays. Is this really 200μA?

    Also, the switch is this a momentary push button or a on/off switch.?

    Option 1 is of course missing a capacitor that assures the first relay to remain on when the left 555 turns off. After the output of the second 555 turns on it will power both relays. These are two 5sec timer.
    [​IMG]

    Option 2 assumes that both timer outputs raise their output voltage completely synchronized when they are both triggered at the same moment. Maybe one of the 555 gurus can confirm or refute this. The upper timer is a 10s timer the lower is 5s. The upper timer will turn on one relay for 10s and the other one 5 seconds after they have been triggered.

    [​IMG]

    Depending on your precision requirements this can be made simpler or more complex.

    Also , can you tell us what exactly you want to switch and if it has to be galvanically isolated from the timer circuit?
     
    Brownout likes this.
  9. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Hi BurtS,

    this is a response to your PM.


    1st circuit:
    turns on a 5s timer. This can power a relay. When the timer ends it will trigger a second timer. The second timer powers the first relay and a second relay too. When this timer ends both relays turn off.
    What needs to be addressed is the fact that there is a short moment where the first relay has no power (when the first timer ends). This can be easily accomplished.

    Second circuit:A 10s timer powers one relay. A 5s timer which is triggered at the same moment as the the 10s timer sinks current through the load (turning it on if it was a relay) when it ends. So it's the same function as the other circuit.

    Please tell exactly what load you need to switch with the relays so we can decide if it really needs to be a relay.
    And also tell us if the switch is a momentary push-button or a latching switch (remains on or off)
     
  10. burtS

    Thread Starter Member

    Mar 6, 2008
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    Praondevou,
    Many thanks for the clairfication. the relay controls a 24vdc solenoid that pulls about 200ma. the solenoid coil and the relay coil will be dioded.
    Is there any advantage of one of these options over the other? In any case it sound like just what is needed.
    Also: can I run this using a 9v battery (and 5v relays, 500 ohm coil)? Operation will be relatively infrequent (e.g. 20-30 times a day).
    cheers,
    Burt
     
  11. burtS

    Thread Starter Member

    Mar 6, 2008
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    0
    sorry: I mean ma: the solenoids pull about 200 ma.
     
  12. Mr. Mho

    Member

    Jun 15, 2009
    15
    3
    Burt,

    Here's another circuit you might try - it's simpler and lower power - important if you want to use a 9V battery. It uses a new version of the 555. It has an internal decade counter that can be programmed from 1 to 1,000,000... which makes it easy to generate long delays. (You can use a 10nF timing capacitor instead of 100uF.) Here's how it works...

    When the CSS555 is triggered, the 555 starts to oscillate at about 1KHz. D1, R2 & C2 form a peak detector and turns on MN1 right after the trigger input goes low. (No need to generate a narrow pulse, the IC will not retrigger until the cycle is finished.) If the internal counter is programmed for 10,000, the cycle will last 10 seconds. Using the "Astable" mode, the Timer Out pin goes high at the 50% count (5,000), or after 5 seconds. After 10 seconds, the terminal count is reached and Timer Out goes low.

    The CSS555 only draws about 5uA, so when the circuit is not triggered, the total current will be about 15uA... 5uA for the IC and 10uA through RA. MN1 and MN2 drive your relays.

    You can purchase the CSS555 at Jameco. It will need to be programmed - you might want to contact CSS for a pre-programmed sample. Good Luck!
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    The assumptions look pretty valid to me. Never tried anything quite like these configurations, but I think I would go with diode gates (power diodes that is).
     
  14. Wendy

    Moderator

    Mar 24, 2008
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    Interesting. I'm not a fan of this chip, I predict it will confuse a lot of beginners. Since it is not a drop in replacement it isn't even a real 555.

    CMOS 555 are already designed for low power. There are other chips like the 4060 that have RC oscillator followed by counters, along with several other similar chips. This is a not too good attempt to capture a specialty market

    Thanks for pointing it out, I'll be following this part with interest. Unless it is a lot more compatible than I think it is I suspect we'll see some interesting lawsuits. One of the classic hallmarks of all the 555 types (even the conventional/CMOS 555s) is the interchangeability of the chips.
     
  15. Mr. Mho

    Member

    Jun 15, 2009
    15
    3
    The default configuration of this 555 bypasses the counter so that it IS a drop in, pin-for-pin replacement, just a lot lower power. For this application, using the counter function allows one chip to generate the 5 second AND 10 second delays with much smaller timing capacitors... however, it needs to be re-configured to do that.
     
  16. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Ok, so you need a transistor to drive the solenoid.

    Unless you follow Mho's suggestion I would then recommend to use the first circuit I proposed. LOAD1 will be replaced by a simple RC, a transistor, the solenoid and a freewheeling diode.

    LOAD2 will be a base resistor, a transistor and the other solenoid with a freewheeling diode.

    LOAD1 stays on 10sec. LOAD2 only 5sec. Both loads turn off simultaneously.

    Sorry I don't have anything to draw this up right now. Maybe someone else can help you out.

    It's more or less like the very first picture here:
    http://www.jaycar.com.au/images_uploaded/relaydrv.pdf

    Put the above mentioned capacitor at the base of the transistor, should be good enough to maintain the first relay on until the second timer starts.

    I forgot, use the LMC555 (CMOS), not the normal version.
     
  17. Wendy

    Moderator

    Mar 24, 2008
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    I don't want to hijack this thread, but I didn't see where the RC values set the timing. If it can do this and use the 1/3 and 2/3 Vcc values then it can be a true 555, but my impression is that it depends on it's internal logic way too much, to the exclusion of these functions.

    Like I said, I'll be watching it with interest.

    It looks like the OP is pretty settled. praondevou invited me over to take a look, since I tend to do a lot with 555 timers. Looks like I got an extra bonus.

    10 seconds is very doable with a standard 555, so is 10 minutes. No real extra circuitry needed.

    praondevou: What software are you using to make the schematics? I'll get with you off line.
     
  18. Mr. Mho

    Member

    Jun 15, 2009
    15
    3
    Actually, it does use 1/3 and 2/3 trip levels, just like a "real" 555. The timing equations are all the same for a 555 oscillator in Astable mode. When the digital counter is set to "1", it functions exactly like a standard 555, but runs on <5uA. I agree, 10 seconds, even 10 minute delays can be realized with a std 555, but it's often cheaper, smaller and more accurate not to need 100+uF caps.

    Maybe check out the spec for yourself???
     
  19. Mr. Mho

    Member

    Jun 15, 2009
    15
    3
    When driving inductive loads (solenoids & relays), I think it's wise to use a discrete NPN or NFET as I show in my schematic. The inductive "kick" can cause some fairly high transient currents that can latch-up CMOS IC's. This often burns them up unless their VDD supply is current limited. Discrete transistors don't have this problem - they don't have the SCR structure in their I/O pins that cause latch-up. With the 2N7000 NFET, you don't have to worry about the drive current. They're rated at 60V, 200mA... should work great.
     
  20. burtS

    Thread Starter Member

    Mar 6, 2008
    29
    0
    Many thanks to Mr. Mho, Bill Marsden and Praondevou for their input. No idea of how to program the 555 Mr. Mho suggests so will go with one of Praondevou circuits but use a transistor to drive the relays that controls my solenoids as load 1 and 2 as I take it that the output of the 555s can't operate my EDR dip type reed relay.. Could use a regulated 12v wall plug type power supply. I assume that the circuit to the trigger is to generate a brief pulse to initiate the timing: could I do the same with a 3 pole switch that will charge and discharge a cap?
    Burt
     
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