help needed on timed shifting outputs

Discussion in 'The Projects Forum' started by beowulf1231, Aug 26, 2008.

  1. beowulf1231

    Thread Starter Member

    Aug 21, 2008
    11
    0
    hi, need help on a circuit diagram

    whats the simplest circuit to achieve my pulse diagram. im thinking of using an astable 555 circuit as the clk input but im not sure wat i can use for my 2 outputs. this is an add-on to an existing project and the available supply is 24v. each output will drive a 24v pneumatic valve.

    thanks in advance
    mak
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    If your clock input can be 4X the frequency for the outputs, a 555 astable clocking a 4017 might work. The 24 supply would have to be stepped down to something between 5v and 15v to power the 555 and the 4017. And then use BJTs or FETs to drive the 24v relays. But the timing should be correct.

    Ken
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Ken,
    If you tied Q4 to RESET instead, you wouldn't need the diodes for ORing the Q4/Q6 outputs, nor would you need either pull-down resistor.
    Also, you mis-labled the Q2 output as Q0+Q4, rather than Q2+Q6

    I'm attaching his original requirement as a PNG instead of a .doc to make it easier to view.
     
    Last edited: Aug 26, 2008
  4. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Right you are. Grabbed the schematic from an old project and didn't go over it closely. :(
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Now that I'm looking at it again... ;)

    If you put the diodes back in to OR the outputs of Q0 and Q2 together, with a 10k resistor to ground - you'd have the exact sequence of a main clock and two offset clocks that our OP was looking for, sans the drive transistors for the valves. But we don't know if he really needs the main clock or not. :confused:
    If unneeded, best to leave it off.

    No mention of how much current the valves need. It would help a lot to know that.
     
  6. beowulf1231

    Thread Starter Member

    Aug 21, 2008
    11
    0
    Thanks for the replies Ken and SgtWookie.

    I dont really need the main clock pulse, and I'll be using a solenoid valve rated at 0.5W/24Vdc.

    Is it ok to use an LM7805 to power this circuit w/out affecting the 24V supply? Will it be sufficient enough to power the circuit plus two drive transistors?

    I'm also thinking of using Q0+Q1+Q2 and Q4+Q5+Q6 for my outputs as I might need a longer duration for the HI pulse but maintaining the LO pulse in-between the two outputs.
     
    Last edited: Aug 27, 2008
  7. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Since a 555 and 4017 draw so little current, this should be no problem for a 7805. You could also use a 7812 or a 7815 since both ICs can work at those voltages.

    Whatever combination works. :)

    Ken
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    As far as the regulator, I agree with Ken that a 7812 or 7815 would be preferable to a 7805.

    You've mentioned that your 24v valves require 0.5w current. Do you have to source this current, or can you connect one side up to 24v, and sink the current from the other side?

    If you can sink the current, you might consider using TIP120 transistors. These are Darlington transistors, and they will amplify the base current considerably.

    You will need a 10k resistor from the output of the 4017 [LM317,previously mentioned, meant 4017] to the base of the TIP120. Otherwise, you risk burning out the 4017 due to excessive current.

    You should also put a diode across the valve's electrical connections, cathode towards positive. This will snub the reverse EMF produced when the coil is de-energized. Otherwise, you may blow the transistors due to the very high peak voltages.
     
    Last edited: Sep 5, 2008
  9. beowulf1231

    Thread Starter Member

    Aug 21, 2008
    11
    0
    Been out-of-town for a few days, thus the late reply. Thanks again Ken and Sgtwookie for the help extended. Will try the circuit next week on a breadboard, hopefully everything goes well.

    By the way, is the 10k resistor like a pull-up resistor? Can I use it together with the 10k pull-down resistor?
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It's been over a week since I last looked at this, and have no clue where my LM317 reference came from. :confused:

    If you are going to "OR" the outputs from the 4017 Johnson counter together (to lengthen the amount of time that one valve stays on), you will need to use diodes, the anodes of which are connected to the outputs of the counter, as shown in KMoffet's first schematic.

    The cathodes are all connected together, and from that junction, you connect a 10k resistor to the base of the TIP1xx transistor. This limits the current output of the 4017 to a safe level. The TIP1xx transistor's base is the ground path for the output; it's not necessary to use a pull-up or pull-down resistor in this configuration.

    Diodes are necessary because all but one of the 4017's outputs are low at any given point in time.
     
  11. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Sarge,

    I'd put a pull-down at the junction of the diodes or at the base of drive transistor. Otherwise when the outputs of the 4017 are low, the diodes are effectively reverse-biased and the base of the transistor is left floating. It may not be a problem, but a pull-down assures that the transistors are turned off.

    Ken
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Ken,
    Take a look at at TIP1xx datasheet; you'll see that they have intrinsic pull-down resistors. The input transistor usually has an 8k pull-down on it.

    I feel that having an additional pull-down resistor may severely limit the ability of the 4017 to turn on the Darlington.
     
  13. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Ahhh...learn something new every day. :)

    KEN
     
  14. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    And that is precisely the reason for this Board :D
     
  15. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I just happened to remember a 4017-based multi-timer circuit I whipped up for someone several months ago. You might find this useful for your application, as it will allow the setting of solenoid 1 time, off time, solenoid 2 time, off time to be of arbitrary duration and individually adjustable.

    As drawn, the schematic allows for roughly 2/3 second to 25 seconds per event.
    If a 40106 Schmitt-trigger hex inverter were used instead of the 4093, a pair of the inverters (in series) could be used to drive the bases of the TIP120's to provide more drive current without affecting the RC times.
     
  16. beowulf1231

    Thread Starter Member

    Aug 21, 2008
    11
    0
    Ahh, everything's much clearer now. Thanks again Ken and SgtWookie.

    I've made a diagram of what we have been discussing about. Pls. feel free to scrutinize the diagram. Maybe there's more improvement that can be done on it.

    Thanks again guys.
     
  17. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Just a quick glance...Pin 16 (+V) and pin 8 (common) on the 4017 missing. ;)

    Ken
     
Loading...