New to the forum, got a question

Discussion in 'Electronics Resources' started by bentomo, May 6, 2012.

  1. bentomo

    Thread Starter New Member

    May 6, 2012
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    Hello, I'm new to the forum, but I do have a little experience with electronics. I'm going into electrical engineering this fall so until then I may need a little help with this circuit.


    I've got a signal on a circuit that needs to be normally connected to ground, and I push a switch and the signal is connected to 1.9v power.

    Now I have to push this button every time I turn on the circuit. What I need is another circuit that pushes the button automatically so I don't have to switch it every time.

    I was thinking a 555 monostable circuit with a NOT gate and a couple transistors acting as the switch.

    I just can't figure this one out.
     
  2. DerStrom8

    Well-Known Member

    Feb 20, 2011
    2,428
    1,328
    Hi bentomo. Welcome to AAC! :)

    A 555 monostable would still have to be triggered, usually through a switch. Astable would switch it repeatedly, with set delay times. You don't mention what the circuit is intended for. When and how often do you want it to turn on?

    If you use a 555 astable, it will turn the output on for a set amount of time, then turn it off, then turn it back on. You don't need a NOT gate--just an NPN transistor. The output of the 555 will be on the base, the collector connected to 1.9v, and the emitter connected to the other part of the circuit (the part you're switching). That's about all I can say with that amount of information.

    Good luck!
    Regards,
    Der Strom
     
  3. bentomo

    Thread Starter New Member

    May 6, 2012
    14
    0
    So I wouldn't want to use a 555 timer, there's no way to make it pulse once with a low or high state and then stop after that?

    I've looked into timed relays but they're way to big and expensive.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    When it comes to pulses the 555 is a pretty versatile tool. Are you wanting this pulse to happen on power up?

    [​IMG]

    That is what this does. The 555 goes high, then powers down and doesn't restart until power is cycled.
     
  5. bentomo

    Thread Starter New Member

    May 6, 2012
    14
    0
    That seems to be what I'm looking for.

    I ultimately need a low high of 2 seconds a low of 2 seconds, then go to high and stay there.

    That delay is the hard part I can't figure out.
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    So how do you want to reset the circuit back to low to restart the cycle? Turn the power off?
     
  7. bentomo

    Thread Starter New Member

    May 6, 2012
    14
    0
    Yes exactly, sorry I haven't been completely specific. It needs to be triggered when the circuit is power on, and then reset when the circuit is powered off.
     
  8. WBahn

    Moderator

    Mar 31, 2012
    17,720
    4,788
    I think there is a type ("need a low high") that is confusing me.

    Are you saying that you need the following sequence to happen:

    1) Power up
    2) Hold signal HI for 2 seconds.
    3) Bring signal LO for 2 seconds.
    4) Bring signal HI and leave it there until powered off.

    How important is that initial HI? Would the following be unacceptable?

    1) Power up
    2) Hold signal LO for 4 seconds.
    4) Bring signal HI and leave it there until powered off.
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    Is the 1.9V power the only power you have?
     
  10. bentomo

    Thread Starter New Member

    May 6, 2012
    14
    0
    This is the wave I need.

    1) POWER ON
    2) HIGH 4 seconds
    3) LOW 2-3 seconds
    4) HIGH

    And then reset when turned off.

    The delay is very important, or else the circuit won't register the LOW at all.

    Also my options for power are 8.2v-7.4v coming from a lithium ion battery, 3.3v from a TI regulator, and 1.9v from another switching regulator.
     
  11. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    Below is a circuit that should do what you want. It uses a 555 as an astable oscillator and a CD4017 Johnson counter to generate the sequence.

    The 555 is set to count with about a 1 second period. The value of R5 may have to be changed slightly to get the desired period after you build it, depending upon the tolerance of the components. Because the first startup pulse is slightly longer than the rest of the pulses, due to the initial charging of C1 from 0V, the following periods are slightly shorter than 1s. Thus the first four counts ORed together give 4 seconds high and the three count gap gives about 2 3/4 seconds low. The last pulse then inhibits the counter clock and the output will stay indefinitely high.

    The schematic doesn't show power and ground to the ICs so you will need to supply that from the part schematics. I show a 5V supply (V2) but you can use 8V lithium source with little change in the circuit operation. Remember to add a 0.1uF decoupling capacitor between power and ground on both ICs.

    You can use a couple of transistors or a relay to connect the 1.9V power. If you need help on that, post the value of current you want to switch.

    555 Sequencer.gif
     
  12. bentomo

    Thread Starter New Member

    May 6, 2012
    14
    0
    Thank you so much!!!!


    I'm ordering some new parts from mouser and digikey soon so I'll pick up a johnson counter too.

    I haven't heard of this kind of component so it's time to do a little research. :D

    Two questions though.

    Do I just power the IC's normally and this is set up to trigger automatically?

    So is the circle "clock inhibit" and the one below it "clock" inside the johnson counter?
     
    Last edited: May 13, 2012
  13. MrChips

    Moderator

    Oct 2, 2009
    12,432
    3,360
    There is common confusion and misuse of the circle symbol.
    The circle symbol represents an inverter but the purpose is much more than just an inverter.

    The circle represents NEGATIVE LOGIC.

    Now what the heck is that?

    In POSITIVE LOGIC, a high voltage activates a defined function.
    In NEGATIVE LOGIC, a low voltage activates a defined function.

    It is very common to find the RESET or CLEAR function defined using NEGATIVE LOGIC.
    To be consistent, in such cases, diagrams must show the input with the circle symbol
    as well as the words "RESET" or "CLEAR" must have a bar drawn over the letters (overstrike). When the overstrike is not available in text documents sometimes people use different schemes such as:

    RESET(L)
    RESET'
    RESET_N
    RESET*
    *RESET
     
  14. bentomo

    Thread Starter New Member

    May 6, 2012
    14
    0
    So what are those 3 pins going into the johnson counter on the left side?
     
  15. bentomo

    Thread Starter New Member

    May 6, 2012
    14
    0
    Well I order a few counters, I guess I'll just assume that the circle is clock inhibit and the other is clock.
     
  16. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    Yes, the one with the circle is the Clock Inhibit input. I use that input to stop the count at the end of the sequence.

    Edit: The third input is the Reset.
     
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