on/off toggle circuit

Discussion in 'The Projects Forum' started by dan@tsasystems.com, Jul 11, 2011.

  1. dan@tsasystems.com

    Thread Starter New Member

    Jul 11, 2011
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    Currently using a momentary switch to a Dual DType Flip Flop MM74C74 and a Hex Inverting Schmitt trigger MM74HC14 to toggle our 9VDC power circuit on and off.

    Does anyone know of a single component that can replace the two ICs?
     
  2. praondevou

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    Jul 9, 2011
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    Any schematic, more details ? ;)
     
  3. dan@tsasystems.com

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    Jul 11, 2011
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    PWR SWITCH to left is momentary low to change states.
    +BATT = 9VDC
    On/Off in center is switched power

    Pressing keypad on/off switch toggles flip flop U13 which latches Schmitt inverter U18 on and off
     
  4. praondevou

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    It should do the same if u just take out the inverter. Difference: With inverter it will trigger the FF when u press the button. W/o inverter it will trigger the FF when u release the button. Am I right?
     
  5. praondevou

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    Otherwise you'll have to find a negative edge triggered FF...
     
  6. SgtWookie

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    Jul 17, 2007
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    Well, the cap to ground and resistor pull-up could be swapped around so it's a cap to +V and resistor pull-down, but the big thing is the Schmitt trigger input for the inverter. Without that, the slow-rising edge might cause problems.

    A microcontroller could handle the debouncing and toggling, but I don't know of one offhand that would run efficiently from a 9v supply.
     
    dan@tsasystems.com likes this.
  7. dan@tsasystems.com

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    Jul 11, 2011
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    I believe that statement to be true. The schmitt inverter creates a signal we use for CLK at the FF. Each press of the switch sends a low to the Schmitt trig inverter, which allows one positive going pulse from the Schmitt trigger inverter to change states of the FF.

    It appears my original statement was less than correct.
    Flow would be
    N.O. Sw sends low to Schmitt trigger inverter
    Inverter triggers positive going (CLK) pulse to FF
    FF transfers logic level on each positive going CLK input

    Circuit is clean and functional but the FF is now available in only a 6V version. 15 V FF just went obsolete and the drop-in replacement is only 6V and does not hold up in our circuit.

    Another nicety of this circuit is very little current sink when "Off" allowing longer batt life.

    That is why I am seeking a low current Pwr controller. Most I have located continue to sink current and discharge Batt when in off.
     
  8. dan@tsasystems.com

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    Jul 11, 2011
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    Circuit is functional and works wonderful in low power circuit - 5VDC and under, if anyone else needs a nice clean low voltage switch circuit. Our equipment runs just over 6V and the current offering of FF are not holding up.
     
  9. dan@tsasystems.com

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    Jul 11, 2011
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    We are also looking at micro controllers, but the old solution (inv and FF)was relatively simple and in-expensive that I thought maybe someone else had a nice easy solution out there.
     
  10. SgtWookie

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    Jul 17, 2007
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    Well, have you looked at 4000 series CMOS? They're good from ~3v to ~16v, many go to 18v.
    The 4013 is a dual D f/f, the 4106 is a hex Schmitt trigger inverter (the pins are different from the 74 series though) The 4093 is a quad Schmitt trigger NAND that can be used as an inverter by tying two of the inputs together.

    Basically, the 4093 and the 4106 are the only two 4000 series CMOS IC's that I can think of offhand that have Schmitt-trigger inputs.

    Various manufacturers use different prefixes; for example:
    CD4013
    HCF4013
    HEF4013
    MC14013
    There are others, but that's just to give you some to work with.
     
    Last edited: Jul 11, 2011
  11. ifixit

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    Nov 20, 2008
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    Hi,

    Try this idea for one part solution. You could use other CMOS parts in a similar maner. I haven't tested it.

    Regards,
    Ifixit
     
  12. dan@tsasystems.com

    Thread Starter New Member

    Jul 11, 2011
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    We are looking at the 4000 series CMOS FF (mainly On-semi and TI), but the pin out is different than the original 74HC14components - and if your going to spend money to rev a board - go big or go home. That's why we are evaluating options.

    There is also the problem of physical component similarities between the 74 series and the 4000 series, ensuring our techs don't swap them board to board if we change the pin assignments and layout to accommodate the 4000.

    But thanks for all the good advice. It is nice to hear I am looking at solutions others would pursue.

    Ifixit - can you expand a bit on your statement "You could use other CMOS parts in a similar maner"
     
  13. SgtWookie

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    Jul 17, 2007
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    Here's an idea I swiped from Bill Bowden:
    http://www.bowdenshobbycircuits.info/page9.htm
    He called his "Relay Toggle Circuit Using a 555 Timer", but this has no relay and a few less parts.

    See the attached circuit and simulation. I suggest you use a TLC555 or other CMOS 555 timer. bjt (transistorized) 555 timers will be too power hungry. S1 is a normally open pushbutton switch. The size of C1 might need some adjustment, depending on how badly your switches bounce. R1 and R2 must be significantly smaller than R3 or the circuit may oscillate.

    Note that I did not show a cap across the Vcc and GND pins of the 555 timer; an 0.1uF ceramic or poly metal film and 1uF or larger aluminum electrolytic are required.

    [eta] Oops, I think the quiescent current is going to be a killer; could be up to around 350uA or 0.35mA. Back to the drawing board.
     
    Last edited: Jul 11, 2011
  14. Wendy

    Moderator

    Mar 24, 2008
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    555 Bistable Multivibrator

    Use a CMOS 555 and a series pass element such as a transistor. I didn't notice any current values, so a small transistor would probably work.
     
  15. SgtWookie

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    Jul 17, 2007
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    I didn't see Ifixit's reply nor yours before I posted the 555 circuit.

    Ifixit, there are a couple of "sticky wickets" with your proposal:
    1) The 10k pull-down resistors for S and R on the left F/F will mean a constant 0.9mA current from the supply. I think our OP is looking for extremely low quiescent current.
    2) Our OP has only specified that the switch they're using is a N.O. momentary; changes to the type of switch may require an expensive redesign of the housing. Just a SWAG, but I'll bet they're using one of those SMT/SMD tactile pushbutton switches; inexpensive, compact, reliable, and don't seem to bounce much if at all. I have yet to see something like that available in SMT - but this may be a non-issue.
     
  16. SgtWookie

    Expert

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    Bill,
    The quiescent current is a killer for even CMOS 555's; it'd be somewhere around .2mA to .3mA for the timer alone. Transistors are just out the window due to the current requirements.
     
  17. ifixit

    Distinguished Member

    Nov 20, 2008
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    Hi dan at tsasystems.com,

    74AC74, 74ACT74, 74HCT74 unless they are obsolete as well. S & R function are active low, which would alter the circuit a little.
     
  18. dan@tsasystems.com

    Thread Starter New Member

    Jul 11, 2011
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    We are using a tactile button pad. Normally Open contacts

    Quiescent current draw is a very important parameter.

    Circuit is in a handheld radiation detection monitor. These get left in the guard shack for days until pulled out and must turn on when needed. I cannot change operation procedure as the end users are mostly trained in their operational capabilities.

    I will take some time to evaluate the 555 components but I think you all have answered most of my questions on them already.
    I also have visited other threads where they are being discussed.

    Sgt Wookie - good stuff you are offering up - and everyone else too. Thanks.
     
  19. Wendy

    Moderator

    Mar 24, 2008
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    Wookie, you can use a logic MOSFET with this circuit, and I've build circuits that worked for over 6 months using CMOS 555s and LEDs. It won't draw 0.1ma if you select really large resistors and a small capacitor. I figure between 1-10µa. Give me some credit please.
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    Bill, I'm just going by the datasheets. I've looked at the ICM7555 and the TLC555's - if you know of a CMOS 555 that has a specified quiescent drain current below 10uA then please tell me, because I can't find one. The ICM7555's quescent current isn't documented at 9v; just 5v and 15v, but doing a bit of extrapolating puts the worst case quiescent current at over 200uA.

    In the CMOS versions I'm familiar with, the voltage divider resistors (for the threshold and trigger input levels) are around 100k each for 300k total; with a 9v supply that's 30uA right there.

    With the kinds of products our OP's building, they have to be quite certain that they'll work when the button is pushed. The quiescent current simply must be documented in a manufacturer's current datasheet, and the part must not be end-of-life.

    With a 300uA quiescent draw on a 500mAh battery, I calculated that the battery would last for 69.4 days before being exhausted. If they actually used the device during that time, battery life would be much shorter.

    [eta]
    Ok, a Fairchild CD4013B has a max 2uA quiescent current @ 25°C when Vdd=10v.
    Maybe a bit of tweaking on Ifixit's circuit would do the trick.
     
    Last edited: Jul 11, 2011
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