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Low cycle Pulse generator ?

Discussion in 'General Electronics Discussion' started by WJ2002, Feb 4, 2017.

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  1. WJ2002

    WJ2002

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    Jan 30, 2017
    I need to build a 12v pulse generator to trigger a delayed OFF relay.

    The plan is to use a 555 circuit, but can I build one to fire a 1sec pulse, every 10 seconds or longer ?

    Or is there a better solution ?

    The feed for the circuit will be an unregulated 12V car supply from the starter solenoid during cranking !! Sorry.
     
  2. Minder

    Minder

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    Apr 24, 2015
    You may need a 556 but it should be possible,
    Do you use any picmicro by any chance?
    M.
     
  3. Bluejets

    Bluejets

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    One can buy 555 delay timers on Ebay for a couple of dollars but you arrangement sounds like it needs to repeat indefinitely during start.
    The one M refers to is a dual timer in one casing.
     
  4. WJ2002

    WJ2002

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    Jan 30, 2017
    Thanks for the replies.

    M. I don't use picmicro ? And TBH, I don't even know what it is. It's been a long time since I messed around with electronics.

    Bluejets - I have seen the 555 timers on Ebay. But, here is the issue:

    The idea for the circuit is to fire a cold start injection valve on an old car. There is a separate Thermo Time Switch (basically a bimetal switch), which fires the valve for a certain period depending on how cold the engine is (4 - 10 seconds). But when the engine is HOT the bimetal circuit is open and then the valve is fired from a separate circuit for just 1sec. The original circuits are now nearly 50 yrs old Bosch units and most have failed and those that haven't (yet), are immensely expensive. Most people bypass, them with a crude switch on the dashboard, operated manually. I would like to automate the process, but try and improve on the original.

    This why I want the PULSE and the low duty cycle.

    Ignoring the Thermo Switch, assuming it is open circuit (i.e. Engine HOT)

    A delay relay will operate with a 12v trigger, but the delay won't start until the trigger is released. If I use the cranking 12v supply, this will be ON, as long as I crank the engine, so the relay wont release until I stop turning the starter motor. This is a problem, as the valve will be operating during the whole start cycle and will flood the engine.

    So, I thought, if I can operate the delay relay with a short 12v pulse, it will see the pulse, then as long as I set the delay for 1 sec, it will go off, hence only firing the valve for the required period. The reason for the low duty cycle is I don't want more than 1 pulse during cranking. It would be odd to be trying to start a hot engine for more than 10sec, in this instance there may be a separate issue anyway, if that makes sense ?

    Please tell me if I'm trying to reinvent the wheel, I have limited electronic project experience !!
     
  5. Alec_t

    Alec_t

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    Jul 7, 2015
    So you want a single 1S pulse to occur during cranking. How soon after starting cranking should the pulse occur?
     
  6. WJ2002

    WJ2002

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    Jan 30, 2017
    It's not critical, but as soon as possible I guess. Ideally, the pulse should occur at the same time as the start signal.

    So the valve will fire the fuel, at the point of the starter engaging. But like I say, it's not entirely critical, just during the cranking process, but better as early as possible.
     
  7. AnalogKid

    AnalogKid

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    Jun 10, 2015
    When the ignition key is turned to the start position, cranking starts and the pulse circuit immediately makes an output for one second. yes/no

    If the ignition key is released before the 1 second is up, the output pulse ends immediately. yes/no

    If the ignition key is held on for a long time, the output pulse starts again in 10 seconds (9 seconds from the end of the previous pulse. yes/no

    I'm not a fan of multi-second 555 timer circuits, but one CMOS 555 (plus an output driver) should be able to do this. If all three answers are yes:
    Configure the 555 as an astable oscillator, 1 second on, 9 seconds off.
    Enable the 555 circuit with its reset pin.

    What is the 555 circuit driving? What is the peak current? Does the 555 circuit switch 12 V to the device, or switch its low side to GND?

    ak
     
  8. WJ2002

    WJ2002

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    Jan 30, 2017
    Hi Analogue. Yes to all 3 questions.

    The pulse will be used to activate a relay, which in turn will activate the start valve, so peak current isn't an issue - I think ?

    I'll look into using the CMOS555 device. What is the advantage over a 555 ?
     
  9. Alec_t

    Alec_t

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    Jul 7, 2015
    It can pull its output almost to the +ve rail.
    It draws less current.
    The disadvantage is that it can't source or sink as much current as the bipolar 555, so will probably need a transistor to drive an automotive relay coil.
     
  10. AnalogKid

    AnalogKid

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    Jun 10, 2015
    IIRC, I think the difference is 200 mA vs. 300 mA, but I'd add a driver transistor no matter what. No matter how tough the part is, IC output stages and automotive power quality do not mix well.

    Is the relay tied to +12 V, and you are switching the ground side, or is it grounded and you have to drive it with +12 V?

    ak
     
  11. WJ2002

    WJ2002

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    Jan 30, 2017
    Thanks for the advice so far guys, really useful stuff.

    I spoke to one of the engineers at work today who also advised the use of a driver transistor.

    The coil side of the relay will be permanently grounded and the 555 will be providing the 12v trigger to activate it. On the switch side it will be switching the 12V rail. Is that what you're asking ?
     
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Is this carved in stone or can it be modified with one end of the coil hard wired to +12V?

    Chris
     
  13. WJ2002

    WJ2002

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    Jan 30, 2017
    Hi CDRIVE. No, nothing is set in stone.

    I'm here to be advised. I have no set ideas, so go ahead. I just assumed that would make sense, but am open to all suggestions.

    Thanks
    Wayne
     
  14. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    I asked because switching the relay coil to GND (low side switching) affords configuring the transistor as a common emitter amplifier. When saturated It will deliver nearly full battery voltage to the relay coil.

    Chris
     
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