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Delay off circuit

Discussion in 'General Electronics Discussion' started by round, Apr 13, 2016.

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  1. round

    round

    11
    0
    Apr 13, 2016
    Hell all.
    New here and found forum looking for an answer to my question.

    I need a circuit that when a 12v supply is switched on and stays constant the output will turn off after around 5 seconds and atay off whilst the 12v is constant.

    Once the the supply 12v is eventually switched off I want the circuit to reset ready for the next time the 12v is applied.

    I have looked into 555 and op amps but I'm getting no where.

    Any help much appreciated.

    I hope I made sense.

    Many thanks
     
  2. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    Step in the right direction!
    You need a 'monostable' or 'one-shot' configuration for the 555 timer.
    When triggered, it's output stays tripped for a predetermined amount of time then returns to it's neutral state.
     
  3. AnalogKid

    AnalogKid

    2,412
    686
    Jun 10, 2015
    A 555 can do this. So can one opamp, or one comparator, or two transistors, or a logic gate. It also can be done with a counter or a microcontroller, but that is overly-complex for this task. Basically, the 12 V charges up a large capacitor through a large resistor. When the voltage hits a certain level, the pass transistor, or whatever is passing the 12 V to the load, is turned off. Because the voltage across the capacitor is changing so slowly, it is important to use a circuit hat has some form of hysteresis to make sure the output is turned off cleanly rather than bouncing around before settling into the off state. All of the things I listed have this property.

    So one question is what kinds of components do you have access to, or are familiar with?

    Also, what is the output current you are switching?

    ak
     
    Arouse1973 and Gryd3 like this.
  4. round

    round

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    Apr 13, 2016
    I have some components and a maplin nearby.

    Currently have op amps: LM358, LM358P, LM393, 4558D and TLO72CP

    555: NE555P --> 12AT68M and 15AT78M

    I understand op amps a little more but still a novice.

    I have a wide selection of resistors and some capacitors.

    I have a small selection of transistors: 2N907, 2N7000, 2N3904, BC547b, 2N3906 and 2N2222

    Various diodes: IN4007 etc

    I found the circuit below but it doesn't work... If the led lit after a delay I assumed I could then replace with a normally closed relay.

    I only had a 12v supply so I used a voltage regulator which dropped input to 6.3v (which I assumed would be fine??)

    Required to switch another 12v circuit off.

    Thanks for replies.
     

    Attached Files:

  5. ramussons

    ramussons

    367
    71
    Jun 10, 2014
    If the highlighted words are without much emphasis, well fine. You have a simple solution. If, however, "stays constant" is further emphasised, you may have a far more complex solution.
     
  6. AnalogKid

    AnalogKid

    2,412
    686
    Jun 10, 2015
    Again, what is the current you are turning off after the delay? 20 mA, 1000 A, 1 uA...? An LM393 (actually, 1/2 of an LM393) is perfect for this, but it can't switch the output current directly. To select the pass device, got got gotta know the current.

    OK, re-read your last post. The circuit is very simple, but I still don't have clear image of the overall system. If the timer is powered by 12 V, and it is to control a relay that controls another 12 V circuit, are the two 12 V sources the same? If not, can they share a common ground? I ask because if they can, then the relay probably can be replaced with a power transistor, which usually has better long-term reliability.

    Worst case, timer circuit drives a relay, and the relay controls whatever. If this is the way it has to be, they we still need to know the voltage and current of the circuit being switched by the relay so we can pick the right relay. Depending on that, the timer circuit still might need a pass transistor to drive the relay coil.

    ak
     
    Last edited: Apr 13, 2016
  7. round

    round

    11
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    Apr 13, 2016
    Ok
    It's for a vehicle.

    With ignition on I want to put another existing switch (currently manual) to ground for a few seconds.

    The current switch literally has two wires. 1 comes from ecu and the other direct to ground.

    The current switch once pressed creates a ground which the ecu detects and activates other systems.

    I want to automate this via ignition on.

    The ground on current switch can only be connected for a few seconds as I want to allow the original switch to be an override. If ground is kept at constant the current switch does nothing. It literally needs to be a pulse to ground but up to 5 seconds would be fine.

    So the feed would be 12v from vehicle ignition at no more than 5amps at a guess based on other vehicle circuits (but I will check) I do not want the circuit to ever switch the current switch to ground again for the entire time ignition is live.

    Perfection would be ignition on, 5 seconds later put current switch (feed from ecu) to ground for 1-5 seconds.

    The next step would be once ignition is turned off it puts the feed from ecu to ground again but I am confident I can achieve this with a simple normally closed relay.

    I really hope I am making sense.

    I was initially playing around with a breadboard trying to get an led to turn off after 5 seconds once power was applied which I was going to use to translate to the ground close. which is why I mentioned a separate circuit. but in essence there is no power or current on what I am trying to control.

    may thanks
     
  8. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    You need a relay, a transistor and few other little bits. Stick them all together and it will work.
     
  9. round

    round

    11
    0
    Apr 13, 2016
    Getting no where with this.
    Anyone got a diagram I could follow?
    My skills arn't quite at creating a diagram yet.
    Many thanks
     
  10. Colin Mitchell

    Colin Mitchell

    1,417
    312
    Aug 31, 2014
    5secON.gif
     
    round likes this.
  11. round

    round

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    Apr 13, 2016
    Thanks so to check I understand.
    The capacitor is to charge creating the delay
    Diode to prevent it discharging

    The resistor is to ensure slow voltage rise to the transistor? (again slowing the circuit) and once the base and collector are at the same voltage triggers the relay?

    Assuming I need 6v input I'm guessing around a 1k resistor what sort of size cap?

    IN4004 diode?

    Many thanks
     
  12. Bluejets

    Bluejets

    4,274
    908
    Oct 5, 2014
  13. round

    round

    11
    0
    Apr 13, 2016
    Thinking about it neither will work.
    I need the circuit to put my switch to earth for 5 seconds then off again.
    These will switch after 5 seconds but will not return to original state after say a further 5 seconds
     
  14. Bluejets

    Bluejets

    4,274
    908
    Oct 5, 2014
    If you can say exactly what you require it would be a big help.

    For example " I need the circuit to put my switch to earth for 5 seconds then off again." means you need a standard 555 with a trigger.

    However this, "These will switch after 5 seconds but will not return to original state after say a further 5 seconds" to me means you need a 555 with a start delay and possibly a trigger.
     
  15. round

    round

    11
    0
    Apr 13, 2016
    My earlier reply stated what the circuit is for and how I need it to perform.
    Sorry if it didn't make sense.

    It's for a vehicle.

    With ignition on I want to put another existing switch (currently manual) to ground for a few seconds.

    The current switch literally has two wires. 1 comes from ecu and the other direct to ground.

    The current switch once pressed creates a ground which the ecu detects and activates other systems.

    I want to automate this via ignition on.

    The ground on current switch can only be connected for a few seconds as I want to allow the original switch to be an override. If ground is kept at constant the current switch does nothing. It literally needs to be a pulse to ground but up to 5 seconds would be fine.

    So the feed would be 12v from vehicle ignition at no more than 5amps at a guess based on other vehicle circuits (but I will check) I do not want the circuit to ever switch the current switch to ground again for the entire time ignition is live.

    Perfection would be ignition on, 5 seconds later put current switch (feed from ecu) to ground for 1-5 seconds.

    The next step would be once ignition is turned off it puts the feed from ecu to ground again but I am confident I can achieve this with a simple normally closed relay.
     
  16. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    You can take this apart and think of it in different components.
    You need :
    A) A Timer to control your output. "It must begin in the off position, then when switched 'on' with a trigger, will remain on for 5-seconds before switching off. Must NOT switch on again after this point until reset."
    B) Your output... regardless of switching a load to ground, or providing power, you can hook up a relay to the timer.

    The Relay can be a Single or Double Pole. (Switch one or two things at the same time)
    The Relay can be Single or Double Throw. (Simply switch on|off, or 'Toggle' from one output to another)
    The Relay can have N.O. or N.C. contacts. (Normally Open is common for Single Throw types, and completes the circuit when active. Normally Closed is included in Double Throw and breaks the circuit when active.)

    So, in this case, you could use a Single-Pole, Double-Throw relay. The 'Common' pin goes to your ECU. The N.C. pin goes to the original sensor. This allows your circuit to leave the sensor connected to the ECU when your circuit is 'neutral' . Connect the N.O. pin to ground. When the relay activates, the ECU will be disconnected from the sensor and connected to ground instead. The timer will control the relay, the relay can be wired to manipulate what you want.
     
    round likes this.
  17. CapSteel

    CapSteel

    13
    2
    Mar 7, 2016
    Here's a link to another idea for a one-shot circuit:

    http://pinvolt.com/960gs/xSite/archive/R11/R11.html

    Looks like the input/output are digital logic levels (0V/5V), so would need to add maybe a voltage divider on the input to bring the 12V down to 5V and the output would go to a buffer/driver to provide more current drive. Just another idea to think about. 555 timer may be simpler.
     
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