12v pulse into 12v supply help needed

Discussion in 'The Projects Forum' started by nooby, Nov 16, 2013.

  1. nooby

    Thread Starter New Member

    Nov 16, 2013
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    Hi guys and gals, sorry if this is simple or old news but I have looked over the forum along with the rest of the internet and find myself running in circles to find a solution.
    I have chosen this forum to join and ask for help so please be gentle and I thankyou in advance for any help.

    I have limited electronics knowledge but can make a circuit up and do understand what components are, I just might not understand certain phrases or know what long words mean lol.

    Onto my issue,

    I have a video doorphone that has an unlock feature. The equipment was bought 10 years ago and only just being fitted.

    There is a lock\unlock feature whereby on pressing a button on the inside monitor the door unit will operate a lock on the door to allow entrance.

    The unit is a Chinese special and did not even come with chinglish instructions to follow (didn't even come with the locking system).

    I have bought and installed a 12v locking strike plate. This strike plate allows the opening of the door when power is applied, at all other times it is fixed closed (fail secure).

    The unit has an output to connect to the locking system the user chooses which emits a very brief 12v output when the unlock button is pressed.
    This output is powerful enough to operate the solernoid on the latch (I can hear it activate) but as it is a split second instance it is not open long enough to allow a user to open the door.

    I have been working on the assumption that I merely need a simple circuit to lengthen that '12v pulse' so its on long enough to allow the lock to operate.

    I have looked at a lot of 555 timer circuits but can only find ones that use a switch to either start the timer or reset the timer. I have looked at monostable multivibrator circuits but they seem to run from a negative trigger which I admit I don't even understand fully.

    Is anyone willing to help me out a little and hold my hand?
    I want a circuit that will take a split second 12v output and turn it into an adjustable length 12v output?

    I think I can get a constant 12v supply from the board to run the lock solernoid or I could use a separate transformer either way im not worried about getting the 12v supply needed for the locking latch.
    I perhaps only need the pulse converting to a 1-10 second constant supply as once the door is open the lock itself will close again without the latch being operable.

    Perhaps I am trying too hard? maybe just a capacitor across the latch to hold some power in long enough is needed but that's above my knowledge level.

    Sorry for such a noob question, I know most of you will be able to do this in your sleep so again I thankyou in advance for taking the time to read this and maybe offer a little help.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Something like this perhaps? The choice of Q2 will depend on the solenoid current. Do you have the spec? If the current is <200mA Q2 may be unnecessary.
     
  3. nooby

    Thread Starter New Member

    Nov 16, 2013
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    Hi, thankyou so much for your reply.

    Unfortunatley I do not have the specs of the latch, I will try to get them but it will be Monday before I can ring the supplier.

    The coil is rated at 5-12v, is there a way I can read its current rating with a meter? im not sure I can but if you could tell me a method I am happy to do it.

    Ill be honest and say it may be quite hard to get its rating as I bought it from a supplier and I doubt they will have the specs, I can try to contact the makers.

    Would a high current transistor work such as a capacitor swap can be a higher voltage? or maybe im getting mixed up.

    On the signal in part of the circuit would the negative be left open or connected to the ground of the circuit?

    Sorry for being so new.

    I really apprechiate your help.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    I agree a 555 monostable (as alec_t has shown) is the correct strategy to turn a brief pulse into a longer one. The 555 will trigger on the negative-going edge of a pulse.

    As a small thing, you could consider eliminating Q1. It's purpose is to reverse the logic of the pulse, but it sounds to me like your pulse is so short that it would be OK to trigger the latch when the pulse shuts OFF instead of when it turns ON.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Personally, I'd use a MOSFET such as IRF540N (which I have on hand). Cheap, and huge current rating for this application. But there are many other choices such as a darlington.
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Here is my version:

    I am guessing that the solenoid DC resistance will be low, less than 5Ω, which means that it will take a couple of Amps to operate it. You can test it with an Ohmmeter.

    My version uses an NFET to switch the coil to minimize heatsinking as would be required by an NPN transistor.
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Connected to ground.
    As MikML says, you can measure the coil resistance with a meter (DMM). That will be useful info.
     
  8. nooby

    Thread Starter New Member

    Nov 16, 2013
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    Thankyou for your replies, some interesting reading.

    What would be the pros and cons of the two circuits? I am unsure which to choose but it is correct that the 12v spike is so short it will be fine to activate the timed 12v after the spike has already ended.

    I shall measure the coil resistance tomorrow as I am away from home right now.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    One question, you mention the lock requires a longer pulse, long enough to enable the door to be opened and then the latch returns, if the door is open at that point, how does the latch return into the striker plate as in the closed position when closing the door? This could not be like a dead bolt type, it could work if it had a curved side as in an ordinary door latch,often a magnetic latching solenoid is used in this application, but these require a latch and a un-latch pulse.
    Max.
     
  10. nooby

    Thread Starter New Member

    Nov 16, 2013
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    Its a strike plate that has a moveable plate that when the solernoid operates the latch on the door is allowed to 'push/pull' out of the strike.
    When the latch pushes past the strike the strike is returned shut by a spring and the solernoid springs back in place.
    The latch on the door is a normal nightlatch which has an angled latch which when pushed past the strike it is forced inwards to allow the door to shut against it.

    They are very common across the uk, I know a lot of commercial properties use magnetic latches but I could talk all day, im a locksmith lol.


    http://en.wikipedia.org/wiki/Electric_strike

    http://www.yale.co.uk/en/yale/couk/ProductsDB/?groupId=2872&productId=58956
     
    Last edited: Nov 16, 2013
  11. #12

    Expert

    Nov 30, 2010
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    Measure the resistance. Everybody is waiting for the specification so they can figure out the best answer.
     
  12. nooby

    Thread Starter New Member

    Nov 16, 2013
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    Yes i appreciate that but as i say i am not at home. The earliest i could have done it was today but i am still away with work. I will be back tomorrow now.
    I will post the resistance asap.
     
    #12 likes this.
  13. Alec_t

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    Sep 17, 2013
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    Incorporating the various suggestions above, the circuit could look something like this.
    D1/R1/C3 have been added to reduce the likelihood of U1 power supply glitches when the lock activates. R2D2 (wasn't he in Star Wars?:)) reduce the positive voltage spike which would otherwise be fed to the trigger input of U1. R4C2 prevent spurious operation of the lock (if that's a concern) at power-up.

    Edit:
    BTW, have you checked that your original controller doesn't have a trimmer/knob somewhere to set the pulse duration?
     
  14. nooby

    Thread Starter New Member

    Nov 16, 2013
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    1
    Hi, thanks for that very helpful.

    I have gone over the hardware and software but not found any way to adjust the delay. Im as supprised as anyone that there isn't an adjustment but it doesn't seem there.

    I will get the coil resistance tomorrow afternoon and start to get some pieces together.

    Thanks for all your help so far. Much aprechiated.
     
  15. nooby

    Thread Starter New Member

    Nov 16, 2013
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    With a meter across the wires of the strike it reads 30.6 ohm.
     
  16. MikeML

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    Oct 2, 2009
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    So at 12V, that would require I=E/R = 30.6/12 = 2.55A, so the NFET is the way to go for switching it...
     
  17. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    :confused:....er, I= 12/30.6 = 40mA. So the 555 could perhaps directly drive the coil (albeit at ~10V) without the need for the FET or a BJT ?

    :rolleyes: Edit: Ignore that. Current = ~400mA :)
     
    Last edited: Nov 18, 2013
  18. MikeML

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    Oct 2, 2009
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    Alec, I dont know about you, but it is too early here...

    Actually, it must be early there, as well.

    It requires 0.4A
     
  19. Alec_t

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    Sep 17, 2013
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    Oops. It must be catching. Quite right, 0.4A. I've edited my previous post.
     
  20. nooby

    Thread Starter New Member

    Nov 16, 2013
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    So would the above circuit still be the best way to go?
    Thanks for all your help.
     
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