12v DC pulse with 555?

Discussion in 'The Projects Forum' started by spaspeckerthedull, Nov 12, 2012.

  1. spaspeckerthedull

    Thread Starter New Member

    Nov 12, 2012
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    hi all - long time lurker, first time poster that needs help with a circuit.

    basically, I am looking for a bit of hand holding here as whilst I have an interest in electronics and wiring/projects, I have a very limited understanding of the subject so would request a bit of patience for this noob!!

    the application I have in mind is a simple timed pulse circuit. I want to be able to push a button and have a pulse drive a solenoid for approx between one quarter/one half of one second. This needs to happen every time I push the button. If it helps I can link to the solenoid I am going to use HERE, and a possible solution I have found HERE

    any and all help would be much appreciated
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Just some comments: You need to determine the current requirement of your solenoid. I don't think it's anywhere near the capacity of the Velleman relay, which is likely overkill, but you should be sure.

    Your timing interval is much shorter than the Velleman is designed for. You could alter the kit to use a smaller timing capacitor, and otherwise I think it will be fine. For instance, change C2 to 1µF instead of the 100µF.

    At short times, you have to realize that the relay on that board and the solenoid are both mechanical devices with finite movement times. I question whether you could get precise control down at the 100ms range.
     
  3. spaspeckerthedull

    Thread Starter New Member

    Nov 12, 2012
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    many, many thanks for the quick reply - i've been searching for the current requirement info you mentioned and can find nothing at all, only some details for the same unit but in 24v. The result claimed power at 9.9w, and current at 0.41a. I don't know if any of that is relevant but it's all I could find!!

    I've also found this circuit (scroll down to "555 timer monostable one shot circuit" which is 11th from the top) on another site, not sure if it's the same as what I require but it reads to me like it is

    I understand what you're saying about the mechanical parts, my trouble is I have seen a circuit like this running in an application I use to own, unfortunately I cannot remember precisely how long the solenoid pulsed for but it seemed to be around half a second. Absolute timing is not critical, near enough will do. The solenoid will be striking a valve to open and allow the passage of a gas, in this case CO2, so around one quarter to one half of a second should be more than sufficient

    I think i'm going to plump for the velleman kit with the cap requirement you suggested - what's the worse that could happen!?!
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    At 9.9 W, & 12 V =.825 A, 3A relay should be fine, providing a snubber, reverse connected diode, is used across solenoid.
     
  5. spaspeckerthedull

    Thread Starter New Member

    Nov 12, 2012
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    just spotted something else - am I right in thinking that off the velleman kit the relay contacts at A B C on the pcb won't give me a voltage output? and if so i'll need a seperate power supply for my solenoid?
     
  6. spaspeckerthedull

    Thread Starter New Member

    Nov 12, 2012
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    UPDATE

    OK - bought and assembled the kit, replaced C2 with the cap as suggested by wayneh, but didn't get the results I expected or wanted. The pulse time delay is more easily and accurately adjusted using the value cap as suggested, but the circuit doesn't behave as I desire:

    to clarify - the circuit stays "latched" (is I think the correct terminology) until the start button is released after which the timer delay activates then switches off after the chosen time using the adjustable pot, and I get no voltage out from A B C to drive my solenoid

    I know this can be done, as years ago I have owned an item previously that did this, but maybe I haven't phrased my question correctly so here goes - what I want to be able to do is press a button, momentarily, which will then create a 12v+ pulse to drive a solenoid for approx 1/4 - 1/2 of one second, which will perform the same action each time I press the button. The only scenario in which I can compare the action I want to replicate is that of a paintball gun - you press the trigger, you get your pulse which does whatever it does inside the gun, which fires a paintball. I am trying to replicate the same action - you press a button, you get a pulse?

    any further suggestions? and please bear in mind that buying a paintball gun and tearing it down to cannibalise its innards is NOT an option due to the prohibitive cost of the gun in the first place!!
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Here is an article on monostables I wrote for the AAC book...

    555 Monostable

    It explains why you are having your problem.

    A possible variation would be something like this, though I would stick with the Velleman kit.

    [​IMG]
     
  8. spaspeckerthedull

    Thread Starter New Member

    Nov 12, 2012
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    Bill_Marsden - many thanks for the reply, but it could not be further over my head.

    I must explain that I have no idea how to read circuit diagrams and am completely unfamiliar with the terminology employed. I can assemble with clear instructions, such as the kit that I bought today, and can basically solder, but that is about it. I know many people reading this will now be thinking "jeez what a dumbass" but i'm afraid that's about the size of it.

    I do not know why this kit will not do what I want, I do not know why the circuit stays "live" or "energised" or whatever the terminology is for the duration of the start button press until it is released and then the timer starts, and I do not know why I do not get voltage out of A B C. I have found on the velleman site that the contacts A B C are "simply switches and need an external source" - whatever that means.

    I'm not one to give up easily but it does look like this is going to be completely beyond me. Once again, many thanks to those who tried to help but I think I will cut my losses and just try to come up with a mechanical solution instead. Please don't think "serves you right you quitter" - you must understand that I am not and never have been an electronic engineer who came to this forum with an understanding of electronics to "chew the fat", and I must make clear I didn't expect someone to come along and say "oh yes, this is what you need and this is how it goes together" although that would have been nice!!
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Don't give up yet! I think you're very close. With reference to the figure 15 schematic in the Velleman manual, have you connected A, B, or C to the power supply? As drawn in the schematic, all 3 poles are unconnected. I believe you'll want "A" connected to +12V, and take your solenoid signal from either B or C, whichever is NOT normally connected to A except when the relay is activated by a timer cycle.
     
  10. spaspeckerthedull

    Thread Starter New Member

    Nov 12, 2012
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    hi wayneh -

    yes, I connected "A" to a 12v+dc, and put my meter across "A" and "C". The result was an output for the duration of the pulse. Very good. The problem now is that this output is active for however long the "start" button is pressed, and the timer circuit only starts counting down when this "start" button is released. This means that the solenoid will be "pushing" for the duration of the "start" button press PLUS the subsequent timer pulse. This is not good, as in my application it may be that the "start" button could sometimes be held down inadvertently by other users unaware of this happening.

    Just "tapping" the start button means the timer cycle/pulse gives me what I want as an output, but I could do this just by wiring the solenoid straight to a button and power supply and cutting out the kit altogether, then actioning the solenoid by simply tapping a button - i'd get much the same effect and could lengthen the solenoid action manually just keeping the button depressed

    what I want is 1xbutton press to give me 1x12v+dc pulse for a set duration (the approx 1/2 second previously mentioned) without the circuit being live for the duration of the button press

    does any of this make sense as i'm just about done with this, and am already drawing out a mechanical solution - now that IS something I can understand!!
     
  11. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Here is a circuit that outputs a ten second pulse whenever the button is pressed regardless of how long the button is held down. The pulse can be shortened to 1/2 second by changing the value of C1 to 10 μF and R1 to 47kΩ.
     
    Last edited: Nov 13, 2012
  12. wayneh

    Expert

    Sep 9, 2010
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    The middle circuit Bill provided shows a similar solution. The idea is you use a capacitor to limit the length of time that a negative going trigger signal (ie. the pressing of a switch) is passed to the timer's pin 2 trigger. The signal ceases once the capacitor discharges it's relatively small burst. You'll want the smallest capacitor that still allows a trigger event to pass. I'd start with 0.01µF - 0.1µF or less. That pulse will be short - microseconds - compared to your timing sequence, 100s of milliseconds.

    I hope you can implement this without too much hacking of the PCB. You may just want to do it off-board.
     
  13. spaspeckerthedull

    Thread Starter New Member

    Nov 12, 2012
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    it's great to see people still trying to help me out with this, I really do appreciate it

    I think I understand about adding a capacitor to the circuit, is there any chance you could "X marks the spot" for its location on this diagram? I reckon it should go between SW1 and R41K?
     
  14. wayneh

    Expert

    Sep 9, 2010
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    One end of the cap must touch ground when SW1 closes, the other end must have a connection to pin 2.

    You'll need an R3 also (see Bill's middle schematic) to recharge the capacitor after you let up on SW1. It will connect to the end of the cap that faces SW1. I think in your case this could be a large value, say 100K. That would still charge the cap in plenty of time to allow the next firing. But 1K or 10K would all be fine.

    Since R4 is 1K, if you use a 0.1µF capacitor the time constant RC for the trigger pulse will be 100µs. I think that's probably fine. A 0.01µF would probably work fine also.
     
  15. spaspeckerthedull

    Thread Starter New Member

    Nov 12, 2012
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    right then - I got my components, a 100k resistor and a 0.1µF capacitor.

    Is the following correct? I connect the negative leg of the capacitor to the negative side of the "start" switch SW1. I connect the positive leg of the capacitor to pin 2 of the 555.

    I cannot figure out the connection of the resistor? I connect one side "to the end of the cap that faces SW1" I take it this is C1? also, which side of C1? and where does the other side of the resistor connect? please be aware I simply cannot read wiring diagrams/schematics and while I truly appreciate your efforts it is pointless to keep referring me to bills drawings - I just do not understand them despite spending hours over the last 2 days trying to teach myself how.
     
  16. wayneh

    Expert

    Sep 9, 2010
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    Does your capacitor really have a polarity? The connection to SW1 should be to the side opposite ground, so that closing the switch touches that end of the cap to ground.
    The resistor supplies power from +12V to the same side of the cap that is connected to the ungrounded side of SW1, the side that gets touched to ground when you close the switch.
    And I find it too much trouble to parse words into a mental picture, which is what I have to see in order to think about this stuff. Everybody's different.
     
  17. spaspeckerthedull

    Thread Starter New Member

    Nov 12, 2012
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    OK been trying to get this to work as you have explained it to me but i can't - i just simply can't get it, so its time to pack it in as i am just getting more and more frustrated. I have connected the components as you have told me, or i think i have, and the circuit when triggered behaves exactly as it did before the extra capacitor and resistor were added so i am no further on than 3 days ago.

    What really gets to me is that i can just fit a simple push to make switch and by just tapping it can make it do exactly what this kit is doing. I am no nearer the action i want and have no further understanding of what to do. I am aware that some people may be laughing up their sleeve at me but it bothers me not, some things i get some i don't and this is one of them. At least i tried.

    Thanks to all for the effort but i've had enough
     
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