monopulse 555/mosfet circuit not monopulsing

Discussion in 'The Projects Forum' started by danefex, Oct 20, 2011.

  1. danefex

    Thread Starter New Member

    Oct 20, 2011
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    Greetings folks!

    This is my first post on this forum, so before I ask about what I'm looking to do I just wanted to say hello and I hope I am doing this right. I've just read the rules about posting questions on this forum, what to do and what not to do, ect - so if I am out of line in any manner (wrong section, question already posted) please point me in the right direction. I have no problem with constructive criticsm or being corrected. I've also tried searching for an answer here but I haven't found a specifc answer to my problem. Here is my short story and situation.

    I had a friend of mine recently help me build a simple monopulse circuit using a 555 timer that would give me a nanosecond(s) monopulse upon closing the circuit, much like a strobelight effect except only doing 1 strobe each time the circuit is closed. This is for a laser gun project that I've built recently, where I am trying to fire a short laser pulse rather than a laser beam when the trigger is squeezed. The problem I am having is that when I close the circuit, the LED I am testing doesn't monopulse - it just stays on. I am unable to get more information on correcting this problem so I thought to try asking here to see who might be able to help out. I'll try to provide as much information as possible along with a schematic but I must confess I am a novice electronics hobbyist, a lot of this is above what I know about circuitry.

    I'm using 3 14500 3.7V lithium batteries to power the circuit and the laser I'm firing is using a XJ-A140 445nm 1W diode. The mosfet I'm using is a 55V stp80nF55 80A 6.5mΩ. The RS requires 1W .6Ω so I am using 2 1.2Ω in parallel. I believe the rest of the important info is on the schematic, but if I am missing some critical info please ask me & I will tell. All of the rest of the components involved are 1/2W to 1W resistors, transistors and caps. The schematic is attached as a thumbnail, I dont know how to directly post pictures here just yet.

    I dont know how difficult or not this would be to solve for an expert but any help or information would be greatly appreciated. Again, please feel free to ask any questions back about something I may not have covered. Thank you in advance for reading this and your interest! :D

    [​IMG]
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Welcome to AAC! Everything is kosher, you even included a schematic, which is rare.

    You mentioned nanosecond pulse, but the time duration you have set up is 0.011 sec to .517 sec. Bit of a discrepancy there.

    I have some tutorials I wrote for the eBook, plus few more under development.

    The 555 Projects
     
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  3. danefex

    Thread Starter New Member

    Oct 20, 2011
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    Yes indeed, I dont mean exactly 1 nanosecond but moreso in the nanosecond range. I dont think a single nanosecond pulse would even be visible but I've also never achieved it. I am amazed that you can spot something like that as quickly as you have from just looking at the schematic. Thank you for the eBook link, I am checking it right now!
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    I have the equation memorized for the monostable.

    T = 1.1 R C
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    555 timers can get you into the low microseconds, but if you're looking for nanoseconds, you need something a lot faster.
     
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  6. danefex

    Thread Starter New Member

    Oct 20, 2011
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    Nice, thank you sir! They say you learn something new everyday :) After looking through your eBook link on the 555 I still haven't found exactly what I'm looking for specifically for this circuit, but again I'm a novice with circuitry and there's a good chance I'm looking right over it. The 555 monostable multivibrator seems closest to what I want to do, but it's a bit above what I understand with what I am trying to accomplish. I shall continue to read on & I thank you again for the pointer.

    ..and here I thought I was dealing with an unmeasured cluster of nanoseconds, thank you for that clarification. Actually I have a youtube video demonstrating a slightly different 555 circuit for this same purpose, and ironically if I were to get this new circuit monopulsing like the old one - my next question was going to be how I could shorten the monopulse demonstrated. In the video I have the 47k pot turned as low as possible for the shortest monopulse, but it's still a little longer than I wanted it to be. I wasn't aware of anything faster that could do this, do you have any recommendations?

    Monopulse firing laser gun
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Sure. The 10uF 25v cap below pins 7&6 and the 47k pot - replace it with a much smaller cap.

    An 0.1uF (100nF) cap will make it pulse ~100x as fast as it's pulsing now.

    At a certain point, the pulse will be of such a short duration that you won't be able to see it - but it's kind of difficult to tell what the actual pulsewidth will be with the odd current limiting scheme being used with the MOSFET.
     
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  8. danefex

    Thread Starter New Member

    Oct 20, 2011
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    I cant thank you enough for making me aware of this, I will definitely try a smaller cap & see how it affects the output. Now if I used this smaller cap & the monopulse were too short, wouldn't I be able to adjust the timing on the pot to lengthen the monopulse?
     
    Last edited: Oct 20, 2011
  9. SgtWookie

    Expert

    Jul 17, 2007
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    You can try it in steps to see what works best for you, if that's what you'd like to do.

    You say that the current pulse is just a bit too long for your tastes.
    If you replace the 10uF 25v cap with an 0.22uF/220nF cap, you should then get almost the same pulse length with the pot set on maximum resistance; and the minimum pulse will be about 1/48th that length.
     
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  10. #12

    Expert

    Nov 30, 2010
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    That circuit looks so good! Besides that, it is correct for what you're trying to do :)

    There is one point of uncertainty. The first transistor slaps pin 2 down to about 3% of Vcc. The "1.1" equasion depends on the lower trigger voltage being 33% of Vcc. That is why the timing is a bit uncertain, as in, 1.1RC won't be true in this case. More like Time = 2.2 RC. In addition, the speed of the LM, NE, or SA prefix 555 chip is 100 nano seconds rise time and the same for shutting off time. That's one reason you will never get a single nanosecond sized pulse. I don't see that as a problem. Humans can't see a nanosecond size pulse, anyway.

    Then I'd increase the driving section current.
    On the second transistor, the 10k collector resistor is limiting the current driving the gate of the mosfet to about .001 amp. I'd change that to about zero ohms and add a 220 ohm resistor from the emitter of transistor 2 to the gate of the mosfet, then change the base draining resistor on the second transistor to 22k.

    Physically attach the 220 ohm resistor with its body very close to the gate terminal of the mosfet. Then add a 4.7 k resistor from the gate to ground.
    Eliminate the 1N4148 diode.

    Change the 10k resistor on the base of the third transistor to 100 ohms.
    Move the collector connection of the third transistor to the base of the second transistor.

    In summary, the 1k and 22k resistor on the base of transistor 2 gives you 95% of the 555 output voltage on the base. Transistor 3 limits that voltage when maximum current through the load is achieved.

    Transistor 2 has unlimited current ability except that the 220 ohm resistor limits it to .049 amps...much faster than .001 amp! The 4.7 k resistor from the gate to ground wastes a fifth of a milliamp and allows quicker shut off.

    Please question me if I missed anything.

    edit: the 47k in the drawing is labeled wrong. Should be 4.7k
     
    Last edited: Oct 21, 2011
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  11. danefex

    Thread Starter New Member

    Oct 20, 2011
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    I am going to take a crack at this tonight when I can get a few uninterrupted hours of soldering to try this. Right now the circuit doesn't monopulse, the LED stays illuminated when the circuit is closed. Once I figure out why it's doing that I'm going to replace the caps & see what the resut yields. Again, I sincerely thank you for these suggestions/ideas. As I'd said in the OP I'm a novice with circuitry so this information is very valuable to me! :D

    You guys are awesome & extremely helpful!!! I will definitely try everything you've mentioned above this evening & see what I get from these changes. Thank you thank you thank you SgtWookie & #12!!!!

    ..does this forum have a donations page anywhere? I'd like to contribute for the help & information I'm getting from you folks, I am sincerely grateful for the time taken to address my little problem and it is the least I can do right now.
     
  12. Georacer

    Moderator

    Nov 25, 2009
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    The site currently doesn't accept donations. Hit the thanks button on the helpful post and it's enough for us.
     
  13. #12

    Expert

    Nov 30, 2010
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    Yep. A, "Thank You" goes a long way here, especially compared to what I call a, "drive by".

    Often, somebody drives by here, rolls a drunken question out of the car, and half a dozen people exert killowatt hours of brain power on it, only to never hear from the original poster again. We don't know if we fixed it, we sound stupid, he forgot our address, or what!

    So, yeah, click on the, "Thanks" button and we all go home happy.

    ps, be sure to double check the current limit determined by the .6 ohm resistor. It's only a calculated limit based on the typical transistor in the third position.
     
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  14. danefex

    Thread Starter New Member

    Oct 20, 2011
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    Thanks again for all the info folks! Unfortunately for me I did not get to take a shot at making any of the suggested changes to my circuit last night so I am sorry I dont have any news yet on what the outcome was. I am part of a wedding going on this weekend & family is in from out of town, so I didn't get the time to work on it that I'd hoped for.... not yet anyways.

    I'm certainly not a drive by visitor here either (nice explanation btw :)) - so I wont be a short term person here suckling from the massive brainpower you've generously offered. I am very interested in where this is heading, so while I know it may take some time to get it right - I'll be around for a bit! Take care for now, I will follow up soon once I can get crackin but it will probably be after the weekend now......
     
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