30 us LED pulse

Discussion in 'The Projects Forum' started by Sedeca, Jul 2, 2014.

  1. Sedeca

    Thread Starter New Member

    Sep 26, 2013
    11
    0
    Hello everybody,

    I would like to ask you for some help on designing a circuit to create
    short LED pulses. I have already spent quite some time reading up on
    similar threads in the forum but as I am a chemist and not an electrical
    engineer I am not quite there yet. So here is what I want to do and what
    I have thought of so far:

    I thought of doing this in two stages, first a simpler device and once
    that works maybe a more complicated one.
    So at first I would want to generate one puls somewhere in the tens of
    miroseconds when I physically pres a button that drives 4- 6 of these
    LED's
    ( if anybody knows brighter red LEDs that work without cooling at about
    the same size I would love to see them :))
    (how many exactly will depend on the light intensity I get and how many
    of them i can fit in the device I am building. as making them less
    bright is always easy lets say 6)

    peak current for those LEDs is 2.6 V and as the duty cycle will be very
    low I would like to use that to get maximum brightness. so that would
    add up to 15.6 Volts.

    to generate the puls I would use a 555 in monostable mode with the
    appropriate resistor and capacitor (something like 10nF and 1kOhm)
    and the output would then drive a n-mosfet that is connected between the
    LEDs and ground.

    Is that going into the general right direction? Is there a simpler
    solution or will this not work?
    if it goes in the right direction here are some more questions:
    1) could somebody point me to the exact 555 needed. I have not yet
    grasped the difference between the variations.
    2)I am also very unsure what kind of mosfet I need. I think that the
    gate impedance has to be low for the mosfet to change quickly enough?
    might I need a mosfet driver (isn't that just another mosfet?)? also I
    chose a mosfet more or less because I know it is a transistor. I am
    aware of my ignorance here. Is that the right type or transistor for the
    task?
    3)What I have not yet figured out is how to convert a (long) physical
    button pressing into a trigger signal that is short enough for the timer
    to work. In another thread I found this
    schematic
    that has a capacitor connected to the trigger pin but I am afraid I do
    not entirely understand how this works. (when the button is pressed the
    capacitor fills and thus temporarily decreases the voltage at the
    trigger pin???)

    4) If we assume I use an 18 Volt power supply (I have one from an old
    laptop lying around) I calculated that I will need a 2.7 ohm resistor in
    series with my LEDs to not exceed their maximum ratings. However as
    these LED's draw a lot of current I am not sure what type of resistor I
    will need. As they are only on for a very short time would any kind of
    resistors that I can combine to generate 2.7 ohms work or do I need to
    look for how much power they can dissipate? (If I would want to drive
    the LEDs at the max rating for continuous illumination (2.2V, 400 mA)
    the resistor would have to be 12 ohms and drop about 2 watts. that
    sounds like a lot. is that correct?

    I now will go and look for a program that paints circuit layouts so that
    It becomes clearer and then attach this here. In the meantime I am very
    grateful for all tips and help. The more complicated version would then
    be to trigger the pulse digitally (arduino most likely) and also to use
    some kind of digital potentiostat as the resistor in the timercircuit so
    that i can vary the pulse length between lets say 10-500 microseconds.

    Thanks very much for your help.

    Sven
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,361
    There are a number of questions and red flags in your post. But that's ok since you admit to having little knowledge about electronics.

    You quote operating voltage of 2.6V for the LED. What is much more important is the operating and maximum allowable current.

    A pulse duration of 30μs is awfully short.

    You need to state what you are attempting to do with this. Give us the whole picture of your project.
     
  3. Sedeca

    Thread Starter New Member

    Sep 26, 2013
    11
    0
    Well I have linked the datasheet in the post. for continuous operation max current is 1 amp. for pulses is is a bit complicated depending on duty cycle and so on but i think with 2 amps it will be safe in the range between 10 and 100 us.

    the whole picture of the project is a bit complicated. Basically the LEDs will be used to illuminate biological samples. the nature of the samples requires the illumination to be short (otherwise unwanted reactions start to happen) and bright to reach saturation of the sample.
    what other details do you need?

    I made a a little circuit design of what I thought so far. Feel free to rip it apart :)

    and thanks again for the help
     
    Last edited: Jul 2, 2014
  4. crutschow

    Expert

    Mar 14, 2008
    13,004
    3,232
    The problem with the 555 is that the trigger must be shorter than the one-shot duration or it will retrigger, and it's difficult to make the trigger shorter than 30μs from a mechanical switch.

    Better to use a non-retriggerable one-shot such as a 74121. That way the input pulse can be as long as you push the buttom while still only generating a single pulse output. The 74121 operates on a 5V supply and has standard TTL output logic levels so will need a buffer to drive the power transistor controlling the LEDs. The data sheet will show you how to connect it to get the desired one-shot period.

    Below is a simulation of an LED driver circuit. I don't have a 74121 model so I just simulated it with a simple circuit, but that should be sufficiently accurate for this application. The N-MOSFET should have no more than about a 5nC gate-charge spec. for good switching speed and be a Logic-level type.

    LED Driver.gif

    Note that I had to reduce the value of the series resistor to get 400mA LED current since all the LED models available in the program have a higher forward drop then the one you spec'd. The value you selected is likely closer to the desired resistor value.

    You likely will need to debounce the push-button output to the 74121 input to avoid generating multiple pulses. An RC low-pass filter circuit should be sufficient for that.

    With that short a pulse duration and a low duty-cycle there should be no power dissipation issues with the LEDs or driver.
     
    Last edited: Jul 3, 2014
    absf likes this.
  5. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,388
    497
    My calculator says that 30 us is 33.3 kHz. I am not sure human eye will detect the led going On and Off. Which makes the whole experiment/project pointless.
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,004
    3,232
    He stated it's to illuminate biological samples, not the human eye. :rolleyes:
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,387
    1,605
    This is actually a very tricky circuit with several complications.

    30uS is damn short: even if you use a non-retriggable one shot the switch itself may very well bounce on and off for longer then 30uS, giving you multiple pulses. So you'll need some sort of debouncer. That said, the simple RC in Sedeca's sketch may be enough.

    Now driving a transistor for a short interval can be tricky: it takes time to turn on and time to turn off too. I'd start with a CMOS one shot (or 555) that you can run at 12V and drive a MOSFET.

    The LM555 (transistor version) over the TLC555 (CMOS version) may be better as it has a stronger drive section, though without trying this either may well work, and the TLF555 is available at most every Radio Shack.

    As the power is only on for a short interval yes the power of the 2.7 ohm resistor is not an issue, especially if you parallel several resistors to get the value. I'd still use 1/2W over 1/4 watt types here.

    As I read the LED data sheet I only see "Forward Current" on sheet 3 as 1000 mA (1 amp) as the Maximim Rating. Max is max, no matter how long, so that is the limit.

    Should work with 12V supply: do add caps at input and output, 0.1 uF min, maybe a 10uF bulk cap too, but something to insure the regulator doesn't oscillate.

    I hope you have access to an oscilloscope to calibrate the time. LEDs react instantaneously so just reading the collector (or drain) voltage is the accurate place to measure when tweaking the timing.
     
  8. Sedeca

    Thread Starter New Member

    Sep 26, 2013
    11
    0
    HI and thanks for all the comments.

    so directly beneath the 1A forward current on page three the datasheet says surge current 2.6A for pulses shorter than 10 us. In the graphs on page 12 it gives the allowed currents for various duty cycles at room and high temperature. From that I deduced that 2.5 amps should be possible but I hope that I will get enough light without going to maximum currents so 2 amps should be ok.

    I hope that I will have access to an oscilloscope in the near future. thanks for the tip how and where to measure...

    Now about chips. I do see the difficulty with the triggering of the 555 and I looked at the other chip suggested. however I saw that this one has the resistor that sets the timing integrated so it is not changeable? I was planing to at some point to exchange the resistor with an analog or digital pot, so that I can vary the timing a bit. So is there a chip that triggers on the edge and has the resistor not integrated?

    I guess I have to decide on a timer chip before choosing the MOSFET?

    Edit: Actually the 74LS121 seems like exactly what I need. Yes?

    Sven
     
    Last edited: Jul 3, 2014
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,981
    744
    or use a MC14538 cmos retrigger /monostable as per page 9 datasheet, this will go down to 10uS to 10Sec.
     
  10. Sedeca

    Thread Starter New Member

    Sep 26, 2013
    11
    0
    Well choosing between those is difficult for me as from the datasheet they both sound good. What is the difference? It seems to me that the MC14538 is multitriggerable that would mean that I run into the problem of making clean short triggering pulses like with the 555? am I right to assume that the 74ls121 does not have that problem?
     
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,981
    744
    no, its single shot or multi shot you set the options, fig 13 page 9 is single shot,the 74ls121 is 5V supply, the MC14538 is 3 to 18V.
     
    Last edited: Jul 3, 2014
  12. crutschow

    Expert

    Mar 14, 2008
    13,004
    3,232
    Below is the simulation of a non-retriggerable 555 one-shot in case you want to stay with a 555. It uses a FF at the input to immediately set the 555 trigger input high at the start of the pulse to prevent the 555 from retriggering due to a long trigger source signal.

    R3C2 is a debounce circuit to lock out any additional inputs for about 65ms. I generated multiple PB input triggers to show how it works.

    The R1C1 values shown gives about a 30μs Out pulse.

    Note that power and ground connections to the FF are not shown.

    555 one-shot.gif
     
    Sedeca and absf like this.
  13. Sedeca

    Thread Starter New Member

    Sep 26, 2013
    11
    0
    ah even more options to chose from ;)

    does anybody know what the typical a short triggering signal is one can achive with physically pushing a button? It seems to me that this r3c2 debouncing circuit might make sense in any way to block the triggering channel fr a few milliseconds. I anever had to physically debounce a trigger yet so i have no clue. maybe my simple rc debouncing thing combined with falling edge triggering might be enough?

    Can anybody give me a pointer to a suitable mosfet for those chips? I assume that the timer chips that work with higher voltage might be better suited for fast switching of a mosfet?

    thanks for the help so far it really seems like its getting there.

    sven
     
  14. Sedeca

    Thread Starter New Member

    Sep 26, 2013
    11
    0
    slightly off topic: Is there any free software around to do this cool drawing and simulating of circuits? it seems really useful to a beginner like me to look if the circuits I design make sense or not...

    I might have some 555 from my flatmate flying around. and I will try to get my hands on this MC14538 and the r3c2 chips. unfortunately I don't have access to the workshop next week so the assembly and testing will have to wait till the week after. but then hopefully I#ll have an oscilloscope around. and I can see how far I get with this.
     
  15. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,797
    1,103
    Yes. LTspice is a free download from Linear Technology. There is a Yahoo LTspice user group which can provide assistance plus lots of free models of components additional to those that come with the download.
     
    Sedeca likes this.
  16. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,981
    744
    Here is the circuit for a one shot pulse using MC14538, no need for debounce, press reset button ready for next pulse. i have not got simulation software to test it, sure others will be able to.
     
    Last edited: Jul 4, 2014
  17. crutschow

    Expert

    Mar 14, 2008
    13,004
    3,232
    A mechanical switch, by itself, can't give a pulse shorter than it's bounce period which is typically in the millisecond region.

    An RC debounce circuit won't work with a 555 since it's level triggered and, as the one-shot period is shorter than the debounce period in your application, the 555 will retrigger. It will work with edge-trigger one-shots such as a the 74121 or the MC14538.

    I use LTspice as do many on these forums and recommend it. It has a somewhat steep learning curve but it's worth the effort if you need to design any circuits, even simple ones. The program has many example circuits you can run to get a feel for its operation.
     
    Sedeca likes this.
  18. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,287
    1,253
    This might work.
     
    Sedeca likes this.
  19. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    Breadboard tested 10 μs, 1A pulsed LED.
    First one shot is a 1 sec. debouncer, sharp trailing edge is differentiated to give a 2 μsec. - pulse at pin 2 of 2nd OS. Timing adjusted to 10μsec. under load. LED driver was surface mt FZT849 med power NPN, also tested with sm, N ch, MOSFET 3055V. One disadvantage is pulse is delayed by 1 sec.
    Error in labling on print, the FET is 3055V, not FZT849.
     
  20. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    Tried a IRF740, 400V FET, assuming higher input cap. pulse comes out at 5 μs ?, with same 5 μs input to gate that gave 10 μs with NPN. Looks like just about any thing would work. At duty of 1:20 LED [white ] is bright enough to read by.
     
    Last edited: Jul 4, 2014
    Sedeca likes this.
Loading...