LED Flash w/o IC

Discussion in 'The Projects Forum' started by specterdragon, Nov 13, 2007.

  1. specterdragon

    Thread Starter New Member

    Nov 13, 2007
    3
    0
    Hey all!!

    I've been digging in the forum for a basic circuit that will flash an LED on and off without the use of an IC (such as a 555 timer). I'm new to electronics and igured that if I could build something like this from scratch, it might make more sense to me. Assuming this is possible, does anyone have any sugestions or scematics laying around that will accomplish this goal?

    Thanks!
    Specterdragon
     
  2. Art

    Distinguished Member

    Sep 10, 2007
    785
    61
    Two transistors and a capacitor, but unfortunately, don't have the schematic,
    but it was an early Dick Smith Funway project.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Look for hobby circuits with unijunction transistors, like the 2N2646. You can make a relaxation oscillator with two resistors and a capacitor.
     
  4. Eplanet

    Active Member

    Oct 25, 2007
    42
    0
    hi
    this is a simple LED flasher but this is one LED.but if you want to use 2 LED you must use 2 capacitor.
     
  5. specterdragon

    Thread Starter New Member

    Nov 13, 2007
    3
    0
    Thanks!! This gives me the basics. Now for the tricky part.... traying to understand it!
     
  6. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    here's how it works, based on the schem presented by "eplanet":

    1. R3 is the current limiter for your LED
    2. R2 & C3 are your timing components, it determines the flash rate
    3. R1 is your Q1 bias resistor
    4. D1 is rather optional, it can be omitted. if used, it certainly will limit the feedback to Q1 to 0.65v and also will result to a higher duty cycle
    5. Q2 is your LED driver

    moz
     
  7. specterdragon

    Thread Starter New Member

    Nov 13, 2007
    3
    0
    Thanks for the assist, Art! That actually helps me! LOL (Yes - I'm completely new to this :) )

    OK, Ephanet... can you help me understand the math a little?
    I tried redrawing the circuit, taking out the transistors (Q1&2) and the diode (D1), just to try and understand the basic flow of the circuit. (With the diode in place, I didn't see a path for the cap to charge, but I'll get back to that in a moment).

    Using the values of R2=10k & C1=1uF (from one of the 12V models), that means the cap charge/discharge rate is 0.01 sec from empty to full. Did I miss something? At that rate, assuming the cap fully charges then fully discharges continuously, it could cycle 3000 times/minute (empty to full to empty again). The model states a rate of approx 60 cycles/min. Based on this, the charge time would have to be around 0.5 sec, meaning that either R2 should be 500k or C1 should be 50uF. So, obviously I'm missing something critical here!! :confused:

    Now, placing D1 and Q1 back in (and not accounting for Q2 yet), it appears that C1 is charging from what little current is flowing from the base of Q1. I can't find anything in my references stating that the amount of current or voltage changes the charge/discharge time of the cap, so C1 would still reach 12V in 0.01 sec, even with this small current? Is that right or am I misunderstanding the flow?

    (BTW: This is going to sound completely "noob" to the gurus out there... but what is the voltage across Q1EB? I may not have gotten to that part in studying transistors yet, but haven't seen anything that tells me what the voltage/resistance characteristics are to calculate the current.)

    Thanks again... this was good info and is already helping me understand with a more "practical" example then what I'm finding in books. :D
     
  8. Eplanet

    Active Member

    Oct 25, 2007
    42
    0
    hi specterdragon
    first thanks of mozikluv this is exactly

    specterdragon if your means of math is mathematic you should read many book and many practice. and about circuit in the flasher circuit that haven't any ic similar to 555 you can change time with first capacitor and second resistor because capacitor charge and resistor will be discharging capacitor. resistor and capacitor determine one frequency with together similar to crystal component do you know about what is within crystal? crystal made with capacitor and resistor that with together make frequency.
    if you use 12v 140 per min have 0.47uf capacitor this is very small and must very fast charge and discharge. is it true??
    now if you understand this simple flasher you can practice with 2LEDs flasher and test it with breadboard and geting good result
    good lock
    Eplanet
     
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