Transistor as switch -problem

Discussion in 'General Electronics Chat' started by athulascc, Feb 18, 2015.

  1. athulascc

    Thread Starter Member

    Aug 15, 2014
    86
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    here is my led flasher circuit which is connected to timer.
    I need this circuit to flash LED for 30sec and off.
    I have used transistor to enable the flashing cct.
    is that correctly done?
    Capture.JPG
     
  2. athulascc

    Thread Starter Member

    Aug 15, 2014
    86
    1
    I am going to switch whole flasher circuit. I am not sure it will works or not. I know there is not included base resistor, I can add it.
    if there is any different method than this inform me.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    The concept should work. Practically you need a resistor in series with the base of Q1.

    Without knowing what voltage this thing runs off of one could only guess at a value.

    My guess is 500 to 1500 ohms.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,415
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    I would not use Q1 in the GND circuit of the 555 timer.

    Why not connect the output U1-3 to RESET U2-4?
     
  5. Reloadron

    Active Member

    Jan 15, 2015
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    232

    I agree, I would enable the second 555 using pin 4 rather than what you have.

    Ron
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    I also agree. Switching power/ground has its uses, but you need to be pretty careful and deliberate about doing it because you become responsible for knowing how the I/O pins of a chip that is not powered properly will behave.
     
  7. athulascc

    Thread Starter Member

    Aug 15, 2014
    86
    1
    ok
    if reset pin is one circuit works fine
    when it 0 flasher not working
    I understood that,
    but I changed circuit to this before see the comments
    what do you think about this idea? Capture.JPG
     
  8. athulascc

    Thread Starter Member

    Aug 15, 2014
    86
    1
    I have another question.
    I do simulation in proteus 8.
    But when I draw both 555 ics timer does not work .
    it only on but do not off in 30sec.
    but if I delete whole flasher part it works well.
    also even when I do not give power to flasher it works,
    what's wrong.
    may I neglect simulation results?
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    Once again you are trying to switch the power to a part. Do you KNOW how a 555 is going to behave when it is part of a circuit but is not powered? Do you KNOW that the simulation model is going to accurately model that behavior?

    If the answer to either of these is, "No," then don't do it!
     
  10. ScottWang

    Moderator

    Aug 23, 2012
    4,850
    767
    The pin 8 connecting to +Vcc.
    Disconnect R5 from Vcc, the output of first 555 connecting to the R5 of the second 555.
     
  11. athulascc

    Thread Starter Member

    Aug 15, 2014
    86
    1
    ok do you recommend it now? Capture.JPG
    thank you all
     
  12. ScottWang

    Moderator

    Aug 23, 2012
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    You should move R5 not the reset pin.
     
  13. Reloadron

    Active Member

    Jan 15, 2015
    963
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    Also as drawn your second 555 which is configured as an astable multivibrator will have the following approximate characteristics:

    Frequency: 48.1 Hz.
    Cycle Time: 20.790 ms
    Duty Cycle: 66.67%
    Time High: 13.860 ms
    Time Low: 6.930 ms

    Those LEDs will be flickering very fast and likely just appear to be on to many. The 555 output will not be symmetrical with 2/3 of the cycle time one LED on and 1/3 of the cycle time the other LED on. Just so you are aware of that.

    Ron
     
  14. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    I'd leave it where it is.
     
  15. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,967
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    make R4 a 47K, that will give you a 2Hz flash .
     
  16. ScottWang

    Moderator

    Aug 23, 2012
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    I knew some people like to using reset pin to do the control thing, maybe you are the one, I don't know why , but I like to using R5, maybe it's a habit from long time ago.
     
    Brownout likes this.
  17. Reloadron

    Active Member

    Jan 15, 2015
    963
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    OK, I am obviously missing something here with the reference to R5. The circuit in post #11 reflects using two 555 timers. The first, on the left is configured as a "One Shot" where when the start button is pressed and released the first 555 on the left will have its output on pin 3 swing high and it will remain high for the duration of its R*C timing network. The original poster wants to use this high out from the one shot to enable the second timer. The second timer is another 555 configured for an astable mode. The idea here seems to be to have a pair of LEDs on its output alternately blink while it is active or enabled.

    The free running frequency of the second stage, the 555 running astable is determined by:
    • Increasing C1 will increase the cycle time (and hence, reduce the frequency).
    • Increasing R5 will increase Time High, but will leave Time Low unaffected.
    • Increasing R4 will increase Time High, increase time low and decrease the duty cycle (down to a minimum of 50%)
    Now would that be correct? That is how I see things working. So what is this thing with R5? The only thing R5 does is increase the On Time of the output pulse. I do not see R5 as being able to enable or disable the second stage timer. What am I missing with this R5 part of the discussion?

    Ron
     
  18. ScottWang

    Moderator

    Aug 23, 2012
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    >I do not see R5 as being able to enable or disable the second stage timer.
    If R5 connected to GND then the second 555 will stop and that is the first 555 want to do.
     
  19. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    Ah, so you are saying to ground R5 to disable the second timer. Place the junction of R5 and R4 at ground. Thank you as now I understand what you are suggesting.
    Left me thinking move it to where? :) Now I understand what you are suggesting.

    Thank You
    Ron
     
  20. ScottWang

    Moderator

    Aug 23, 2012
    4,850
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    What I knew the methods to control a 555 astable to stop generate the pulse as :
    1. Using a PNP bjt to control from Vcc(pin 8) with a low level in base.
    2. Using an NPN bjt to control from Gnd(pin 1) with a high level in base.
    3. Control the reset pin (pin 4) to Gnd.
    4. Control R1 to Gnd. (see astable circuit)
     
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