Help w/555 to 555 timer problem

Discussion in 'The Projects Forum' started by LesIzMore, Mar 17, 2014.

  1. LesIzMore

    Thread Starter New Member

    Mar 17, 2014
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    I need a circuit that develops rapid pulsing of an medium high power LED group, then a pause and repeat of rapid flashing (adjustable.)

    I have wired two 555 timers using around a one second pulse from timer 1 to control timer 2 using the reset pin 4 on timer 2. Timer two is adjustable for the rapid flashing. Timer two pin 3 is output to a driver transistor.

    What is happening is I get the rapid flashing through a driver transistor, but the pause cycle is a high, causing the LED circuit to remain on between rapid flash cycles.

    I would like the pause cycle to be no power to the LED circuit for at least .5 seconds. I tried tying reset pin 4 of timer 2 low via a 1K resistor, but that didn't work - unless I fat fingered the connection.

    The circuit is driven by a nine volt battery.

    What am I doing wrong?
     
  2. AfdhalAtiffTan

    Active Member

    Nov 20, 2010
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    It's hard to get basic astable 555 circuit to give low duty cycle.

    Since you supply us with no circuit diagram at all, this is my best guess:

    You need to add a diode between pin6 and pin7, with the anode facing pin7.

    This will bypass the charge resistor.

    The equation now would be t=0.693*C*Rx, R1 for on-time and R2 for off-time.
     
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Try this for independent adjustment of #1 gap timer; assuming LED's are on when #2, pin 3 is high.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Is your transistor being used as a high-side switch for the LEDs, or a low-side switch?
     
  5. LesIzMore

    Thread Starter New Member

    Mar 17, 2014
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    The transistor when active provides a ground to the LEDs and is active when a high is presented to the base.

    I originally thought the transistor was part of the problem, but the problem is visible when just the output of of 555 #2 is applied to a LED.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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  7. AfdhalAtiffTan

    Active Member

    Nov 20, 2010
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    So you're using NPN low-side switch.

    I suspect it is the 1st 555 giving long on-time.
    You'll need to use this circuit for the first 555:

    [​IMG]

    Then, use this excellent calculator. (Note: use the lower ones)

    As shown, you need to add a diode across pin6 and pin7.

    This will help the 555 to turn-off sooner.

    Could you provide us your schematic?
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Which pin of the first 555 are you using to control the reset pin of the second 555 ?
     
  9. LesIzMore

    Thread Starter New Member

    Mar 17, 2014
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    I'll have to draw up the current circuit arrangement. The transistor is a 2N4401 but isn't a part of the problem.

    The first 555 is the slow timer and uses pin 3 to pin 4 of 555 #2. The circuit is inconsistent in operation. 555 timer #2 is an old stock device from 1978. I tried a new recent production in it's place and the operation changed to no negative or positive pulse between the higher frequency pulses - output of 555 timer #2.

    I have tried tying low the interface between 555 devices, no change. I tried a resistor of 1k in series pin3 to pin4 of the two 555 devices, no change. I tried tying the output of 555 #2 low with no change.

    Perhaps using two 555 timers isn't the best way to accomplish the desired output.

    This is for a simple visual effect associated with a pinball machine modification. The frequencies are not a big factor, just some off time with some on flickering, off again, then repeat. 1 second intervals are ideal.
     
  10. LesIzMore

    Thread Starter New Member

    Mar 17, 2014
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    Thank you all for your attention, comments and information. The circuit is now working as intended, and is consistent. I know I owe the circuit diagram, but as I was doing some further testing, I determined that lowering the voltage from 9VDC to 6VDC the circuit works as expected.

    The circuit diagram is nothing unusual as I have 555 timer 1 operating at 1 second pulses.
    555 timer 2 is operating a adjustable pulse rate of around 5 pulses per second.

    I am working on drawing up the circuit.
     
  11. LesIzMore

    Thread Starter New Member

    Mar 17, 2014
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