555 monostable with multiple inputs and outputs

Discussion in 'General Electronics Chat' started by vinras, May 12, 2016.

  1. vinras

    Thread Starter New Member

    May 11, 2016
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    Hi Friends

    I'm new to electronics and I would like to know is there a way to have more than one inputs (say 3 switches) arrange in some way to 555 monostable circuit and arrange 3 outputs from the pin no. 3. but using just one 555 chip.

    to explain it bit more, let's say I have 3 push buttons and i want to light up 3 different LEDs for pre-determined duration (30 sec)
    when button 1 is pushed LED 1 will be on for 30 secs and then turned off automatically. If I push button 2 the second LED will be on for 30 secs

    instead of using 3 monostable circuits is there a way to use single 555 timer but arranging the output pin 3 with input switches to make it work?

    Thank you
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You could use one 555 timer in conjunction with added logic circuitry and transistor switches; but it won't necessarily be simpler than just using three 555 timers.
    Welcome to AAC!
     
  3. vinras

    Thread Starter New Member

    May 11, 2016
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    Thanks Alec
    I have no problem of using 3 555 timers as they are cheap, but I want to learn if there is any other way to do without using 3 separate circuits with reasonable number of components. So if you can point me to right direction with some existing circuit or things I should try out, that would be really great

    Thanks again
     
  4. Alec_t

    AAC Fanatic!

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    You could:
    Use the buttons to set respective latches,
    OR the buttons to trigger the 555 timer,
    AND the latch outputs with the timer output to get three respective signals for turning on respective transistors,
    Apply the timer output to the anodes of the LEDs,
    Connect the cathodes of the LEDs via respective current-limit resistors to the respective collectors of the transistors,
    Arrange for the termination of the 555 signal to reset the latches.
     
  5. WBahn

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    Mar 31, 2012
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    But it sounds like he wants to be able to hit button 1 to turn on the first LED for 30 seconds and then, perhaps 7 seconds later, hit button 2 to turn on the second LED and then 15 seconds after that hit the third button to turn on the third LED. So all three LEDs might be on at the same time but have to turn off at different times.

    I think the easiest way to accomplish this (using just one 555) would be to use the 555 as a free-running clock and then have a countdown timer that a button sets to an initial value (via an asynchronous load unless the 555 frequency is fast enough to catch any quick button press) and then it counts down from there. When it gets to zero it stalls. If all of the outputs are zero then the LED is blanked, so a NOR gate will do the trick for that. You might be able to use a ripple-carry out signal to control the LED.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    I basically agree with that approach, but I believe your intention in connecting the LED anodes to the timer output was so you could eliminate the AND gates at the timer output and just connect the latch outputs directly to their respective transistors.
     
  7. Alec_t

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    You're right. Fingers got ahead of brain :).
     
  8. vinras

    Thread Starter New Member

    May 11, 2016
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    Hey Alec
    Thanks a lot again, I think Latches is what I was looking for, I understand AND/OR Gates but I was wondering how to keep the pushed button input while the 555 timer's out is on so that final AND output is high during this 30 second time. I will look into latches more and come back if I still have questions. Only question I may find still is to release the latch after 30 seconds, I believe i may be able to reset the latch using pin 3 output when it is going from high to low.

    Thanks again Alec I will check bit more on what latches are - this is the missing link I was looking for :)

    BTW crutschow's reply makes me bit confused because I don't want to latch to keep on for ever I need every bulb to keep on only for 30 seconds (based on R and C value selected), or I misunderstood what crutschow mentioned?
     
  9. Alec_t

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    Crutschow was just saying that AND gates aren't needed.
    You can make a latch from two cross-coupled NAND gates. The negative-going edge of the 555 pin 3 output can drive one NAND input via a cap to reset the latch.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    All you need to do is use the push-button to trigger the 555 timer one-shot and, at the same time, set the latch to turn on the desired LED with the transistor driver.
    Connect the output of the 555 to all the latch reset inputs through an RC differentiator so that he fall-time of the 555 at the time-out generates a pulse that resets the latch and turns off the LED, as you wanted.

    A NAND cross-coupled latch can be SET at one of its inputs by the same negative pulse that triggers the 555, and a negative pulse from the falling 555 output at the other input will then reset it.
    (The NAND inputs are biased high by a pull-up resistor when not active.)
     
  11. hp1729

    Well-Known Member

    Nov 23, 2015
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    Sounds like it could be done with two section switches. One section of the switch on the input mechanically linked to a switch on the output.
     
  12. crutschow

    Expert

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    He wants to use pushbuttons.
     
  13. ScottWang

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    Aug 23, 2012
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    The below circuit does not tested, I left R2 and C1 for you to calculate.

    TLC555_3Vin_3Voout_ScottWang.gif
     
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  14. hp1729

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    Momentary SPDT switch?
    Scott's answer is superior though.
     
  15. crutschow

    Expert

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    The 74HC74 can only supply about 4mA at the output, not the 20mA or so implied by the 120 ohm resistor in Scott's schematic, so if you want more LED current then that you will need to add a transistor driver for the LED.
     
  16. hp1729

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    True you will not get 20 mA out. The output voltage will not go to VCC.
     
  17. ScottWang

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    Aug 23, 2012
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    I'm not sure how bright the TS want, I also considering to adding the bjt, but the situation not clear yet, so I didn't add into the circuit, I can't find the 74HC74, so I used the 74HC08 that they have same current condition and I just did a simple measuring, I used the the current limits resistor as 150 ohms, and I got 9.999 mA. (The first photo shown that the LED has two resistors, I was used 150 Ohms + 1K, because I tried to made it close to 120 ohms, and the current over 10 mA and some more)

    74HC08with74HC74Current_LedLighting.jpg

    74HC08with74HC74Current_LedMeterMeasuring.jpg
     
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  18. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Be aware that 30 secs is a long time for a 555 timer and won't be very consistent. You will need a high value timing capacitor with low leakage, because of the very low charging current. Get a reliable branded one from a reliable source.
     
    vinras likes this.
  19. dannyf

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    Sep 13, 2015
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    There is always a way, as others have pointed out, with obvious limitations.

    Just to note that 555s can be configured as flip flops too.

    A simpler (depending on your perspective) approach is to use a MCU.
     
  20. vinras

    Thread Starter New Member

    May 11, 2016
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    Thanks a lot ScottWang, thank you very much for your time truly appreciate,

    As mentioned I’m totally new to electronics, so starting with small steps, I would have clearly used three 555s for three different buttons and outputs, but I just wanted to know instead of using 3 identical circuits can’t we use single circuit with the help of pushed input to derive the output. This solution might be expensive than having 3 separate 555 monostable circuits, but this solution definitely taught me lot of things.

    Thanks again and I can’t believe how helpful and nice all of you in this forum :)
    @Alect_t - as I mentioned I may not need to keep it 30 seconds on, probably less but the idea is for me to learn something I couldn't figure out myself :)
     
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