555 Astable delay

Discussion in 'The Projects Forum' started by Orbitingful, Jan 23, 2016.

  1. Orbitingful

    Thread Starter Member

    Oct 31, 2015
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    I have built a 555 Astable that has a time period of 60s; it produces a pulse every 60 seconds. However, as soon as the Astable is turned on, a pulse is produced. How can I delay this initial pulse by 60 seconds? I only want the first pulse to be delayed.

    Thanks.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    One way is to use a transistor stage to invert the output from pin 3.
    Unless you can pre-charge the timing capacitor to about 1/3 of the supply voltage, the first pulse period is going to be slightly longer than successive periods.
     
  3. Orbitingful

    Thread Starter Member

    Oct 31, 2015
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    Also, one more thing. I am using the Astable with a BCD counter. At the moment, I have a push switch (debounced with a schmitt invertor) connected to the clock input of the 4510 IC. As it is a Schmitt inverter, the voltage at clock is 5V when the push switch isn't pressed. The push switch is momentary; when it is pressed the voltage becomes 0.

    However, when I connect the output of the Astable to the clock input of the BCD, the push switch doesn't work - even when the Astable is turned off. My question is: how do I get the Astable to stop preventing the push switch from working.
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    To stop the pulse on the 555, put a 560k resistor from pin 4 to pin 8, and a 100uF capacitor from pin 4 to pin1, this will keep pin 3 output low for approx 56seconds.

    If you can supply a circuit diagram of what you have.
     
    Last edited: Jan 23, 2016
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  5. Orbitingful

    Thread Starter Member

    Oct 31, 2015
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    Hi thanks for the reply. I will try the RC network when I have access to my circuit. In the meanwhile, how would I go about solving the issue of the push switch not working when the output of the astable is connected to clock input of BCD counter. My circuit has a slide switch, with terminals connecting to +ve voltage and ground. It has two slide states. The middle terminal is connected to pin 10 of the BCD counter (UP/DOWN). The BCD counter will have two terminals connected to the clock input: the output of the Astable and a debounced push swithch. I want the astable to be disabled when the slide switch is in the +ve state (i.e counting up). To do this, I connected pin 1 (ground) of the 555 IC to the middle terminal of the slide switch. I did this so the Astable would only be activated when the slide switch is in ground state, however for some reason when in the +ve state of slide switch, the push switch isn't working - in either state. When I remove the connection from the Astable output to the clock input, the push switch works fine. Keep in mind, the push switch is momentary and is debounced using an RC filter and a schmitt invertor. This means that when not pressed, the switch is high and only when pressed the switch becomes low.

    Sorry for the long paragraph, but I really want to get this to work! Capture electronics.PNG
     
  6. absf

    Senior Member

    Dec 29, 2010
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    Your push button when pushed is directly shorting your +5v to 0V. And it is also applying +5V to the output of your 555. It would be bad for the 555 chip.

    The center of your slide switch is connected to 0V and should be removed.

    Allen
     
    Last edited: Jan 23, 2016
    atferrari likes this.
  7. Orbitingful

    Thread Starter Member

    Oct 31, 2015
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    Hi thanks for the reply. The switch won't be shorted, as I will connect both sides of the switch on different lanes of the breadboard. Also is it possible then, to have both the Astable and the push switch to be connected to the clock input. The push switch is denounced using a Schmitt invertor, so at rest the voltage at clock is 5V.
     
  8. absf

    Senior Member

    Dec 29, 2010
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    I'd do it something like below...

    555 ASTABLE DELAY.PNG
    Allen
     
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    On your drawing, pin 4 is still linked to pin 8, remove it, pin 4 only connects to the R/C .
     
  10. Orbitingful

    Thread Starter Member

    Oct 31, 2015
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    1
    Instead of using the AND gate, could I use an OR gate instead? Keep in mind the push switch is not there to act as a latching on switch of the astable. The push switch is a momentary push switch, that allows the user to count up via the clock input. When the user changes the state of the slide switch, the Astable count down. The push switch is debounced using a capacitor tied to ground, and a pull down resistor. The other side of the switch is high. The signal of the push switch is connected through a schmitt inverter. This causes the signal to be an active high when not pressed, and low when pressed. When I connect the output of the switch to the clock input without the Astable, pressing the switch momentarily, the BCD counter counts up (I'm not sure it matters which direction the edge trigger). I have tried to invert the signal of the push switch schmitt invertor with no success using a 4011 IC (Using a nand gate to create a NOT gate).
     
  11. Orbitingful

    Thread Starter Member

    Oct 31, 2015
    32
    1
    I've changed the pull down resistor to a pull up - so the output of the schmitt inverter is low when the switch isn't pressed momentarily. But I still can't use the Astable in conjunction with the push switch - it stops it from working. The OR gate doesn't seem to work - and there appears to be some noise. I will use the schmitt inverter with the Astable and see what happens.
     
  12. absf

    Senior Member

    Dec 29, 2010
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    If I understand what you've said in post #10 correctly. You want to manually step the counters while the 555 is still running, right?

    Since the output from 555 has a 58.19% duty cycle. The on time (hi) is 35 seconds and the off time (lo) is 25 seconds. In my circuit, the Push button would only step the counters when the 555 o/p is high. The button is inactive when the 555 is low.

    If you want to manually step the counter during the whole period of 60 seconds, you have to add in extra gates and an extra "Manual" switch might be needed. I need some time to rethink over it.

    Allen
     
    Last edited: Jan 24, 2016
  13. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Even worst for the PSU if not protected for shortcircuits. :(
     
  14. Orbitingful

    Thread Starter Member

    Oct 31, 2015
    32
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    No, I want the Astable only to count down when the slide switch is low. I don't want the Astable activated when the slide switch is high. I have an OR gate, with the inputs being the BCD counter and Astable. When the slide switch is high, the Astable's output should be low (0V). Where should I connect the slide switch to, to do this? I have considered RESET and Ground.
     
  15. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Time for a new schematic that shows exactly what you have built, including the switches, 555, debounce circuits - everything. You still have not implemented Dave's reset suggestion from post #4, and the logic descriptions are too hard to follow.

    ak
     
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  16. absf

    Senior Member

    Dec 29, 2010
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    Agree. I have big problem understanding what the TS really wants from the ONE big paragraph of sentences.

    May be he should have written them in points form.

    Allen
     
  17. absf

    Senior Member

    Dec 29, 2010
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    In that case, just connect the center of the slide switch to pin 4 (reset) of 555 as follows:

    555 STOP CLOCK.PNG
     
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  18. dannyf

    Well-Known Member

    Sep 13, 2015
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    run the pulse train through a 2-input NAND gates. One of the inputs has a RC delay line to mute output on power-on.
     
  19. Orbitingful

    Thread Starter Member

    Oct 31, 2015
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    Thanks everyone for the quick replies. I have produced an updated schematic focusing on just the Astable, as there are still some problems. I have implemented the schematic from Allen, and have added some minor changes such as the inverter. The RC delay is great, but it starts charging regardless of the slide switch state. Would it be possible to get the capacitor to start charging only when the slide switch is 0V?

    Once again, thanks for the help. Capture electronics 2.PNG
     
  20. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Connect R1 to the output of the inverter Ic6a instead of pin 8, and put a diode across it Cathode to the inverter,then when the switch is in the negative position the inverter will go high and charge the capacitor, the capacitor will discharge only when the inverter goes low via the diode.

    Edit you can do away with the inverter and the OR gate, replace them with a cd4093 gate instead.
     
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