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Not getting the ideal 555 output

Varun Dua

Jun 18, 2015
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Hey,
I am providing a 230v ac supply to the attached circuit which gets rectified and the output that I get from the zener diode is a constant 12v dc supply.
The requirement is that there should be just 1 negative trigger pulse that should be applied when the power supply is switched on and the output of the IC555 timer should be there for 500ms. I'm somehow not getting an output which specifies the above 2 criteria's. The output that I get is a positive trigger pulse and the output which remains low till infinity time..

Please help..
 

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Arouse1973

Adam
Dec 18, 2013
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Hello
I am really struggling to see the picture clearly. Would you be able to take a closer screen grab. Separate them into more than one section if you need to. Just make sure you overlap the section a little so we can see where they connect to on the other section.
Adam
 

Martaine2005

May 12, 2015
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Is this any better?
circuit-diagram-jpg.20534
 

Martaine2005

May 12, 2015
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Oh dear.. Me making things bigger for you will have people talking...:rolleyes:
 

Martaine2005

May 12, 2015
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Hi Varun,
Just out of curiosity, have you tried the reset to high instead of low?
 

Arouse1973

Adam
Dec 18, 2013
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Hey,
I am providing a 230v ac supply to the attached circuit which gets rectified and the output that I get from the zener diode is a constant 12v dc supply.
The requirement is that there should be just 1 negative trigger pulse that should be applied when the power supply is switched on and the output of the IC555 timer should be there for 500ms. I'm somehow not getting an output which specifies the above 2 criteria's. The output that I get is a positive trigger pulse and the output which remains low till infinity time..

Please help..

I think you have two problems. First is that you need to provide a small amount of settling time for the circuit. The second is that I think if you hold down the trigger terminal of the timer the output will stay high. The trigger pulse needs to be shorter than the output pulse. You could have a small delay circuit and using a small capacitor in between two resistor pulled to supply produce a negative going small pulse that triggers the timer. You will need a diode across the second resistor to protect the 555 timer. let me see if I can doodle a drawing for you.
Adam
 

Varun Dua

Jun 18, 2015
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Yeah if you could draw something like that.. It would be of a great help.. I'm unable to understand what you meant :p
 

Martaine2005

May 12, 2015
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hey Varun. just found this on the 555.
I don't know if it applies to you but it does switch the high to a low.
  • Pin 6: Threshold – also the negative input to the second internal comparator; it resets the internal flip-flop when the voltage applied to it exceeds 2/3 of VCC, causing the Output to switch from high to low; it also connects directly to the RC timing circuit.
Probably of no use but hey!!!
Martin
 

davenn

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Sep 5, 2009
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and you have a dreadfully small smoothing cap in the PSU section 22uF is nowhere near enough
the AC ripple is likely to be causing all sorts of weird problems

And the zener isn't going to give you anything close to a stable 12V supply

Dave
 

Arouse1973

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Dec 18, 2013
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Here is a modified circuit that shows the principle. See if you can include this in your design. R3 and C3 provide a small power up delay which then turns on the transistors T1 and T2. This then produces a small active low pulse which triggers the 555 timer. The output then goes high for approx. 500 ms. This is only one way of many to do this.
Adam

555_PON_TRIG1.PNG 555_PON_TRIG2.PNG
 

Martaine2005

May 12, 2015
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Here is a modified circuit that shows the principle. See if you can include this in your design. R3 and C3 provide a small power up delay which then turns on the transistors T1 and T2. This then produces a small active low pulse which triggers the 555 timer. The output then goes high for approx. 500 ms. This is only one way of many to do this.
Adam

View attachment 20535 View attachment 20536
Can you make it larger please? Can'y see a darn thing..:D
 

Arouse1973

Adam
Dec 18, 2013
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Sorry I got very long arms and have to sit 6 feet away from the monitor. Looked the right size to me :)
 

Colin Mitchell

Aug 31, 2014
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The problem is this:
Your resistor-fed power supply will deliver about 25mA and this is all being "eaten up" by the LED and other components. You have nothing left for the 555.
Of course the circuit will not work.
 
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