Not getting the ideal 555 output

[Varun Dua]

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..

 

[Arouse1973]

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]

Is this any better?

 

[Arouse1973]

Oh wonderful Martin, thanks.
Adam

 

[Martaine2005]

Oh dear.. Me making things bigger for you will have people talking...

 

[Arouse1973]

Oh dear.. Me making things bigger for you will have people talking...

Yes that I need to wear my glasses

 

[Martaine2005]

Nope...Haha..This is me.

 

[Varun Dua]

Haha my bad.. tried this circuit for the first time..

 

[Martaine2005]

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

 

[Varun Dua]

Reset pin is connected to high

 

[Martaine2005]

Ah yes, missed the link above U3.

 

[Arouse1973]

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]

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

 

[Martaine2005]

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]

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]

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

 

[Martaine2005]

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..

 

[Arouse1973]

Sorry I got very long arms and have to sit 6 feet away from the monitor. Looked the right size to me

 

[Martaine2005]

6 feet away from the monitor

So small then?

 

[Colin Mitchell]

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.