Not getting the ideal 555 output

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varundua

Joined Jun 15, 2015
21
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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AnalogKid

Joined Aug 1, 2013
10,971
Your schematic is very difficult to read.
Since everything has a common ground, why do you have an optoisolator in there?
In you description you use the word "output" in two different ways. Which is it?
Look at 555 pin 2. Are there continuous pulses there or just one edge at startup? The Monostable section of the 555 datasheet has a warning about changes to the trigger input while the output is in its timed state.
This is a non-isolated power supply, and this thread will be shut down by the moderators.

ak
 

Thread Starter

varundua

Joined Jun 15, 2015
21
Alright I'll get the transformer but if considering the circuit ahead of it, I have used the optocoupler along with TL431 since I require just 1 single trigger at PIN 2 of IC555 when my power supply is switched ON and during the whole operation of the circuit no more triggers are produced. That I have somewhat achieved but getting a positive pulse instead of a negative one.

What I'm more focussed on is the output I get from IC555 which should remain high for 500ms when the trigger pulse is applied.
 

MikeML

Joined Oct 2, 2009
5,444
If the trigger pulse low duration is longer than the RC period of the timer, then the output of the timer stays high until the trigger pulse goes back high.

Normally, the trigger pulse must be shorter than the timer duration. Usually, an RC differentiator is used to make the 555 trigger on the down-going edge of the trigger event.

77s.gif 77l.gif

I have no idea what you are trying to do with TL431???
 
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Thread Starter

varundua

Joined Jun 15, 2015
21
TL431 is actually being used to ensure that I get just a single trigger pulse as the power supply is switched ON and no further trigger pulses during the operation of the whole circuit. Is there any other way with which I can achieve this?
 

crutschow

Joined Mar 14, 2008
34,201
But dont you require a negative going trigger pulse to activate the IC555?
Yes. There is a short negative trigger at the start due to capacitor C2 momentarily holding the trigger input low for a few ms when the power is applied.
Notice the V(trig) does not rise quite as fast as the supply voltage.

But as an added note, I would increase the time constant of R1C2 to about 100ms from 10ms (say by changing C2 to 1uF). 10ms seems a little fast for a 50ms power supply rise-time.
 
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Bernard

Joined Aug 7, 2008
5,784
22 uF is excessive on pin 5, may screw up timing. Set up 555 as a 500ms one shot. hold pin 2 low with a cap., charge cap with R to Vcc; RC should be longer than rise of Vcc at turn on. Can add a diode anode to pin 3, cathode to pin 2 to rapidly charge C when 555 triggers so as not to interfear with 500 ms timing.
 

WBahn

Joined Mar 31, 2012
29,932
Alright I'll get the transformer but if considering the circuit ahead of it, I have used the optocoupler along with TL431 since I require just 1 single trigger at PIN 2 of IC555 when my power supply is switched ON and during the whole operation of the circuit no more triggers are produced. That I have somewhat achieved but getting a positive pulse instead of a negative one.

What I'm more focussed on is the output I get from IC555 which should remain high for 500ms when the trigger pulse is applied.
Please show a revised schematic in which you are properly using a transformer to obtain your low voltage supply rail.
 

Wendy

Joined Mar 24, 2008
23,408
Basically a transformerless (non-isolated) power supply is a man trap, a killer. We don't allow those here for that reason. Wall warts are cheap, so are batteries. If WBahn had not responded first I would have just shut this thread down.

The 555 Projects

555 Monostable
 

WBahn

Joined Mar 31, 2012
29,932
And I will still shut it down if the TS doesn't respond back with a revised schematic in a reasonable amount of time.
 

ScottWang

Joined Aug 23, 2012
7,397
Welcome to the AAC forum.

The last thread that I already told you the rules and you knew it, but you still not to give up.

The owners of All About Circuits has elected not to host discussions of transformerless threads.

This thread is against the AAC forum rules, Terms of Service (ToS).

Any kind of over-unity devices and systems
Automotive modifications
Devices designed to electrocute or shock another person
LEDs to mains
Phone jammers
Rail guns and high-energy projectile devices
Transformer-less power supplies
Terms of Service (ToS).

Automotive modifications of any kind are strictly forbidden. Therefore, this thread will be closed.

Please try to understand the reasons behind this action, and feel free to browse and use the forums.

You might find answers to your questions in one of these forums:

http://forum.allaboutcircuits.com/showthread.php?t=54400

Another good forum that shares many of the same membership is http://www.electro-tech-online.com/ .

Good luck.
 
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