Bad Capacitor? 555 Timer Not Turning Off - diyAudio
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Old 13th February 2009, 06:43 PM   #1
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Default Bad Capacitor? 555 Timer Not Turning Off

First, thanks for all of the help so far in this project. I had it working perfectly so I decided to build and solder the components on a board using different identical parts.

Here is the problem:

When the trigger is touched to ground and the variable resistor is set very low (within the first 1 or 2 clicks of the very lowest setting, theres a total of 36 'clicks') the timer turns on (i can tell its on because the relay clicks) but only when the trigger is held to the ground point; when the trigger is held off of the ground point, the timer turns off (the relay turns off).

When the resistor is anywhere above the first few clicks, the timer will turn on when the trigger is touched briefly to the ground point but it will not turn off. The only way to turn the timer off is to either 1.) Turn off the power supply, 2.) put the variable resistor to its lowest point, or 3.) bypass the resistor by short the 2 leads of the variable resistor. I soldered a 10Kohm resistor in place if the variable and it acts the same; its kept turned on.

Would this be the charging capacitor problem? I checked the voltage of the capacitor to ground and it was right at 8volts; i believe the power supply was 12.5 volts. I have used 4 different timer chips and they all act the same. With basic but limited knowledge, I am guessing at the capacitor problem since if the full supplied voltage is presented to pin 6, it turns off. Also, 8V is less than the 2/3 of supplied voltage of 12.5(barely under).

Thanks for reading, Any help on this would be greatly appreciated!

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Old 13th February 2009, 07:39 PM   #2
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Try decoupling the supply rails a bit more. 0.1uF is nowhere near enough, the recommended size in 10-100uF. 555's are notoriously easily to trigger or double trigger when connected the way you've shown, the trig needs decoupling as well or it can keep retriggering itself as its input impedance is very high indeed. Look at the datasheet for better ways to use the trig input, try AC coupling it to your trig contact with 2 pull-up resistors, one either side the cap.

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Old 13th February 2009, 07:50 PM   #3
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Try increasing the rail decoupler, from memory I used 10uF. The trigger input should be fine like that. I ran a tilt switch with the same sort of trigger scheme and never had a problem even using much higher pull-up resustor.
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Old 13th February 2009, 07:52 PM   #4
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I will try your suggestions. I used the .1uf capacitor because thats the size iv seen to use for this type of IC. Also, for the trigger, iv seen many schematics connecting it to ground like i have now.

If the suggestions dont solve the problem, is there anything else that could be wrong? Im just wondering because it worked perfectly hooking it up like this on a different board even without a decoupling capacitor.

Thanks again.
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Old 13th February 2009, 08:03 PM   #5
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The only other thing I'm wondering is if the time capacitor is too big. I think there is a max capacitance recommendation in the datasheet.

edit: just quickly checked my Philips NE555 datasheet and couldn't see anything. I'm sure an old paper copy I have from RS Components gives more information on component choices and includes the recommendation.
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Old 13th February 2009, 08:31 PM   #6
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If you are using an NE555 then you neeed at least 47uF rail decoupling as this IC "crowbars" the supply pretty badly. The circuits you have seen may be using the CMOS version (1455, 7555 depending on the manufacturer!) which don't do this.
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Old 13th February 2009, 09:43 PM   #7
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Thanks for the help so far, i will try all of the suggestions. This is my first time working with an IC like this so there is a lot I am learning.

For the decoupling capacitor, i use a polarized cap, correct? Im only asking because other schematics I have seen mostly display a non-polarized capacitor (such as the ceramic .01-.1 caps).
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Old 14th February 2009, 12:09 AM   #8
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Yep,a polarized electrolytic is fine. Leave the small ceramic cap there also.
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Old 14th February 2009, 03:05 AM   #9
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I tried that but no luck; its still doing the same thing. I measured the output voltage of pin 3 and its putting out 10.95 but the supplied is 12.
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Old 14th February 2009, 06:29 AM   #10
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Something really weird, a decoupling capacitor was actually causing this problem. i took it off and it works perfectly now.
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