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Problems with my 555 circuit


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Hi well basically i have a hobby in RC cars. A few week back i started to make circuits using the 555 timer but now i want to make them on board. i have cut some board and have soldered all the components on. the only change ive made to the circuit is a 100uf capacitor instead of a 10uf one. the circuit does nto seem to flash the LED. it simply just stays on and i checked everything and no legs touch and everything goes where it should any ideas? 

i dont know if this is in the right section but i saw somethign else about a 555 here so i posted it here! 

AsH~ 

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Yay! 555s....

Provided that everything is correct and the only change you made was using a 100uf capacitor, then the circuit is oscillating at a different frequency. In this case, the capacitor will take much longer to charge and discharge. If you look at the light long enough, I wager it will turn off again and then turn back on. To fix this, replace it with a 10uf capacitor.

If I did my math right, the LED should be on 10 times longer and off 10 times longer. So, if originally the LED was on for 1 minute, it will now be on for 10 minutes...

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The capacitor needs to charge to 2/3 of the supply voltage, then the LED will turn off. If the capacitor has a high leakage current then the value of the resistor charging it needs to be reduced or the capacitor will never charge and the LED will not turn off.

Big capacitors have a higher leakage current than small ones. Good quality capacitors have less leakage current than cheap crappy ones.
Whoever said that a 555 can time for a few hours used a fortune worth of low leakage film capacitors, not electrolytics.

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Changing the values of the resistors could certainly solve your problem, but doing so is asking for trouble. By changing the values of the resistors you not only change the frequency, you also change the duty cycle. If you like the way the circuit worked before, your best solution would be to revert back to the 10uf capacitor.

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Ok well i bought some 100uf  onces from ebay lolb ut only cost 2.95 (well i didnt the person im making them for did.) i won tell them and i will buy some 10uf ones. Also can i not use a variable resistor?

You guys hgave been the best out of all the electronic forums ive posted on!

thnx alot

Ash~

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An LED need to have a current-limiting resistor in series with it. Without a current-limiting resistor then the LED shorts the output of the 555 which might blow up the LED, the 555, the battery or all of them. Maybe the 555 kept trying to drive a very high current into the LED and got so hot that it stopped oscillating.

The datasheet for the 555 shows a calculation and a graph to determine the flash rate with different resistor and capacitor values.

Attach your schematic for us to see the resistor values and battery voltage etc.

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Ok so me not having resistors in series with the LEDs could be whats stopping it from working?

None of the rails on my strip board are touching but still i have the same problem it just stays on and i am using a 10uf capacitor.

Will adding 680R Resistors In series fix the problem?

Ash~



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The max current rating for an ordinary LED is only 30mA.
The output current of an ordinary (not the Cmos one) 555 is more than 200mA. Without something to limit the current then the LED or 555 will be damaged. Maybe you are lucky that your battery cannot supply enough current to cause damage by dropping its voltage so low that the max current is much less and the voltage is too low for the 555 to oscillate.

You must calculate the value of a current-limiting resistor but you didn't attach your schematic so we don't know the voltage of your supply, the voltage of your LED or if you have an ordinary 555 or a Cmos 555.
If your supply voltage is 12V and the LED is a 2V red one then the 680 ohm current-limiting resistor will limit the current to about 14mA which is fine.

Since you tried connecting the LED directly to the 555 then both might be damaged.
Now you said, "LEDs". How many? In series or in parallel?

You didn't attach your schematic so we can't determine the frequency of your 555. It might be so high that the LED appears to be on all the time, but it is actually blinking very quickly.

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i didnt put the 680R resistors in when testing it out...

If your battery was strong enough then the LEDs and the 555 are burned out. The resistors limit the current.
You still don't say how many volts is your power supply or battery.

The resistors and capacitor values cause the 555 to oscillate at 10Hz.
The resistors cause one LED to be on 19 times longer than the other, so it will be bright and the other will be very dim. If you swap the positions of the resistors then the LEDs will light nearly equally.
If you increase the value of the 18k resistor or capacitor or both, then the frequency will be lower.
You should add a 100uF bypass capacitor across the supply.
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Wow, 95% duty cycle....

lol... BTW, it's 7.2Hz ;D And considering the duty cycle, the top LED should be on longer than 19 times....

Actually, I am surprised the bottom LED works at all.... I didn't know the 555 produced a negative signal when its output was low. I thought it was 0 volts.... Learn something new everyday! :D

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Hi Bob,
You still didn't say what is your supply voltage.

The output low of a 555 can't go to a negative voltage. It goes to almost ground.
Look at the datasheet for the 555. With a 20mA load connected to ground, the output high goes to 1.4V less than the supply voltage. With a 20mA load connected to the supply, the ouput low goes to 0.2V higher than ground.
That is if you didn't burn it out when you tested it without current-limiting resistors. It tries to provide more than 200mA into the LEDs that have a max rating of only 30mA.

Did you try a better duty cycle by swapping the positions of the resistors?
Did you slow it down?

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Hi Bob,
You still didn't say what is your supply voltage.

Uhh...? I never said anything about a supply voltage...

That is if you didn't burn it out when you tested it without current-limiting resistors.

umm... I'm not sure I have ever burnt out a 555! I've tried, sadly failed!

It tries to provide more than 200mA into the LEDs that have a max rating of only 30mA.

I have known this for quite some time now...

Did you try a better duty cycle by swapping the positions of the resistors?

I didn't have to, I used a circuit that I built a while ago with a 50% duty cycle at about 1Hz...

Did you slow it down?

No, it was already at 1hz and I was just wondering if they would actually oscillate back and forth...


LOL! WOW! I think you might have gotten a tad bit confused here...
I'm not the one with the 555 timer difficulties here... I feel that I am quite proficient with the 555!

And, BTW, I have answered my question with further analis of the circuit. Thanks anyways....

lol, still confused?
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Sorry Bob,
I thought you were the guy who tried to blow up his 555 and LEDs.

I gathered... lol

Ashkwil, might I ask what you need this circuit for? We might be able to suggest a duty cycle and frequency... I know it is for your RC but what part? Does it need to oscillate a certain way to control something inside, or is it a light to tell you the car is on? The voltage source is always nice to know as well...
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