# Cant achive an astable 555 Circuit

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#### mozly

##### New Member
Hi,
before I begin I would like to state that I am a complete newcomer to electronics and I am self-taught so forgive my lack of vocabulary.

I looked up a few tutorials for the 555 astable mode.
like this one: 555 and 556 Timer Circuits

I made the circuit exactly as they told me but I keep getting a steady pulse out of pin3 instead of a square wave. I'm using a 9v battery as my supply and an LED on pin3

Here are a few things I already tried:
1. replacing the 555 timer
2. trying Many diffrent values of resistors and capacitors
3. trying monostable mode(it works)
4. the information sheet says there is an astable mode.

I'm really stuck, If anyone has any suggestions for me I thank you in advance

#### Brick

##### New Member
Have a look at the "duty cycle" parts of the calculation, for a square wave you are looking at a 50:50 (half the time on, half the time off).

FYI: If I recal, at least some, 555 times can't achieve a perfect 50:50 duty cycle, but you can get rather close so you don't need to worry about

#### colin55

##### Well-Known Member
What values did you use and how did you detect pin 3

#### mozly

##### New Member
For resistors Ra and Rb I tried values ranging from 100ohms to 500k ohms (keeping both resistor values the same). and for the capacitor I've tried 100nF to 100uF
To the if Pin3 was giving me a square wave I put an LED there, also tried putting a resistor parallel with a multimeter

#### ericgibbs

##### Well-Known Member
Most Helpful Member
For resistors Ra and Rb I tried values ranging from 100ohms to 500k ohms (keeping both resistor values the same). and for the capacitor I've tried 100nF to 100uF
To the if Pin3 was giving me a square wave I put an LED there, also tried putting a resistor parallel with a multimeter

hi,
Look at this link
https://www.electro-tech-online.com/blogs/ericgibbs/125-simple-7555-cmos-timer-calculator-v1-2.html
Download and install as per the instructions, use Astable V2 for 1:1 square wave ouput.

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#### mozly

##### New Member
THANK YOU SO MUCH.

The problem was that I was trying to observe frequencies too high too see with an LED. turns out that if I used diffrent values for Ra and Rb I'll get something that gives a visual demo

#### mozly

##### New Member
one more question though,
I took the connection to ground from the capacitor on pin6 off and the frequency went up.
Can someone explain whats going on?

#### ericgibbs

##### Well-Known Member
Most Helpful Member
THANK YOU SO MUCH.

The problem was that I was trying to observe frequencies too high too see with an LED. turns out that if I used diffrent values for Ra and Rb I'll get something that gives a visual demo

hi,
So you used the 7555 calculator, any set up problems.?

#### mozly

##### New Member
No problems i just find that weird that breaking the connection to pin6 still gives me a frequency

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