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555 timer trouble

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ChrisPDuck

New Member
Ok i am Making a simple 555 timer circuit on a breadboard using the astable circuit diagram.

R1 is 1Kohm
R2 is 10kohm
C is 470 microfarad

According to my maths this should give me an on time of about 3~ seconds and then an off time of about the same.

BUT, for some reason the LED will just stay lit.

I've put a multimeter across the capacitor, and it will charge up to about 7.5v ( off a 7.8v supply) and then stay charged.

Can anyone help? i'm kinda getting annoyed with it now lol

Thnx

Duck
 

ChrisPDuck

New Member
I've not done this in a while an hence don't have any circuit diagram drawing programs on this machine but i can talk through how i have it connected

( on the 555)
1) 0v
2)+ on capacitor and one side of R2 and Pin 6
3)To LED
4) +v
5) ground via 5.6pf Cap
6)pin 2
7)between R1 and R2
8)+V

The -ve on the capacitor goes to 0v, and the other side of R1 goes to +v

If you know of a program i can draw it with if this is unclear please link away

The LED -ve also goes to 0v
Thanks
Duck
edit: drew one up on my tablet quickly, not neat, but shows what i'm doing

**broken link removed**
 
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Boncuk

New Member
Hi,

if you have a 10µF or a 100µF cap please try them to give shorter pulses. At 10µF the frequency should be about 7Hz and at 100µF it should be 1Hz.

Also check the wiring on your breadboard.

The circuit MUST work.

What value cap is connected to pin5 (CV)? (Should be 10nF).

With the original values the frequency is 0.146Hz at a duty cycle of 52.3%.

Boncuk
 
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ChrisPDuck

New Member
i am using a 5.6pf cap on pin 5 as it is the only one i have around that small.

Replacing C1 with a 100µF cap makes the cap charge faster ( according to multimeter, but it still won't trigger.

I have treble checked the breadboard and even completely removed the circuit and started again a few times to see if it was just an error i couldn't see.

I have tried several of the 555's that i have around - is it possible they are a faulty batch?
 

ChrisPDuck

New Member
ok i'm really at a loss now, i have tried replacing the 555 as well which is the last thing i hadn't tried and it still doesn't work :(

does anyone have any ideas?
 
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ericgibbs

Well-Known Member
Most Helpful Member
ok i'm really at a loss now, i have tried replacing the 555 as well which is the last thing i hadn't tried and it still doesn't work :(

does anyone have any ideas?

hi,
Your circuit, if wired as per your sketch should work.:)

Couple of questions,, are you 100% sure that you have identified pin 1 as top left of the 555, with the small notch at the top and the pins pointing away from you.??

Also that you are counting the pins from pin#1 anticlockwise around the 555, the last pin being pin #8.

What is the value of the resistor in series with the LED.??

EDIT:
using your voltmeter, check and post the voltages on ALL the 555 pins.
 
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ChrisPDuck

New Member
thank you so much, i have realised where my mistake was, i was numbering the pins wrong, i had mistakenly gone 1( at the dot) down to 4, then started from the top for pin 5 instead on continuing anticlockwise.

Thank you soooo much, now i'm happy :)
 

Boncuk

New Member
Hi,

here is a special IC just for you. :D
 

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Karkas

Member
An advice, everytime you make a project always read the specifications of the components, at least the absolute maximum ratings and the connection diagram, you need them unless you know the chip very well.
 

AllVol

New Member
At least he admitted his mistake, instead of just going away as so many do.
 

ChrisPDuck

New Member
thanks, the IC made me smile, i've worked with 555's alot, just not in the past few years, and seems i assumed my memory was better than it is, oops!:D
 

Boncuk

New Member
thanks, the IC made me smile, i've worked with 555's alot, just not in the past few years, and seems i assumed my memory was better than it is, oops!:D

Enlarge the picture, print it and stick it near your workbench to the wall.

It will help you in case of further RAM malfunctions. :D
 
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