NE555 Timing

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Suraj143

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I need to generate an astable frequency of 1Hz with a 20mS high time & balance will be off time.

High time = 20mS
Off time = (1Sec-20mS)

Can this done?

It doesn't needs to be exact 1sec just closer values will be fine.
 
yes . . .
The way to do this is to realise pin7 is in sequence with output pin 3. So you want pin7 to be low for about 1 second.
Connect a resistor between pin 7 and an electrolytic capacitor that takes 1 second to discharge to a point where pin 2 detects a LOW. Now connect a resistor and diode in series across the previous resistor so the electro charges in 20mS and pin 6 detects a HIGH. But firstly put a 1k between pin 7 and posiive rail.
 
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use 1k for pin7 to rail and 10u This gives 75k for 500mS high and 500mS low. This is about 1Hz.

Now use about 2k2 to 3k3 to get 20mS for the resistor connected to the diode
 
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Frank7 said:
f=1.44/(R1+2*R2)*C

Can i use this formula for calculating frequency of 1 Hz?
Here is shematic:
**broken link removed**
Click to expand...

Yes, providing the circuit timing components do not have any diodes across them.
 
I simulate that shematic in link above in multisim 11 with R1=22kΩ, R2=11kΩ, C1=1,5uF i get f≈1Hz
But when i put that values in formula it dosent fit...with formula f=21 Hz

Formula is incorrect or what ?
 
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Frank7 said:
I simulate that shematic in link above in multisim 11 with R1=22kΩ, R2=11kΩ, C1=1,5uF i get f≈1Hz
But when i put that values in formula it dosent fit...with formula f=21 Hz

Formula is incorrect or what ?
Click to expand...

hi,
The formula is correct, see image.

Post an image of your mult circuit.
 

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Do i need to use this C2=1uF ?

In this simulation i think i get f≈1Hz coz LED turn off and on every 1 sec isnt that rigth ?
 

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sry i put wrong pic
Well its almost same thing, LED blink every 1 sec
But in calculation i get bigger freq

If i use formula and put that values for R1,R2 and C1 in simulator my LED will not blink... i dont know why,
 

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hi,
I will do a comparison simulation on LTspice and post what I see.


EDIT:
look at this image , it clearly shoes 21.8Hz, in the Sim and the calculation.
 

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Ok ill use that formula for calculating values. Thx alot ericgibbs

But if f=1Hz and if i put LED at output of Ne555 it should blink 1 time every sec right ?
If f=1Hz then i get 1 impuls per sec
 
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Frank7 said:
Ok ill use that formula for calculating values. Thx alot ericgibbs

But if f=1Hz and if i put LED at output of Ne555 it should blink 1 time every sec right ?
If f=1Hz then i get 1 impuls per sec
Click to expand...

If you want to blink at ~1Hz, use a 22k, 68K and a 10uF
 
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