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555 timer!

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watzmann

New Member
Hi All ,

i need to generate a puls using the IC 555 , actually i don't know anything about the equations of the time and frequency calculation ,

so i have tried it on protous with the following schematic , 10 K,

now i need to get output with T= 0.5 second ,

how much is the resitors and the capacitor i have to connect ,

and is this connection is descriabed as "stable " or monostable ?

here is my schematic ,

thank you
 

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ecerfoglio

New Member
watzmann said:
Hi All ,

i
and is this connection is descriabed as "stable " or monostable ?

here is my schematic ,

thank you
Your circuit is "Astable" (has no prefered state) aka oscilator. It produces a train of pulses or "square" wave.

Pin 4 (reset) should be connected to +V (pin 8)

It is a good practice to add a small capacitor (10nF= .01:mu:F) between pin 5 and ground, and another one between +V and ground (pins 8 and 1).


The other clasical configuration is the Monostable (one state) or one shot (gives one pulse when triggered)

EDIT: Corrected the ".001:mu:F" that AudioGuru detected
 
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audioguru

Well-Known Member
Most Helpful Member
10nF is 0.01uF, not 0.001uf.
The datasheet for the LM555 shows 0.01uF on pin 5 to ground on all the circuits shown.
 

ecerfoglio

New Member
audioguru said:
10nF is 0.01uF, not 0.001uf.
The datasheet for the LM555 shows 0.01uF on pin 5 to ground on all the circuits shown.
You are right, of course it is 0.01uF

I wrote ".001F" without the leading 0, then when I saw only 2 ceros it didn´t "look" bad :confused: :confused:
 

watzmann

New Member
i built the circuit , with R1 = 68K and R2 = 39K C = 0.1 micro
but when i test it , the output pin3 indicate 4.5 volts all the time , i thought it'll flash the volt from 0 to 5 ,

i know i'm wrong , may be the frequncy is so hight , tell me plz
 

audioguru

Well-Known Member
Most Helpful Member
Didn't you do the simple arithmatic that is shown in the datasheet for a 555??

The capacitor charges for 7.4 thousandths of a second and it discharges for 2.7 thousandths of a second. Each cycle takes 10.1 thousandths of a second which is a frequency of 99Hz. It is much too fast to see flashing and it is too fast for your voltmeter.

Replace the 0.1uF capacitor with a 10uF capacitor (use the correct polarity) and the output will make about one pulse per second.

The output of an ordinary 555 goes high to 1.4V less than the positive supply voltage if it does not have a high load current. So if your supply is only 5V then the output goes high to 3.6V. It is shown on the datasheet.
 
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