555: aside from cap and resistor values, how does one calculate/determine the other resistors?

Thread Starter

hbasile91

Joined Dec 6, 2020
125
im a little familiar with the calculation of the cap and the main resistor, which combine to determine length of pulse...but as im trying to really understand the whole circuit, im wondering how the other two resistor values are determined in this circuit... in this case, how did the two 1K resistors get chosen?Screen Shot 2021-02-20 at 5.31.44 PM.png
 
Last edited:

dl324

Joined Mar 30, 2015
12,873
The charging resistance is 471k and the discharge is 470k, so it would appear that the author was trying to achieve a duty cycle of about 50%.

The other 1k is a current limiting resistor. If you assume an LED voltage of 2V and the timer high level output being VCC, you'll have about 7mA in the LED.

It would be helpful if you deleted some of the duplicate images.
 

Papabravo

Joined Feb 24, 2006
16,195
im a little familiar with the calculation of the cap and the main resistor, which combine to determine length of pulse...but as im trying to really understand the whole circuit, im wondering how the other two resistor values are determined in this circuit... in this case, how did the two 1K resistors get chosen?View attachment 231070View attachment 231070View attachment 231070
View attachment 231070
Here is an accessible article on how the component values affect the behavior.

https://en.wikipedia.org/wiki/555_timer_IC
 

Thread Starter

hbasile91

Joined Dec 6, 2020
125
The charging resistance is 471k and the discharge is 470k, so it would appear that the author was trying to achieve a duty cycle of about 50%.

The other 1k is a current limiting resistor. If you assume an LED voltage of 2V and the timer high level output being VCC, you'll have about 7mA in the LED.

It would be helpful if you deleted some of the duplicate images.

what exactly is "charging resistance"?
 

dl324

Joined Mar 30, 2015
12,873
ok, that makes some sense!
If you understand that and that the voltages on the threshold and trigger pins control the charging and discharging, you're well along the path of understanding how the timer works.

For astable mode, the capacitor voltage is usually between 1/3 and 2/3 of VCC. Your 9V supply makes calculating the voltages easy. When the threshold voltage is below 6V, the output will be high. When the voltage gets to 6V, the output will go low, and the capacitor will be discharged until the voltage is 3V.

There's a slight complication when the timer first starts. The first on time will be longer because the capacitor is charging from 0V, not 3V.
 

Zeeus

Joined Apr 17, 2019
615
im a little familiar with the calculation of the cap and the main resistor, which combine to determine length of pulse...but as im trying to really understand the whole circuit, im wondering how the other two resistor values are determined in this circuit... in this case, how did the two 1K resistors get chosen?View attachment 231070
Maybe another thing to keep in mind also is the 1k at 9V (call it resistor R1 )... There is a minimum R1 that has to be used for oscillation. If R1 is below this value then the capacitor can not discharge because the transistor at pin 7 is already sinking it's maximum current from R1.
 
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