hi, I'm building a speed control for a DC motor using 555 IC. I have gone through many designs and articles regarding that, like this 555 square wave generator for example. All those are based on basic astable operations. I have tried them and they work like a charm. But in all of those designs, they are using the timing resistors to change the pulse width. But as I'm aware we can also do the same with the control pin on the 555 timer. After all, it's the control pin right? We can control the pulse width by applying a variable voltage to that pin, for eg using a pot as a voltage divider. But none of the circuits I seen uses that pin. It is always decoupled to the ground through a 100nf capacitor. Is there any specific reason for not using this pin? If someone could clarify that it would be really helpful. Thank you.

 

Hi Tech,
Welcome to AAC.
The Control pin varies the frequency in Astable mode, in PWM the frequency is usually fixed and the Mark/Space ratio is varied.
E
https://www.jameco.com/Jameco/workshop/TechTip/555-timer-tutorial.html

 

hi, I'm building a speed control for a DC motor using 555 IC. I have gone through many designs and articles regarding that, like this 555 square wave generator for example. All those are based on basic astable operations. I have tried them and they work like a charm. But in all of those designs, they are using the timing resistors to change the pulse width. But as I'm aware we can also do the same with the control pin on the 555 timer. After all, it's the control pin right? We can control the pulse width by applying a variable voltage to that pin, for eg using a pot as a voltage divider. But none of the circuits I seen uses that pin. It is always decoupled to the ground through a 100nf capacitor. Is there any specific reason for not using this pin? If someone could clarify that it would be really helpful. Thank you.

The control pin is rather limited.
It sets both thresholds at once. That means that the discharge time is always the same, because the capacitor has to be discharged from its present voltage to half that voltage, and that is always log(2).RC.
It does vary the charge time, but it can never get to zero, because that could only happen with the thresholds below their common mode input range. With the thresholds higher than standard, the charge time does increase, but it does so in a non-linear fashion.
Someone might correct me here, but I suspect that the tap on the resistor divider was only pinned out because it needed a decoupling capacitor too large to fit in the IC, and someone in marketing labelled it “control” because it does sort-of do that.

 

, it's the control pin right?

Yes, but it just connects to between the top two of the three resistors that sets the TRIG and THRS trigger levels.
It has limited "control" functions.

 

The control pin is usually used to modulate the output frequency.

You can use it to change duty cycle with diodes, resistors and feedback from the output but it's rather convoluted and limited.

 

You can use a second 555 timer, feeding it the RAMP wave from the first timer. That was a circuit used about 40 years ago. Then the output from the second 555 will be the PWM signal and it will be adjustable from just a small bit up to 100%.