I have finished the servomotor controller with 2 NE555.

The astable runs at 50 Hz with a 50 % duty cycle
The mosnostable delivers a 1 to 2 ms signal.

Two problems remain :

• The astable delivers a nice signal when it is alone. But when it is connected to the monostable, the front edge of the signal is distorted (See photo).
• When I turn the pot., the servomotor has then a strange operation . It doesn’t turn smoothly. The motor is not stable and oscillates on some angles …

Thank you in advance for your ideas

Philippe

 

Please do not start a new thread.
Continue here and if it turns out to be very different then we can choose to move it to its own thread.

 

Hi,
He did start a new thread, I merged it back here.
E

 

Please do not start a new thread.
Continue here and if it turns out to be very different then we can choose to move it to its own thread.

OK fine Thank you I will remember for the next time

 

Hello Philexium,

I think we may need more information. Initially, this began as a "my 555 output has a spike"...and it has morphed into "I'm developing a servo motor driver with dual 555s and it is unstable".

I do not know what kind of servo motor you have; when I hear (read) "servo", I think of a little RC servo with +V, GND, and a control pin. Is this what you are controlling?

From what I know about servo motors, they adjust their position by assessing the width of a continuously-inputted pulse.

That being said, and because the width of the pulse needs to be fairly precise, I believe your motor is experiencing some noise. When you turn the potentiometer, it isn't nice and smooth necessarily, there may be wiper noise. If there is noise, this will echo through the timers as (for a lack of better wording) phase noise, which will manifest itself as jitter in the motor position.

If you low-pass filter the output of the potentiometer, I believe it would help to smooth out the motor operation. I also do not think the slight distortion at the top is necessary to worry about, as the servo motor is looking at pulse duration, not pulse amplitude. However, if you are concerned about this, you can always clamp the square wave to a voltage that is just less than the lowest "high point" when the square wave is on.

Hopefully this helps!

Paul
KI5VNH

 

I have finished the servomotor controller with 2 NE555.

The astable runs at 50 Hz with a 50 % duty cycle
The mosnostable delivers a 1 to 2 ms signal.

Two problems remain :

• The astable delivers a nice signal when it is alone. But when it is connected to the monostable, the front edge of the signal is distorted (See photo).
• When I turn the pot., the servomotor has then a strange operation . It doesn’t turn smoothly. The motor is not stable and oscillates on some angles …

Thank you in advance for your ideas

Philippe

You don't need your 50/50 mark space ratio from the first 555, you only need the overall time between pulses to be 20mS. I'd make the coupling capacitor from the astable to the monostable smaller, maybe 0.01uF and add a diode, cathode to +ve supply, across the 10k resistor, to discharge the capacitor when it returns high.
See if this helps!

 

Hello Philexium,

I think we may need more information. Initially, this began as a "my 555 output has a spike"...and it has morphed into "I'm developing a servo motor driver with dual 555s and it is unstable".

I do not know what kind of servo motor you have; when I hear (read) "servo", I think of a little RC servo with +V, GND, and a control pin. Is this what you are controlling?

From what I know about servo motors, they adjust their position by assessing the width of a continuously-inputted pulse.

That being said, and because the width of the pulse needs to be fairly precise, I believe your motor is experiencing some noise. When you turn the potentiometer, it isn't nice and smooth necessarily, there may be wiper noise. If there is noise, this will echo through the timers as (for a lack of better wording) phase noise, which will manifest itself as jitter in the motor position.

If you low-pass filter the output of the potentiometer, I believe it would help to smooth out the motor operation. I also do not think the slight distortion at the top is necessary to worry about, as the servo motor is looking at pulse duration, not pulse amplitude. However, if you are concerned about this, you can always clamp the square wave to a voltage that is just less than the lowest "high point" when the square wave is on.

Hopefully this helps!

Paul
KI5VNH

Hi Paul

Thank you for your time and explanations.

• Yes there was some confusion as I posted a new thread and the moderator merged it in the same previous one
• The servomotor is a small one with 3 pins +, - and signal (See enclosed photo)
• The servomotor needs to rotate from 0 – 180 ° a pulse from 1 to 2 ms every 20 ms (See enclosed diagrams)
• The potentiometer !!! Yes the very last thing I was never thinking about. It was a poor quality one. I changed it and it works now ! Congratulations !

The very last question :

• The astable delivers a nice signal when it is alone. But when it is connected to the monostable, the front edge of the signal is distorted (See photo). The controller works fine anyway.
• When you mention a low pass filter for the wiper, what values for R and C ?

Thank you so much for your nice help !

I wish you a happy new year 2023
My best 73’s

Philippe
F1CUJ

 

You don't need your 50/50 mark space ratio from the first 555, you only need the overall time between pulses to be 20mS. I'd make the coupling capacitor from the astable to the monostable smaller, maybe 0.01uF and add a diode, cathode to +ve supply, across the 10k resistor, to discharge the capacitor when it returns high.
See if this helps!

Hi Sarah

Thank you for the idea.
Sorry but it doesn't work. The monostable is not triggered this way

Anyway the controller works now (See reply to PaulEE above)
Best regards

 

You don't need your 50/50 mark space ratio from the first 555, you only need the overall time between pulses to be 20mS. I'd make the coupling capacitor from the astable to the monostable smaller, maybe 0.01uF and add a diode, cathode to +ve supply, across the 10k resistor, to discharge the capacitor when it returns high.
See if this helps!

Sorry Sarah about my previous post !
My apologies yes when reducing the capa to 15 nF and a diode across the resistor it works too !
I didn't put the diode across
But the only problem remaining is that the front edge of the astable is distorted
I just wanted to understand why ...
Anyway as per my reply to PaulEE the controller works
Thanks again

 

Hi Paul

Thank you for your time and explanations.

• Yes there was some confusion as I posted a new thread and the moderator merged it in the same previous one
• The servomotor is a small one with 3 pins +, - and signal (See enclosed photo)
• The servomotor needs to rotate from 0 – 180 ° a pulse from 1 to 2 ms every 20 ms (See enclosed diagrams)
• The potentiometer !!! Yes the very last thing I was never thinking about. It was a poor quality one. I changed it and it works now ! Congratulations !

The very last question :

• The astable delivers a nice signal when it is alone. But when it is connected to the monostable, the front edge of the signal is distorted (See photo). The controller works fine anyway.
• When you mention a low pass filter for the wiper, what values for R and C ?

Thank you so much for your nice help !

I wish you a happy new year 2023
My best 73’s

Philippe
F1CUJ

I apologize, I do not have a lot of time, I am on my way to work soon.

The curvature on the output of the astable multivibrator stage appears to be "RC" in nature. My suspicion is that it has to do with the output impedance of the output pin in conjunction with the C-R filter you have coupling the two. This coupling mechanism tends to act like a differentiator, passing only the changing portions of a signal (dV/dT...). I believe if you were to increase this capacitor, it would improve the top of the waveform, but it is only a quick guess at this time.

As for the potentiometer/filter...I did not initially examine where the potentiometer was in-circuit. It is being used to limit current into a capacitor (variable RC) to control the output pulse-width. I would have to think a bit as to how I'd modify this. Perhaps a small capacitance between the fixed resistor to +5v and other side of that same resistor (connected to top of potentiometer) would help eliminate some noise. The value would have to be smaller...certainly many times smaller than your timing capacitor, I would think.

One way to experiment would be to re-install the noisy potentiometer and look at the voltage in this area of the circuit (R, potentiometer/rheostat/C) and if you add a small capacitor, perhaps as above, you can see if it improved things with the noisy device, which would help you understand whether it would work with the less noisy potentiometer...does that make sense?

When in doubt, try it
Glad you have almost gotten everything working! Interesting little project.

À bientôt!
Paul
KI5VNH

 

hi Philippe,
This is what LTSpice shows,
The Trigger pulse on the Monostable is very wide , slow rise, use a 2n2 not a 100n.

Also, the 0.5mS through 2mS appears to be delayed from the start of the 20mSec Frame period.??

E

 

This is a good point; the trigger pulse going into a monostable should be a very quick pulse mean to reset it. I believe the datasheet recommends that this pulse should be much shorter than the one you wish to generate. This should help stability as well. Good call on that!

Paul
KI5VNH

 

hi Philippe,
Try these values, plot shows 1mS thru 2mSec
The Vmx is just my test voltage, ignore it.
E

 

I apologize, I do not have a lot of time, I am on my way to work soon.

The curvature on the output of the astable multivibrator stage appears to be "RC" in nature. My suspicion is that it has to do with the output impedance of the output pin in conjunction with the C-R filter you have coupling the two. This coupling mechanism tends to act like a differentiator, passing only the changing portions of a signal (dV/dT...). I believe if you were to increase this capacitor, it would improve the top of the waveform, but it is only a quick guess at this time.

As for the potentiometer/filter...I did not initially examine where the potentiometer was in-circuit. It is being used to limit current into a capacitor (variable RC) to control the output pulse-width. I would have to think a bit as to how I'd modify this. Perhaps a small capacitance between the fixed resistor to +5v and other side of that same resistor (connected to top of potentiometer) would help eliminate some noise. The value would have to be smaller...certainly many times smaller than your timing capacitor, I would think.

One way to experiment would be to re-install the noisy potentiometer and look at the voltage in this area of the circuit (R, potentiometer/rheostat/C) and if you add a small capacitor, perhaps as above, you can see if it improved things with the noisy device, which would help you understand whether it would work with the less noisy potentiometer...does that make sense?

When in doubt, try it
Glad you have almost gotten everything working! Interesting little project.

À bientôt!
Paul
KI5VNH

Hi Paul

I replaced the 100 nF capa with a 2,7 nF as @ericgibbs suggested below.
Everything OK to more bad front edge on the output of the astable Everything is now nice and smooth

Thank you so much !
73
Philippe

 

hi Philippe,
Try these values, plot shows 1mS thru 2mSec
The Vmx is just my test voltage, ignore it.
E
View attachment 284151View attachment 284152

Hi @ericgibbs

So nice to read your explanations and LTspice simulation !
I replaced the 100 nF capa with a 2,7 nF as it was the closest value in my stock.
Everything OK no more bad front edge on the output of the astable
Everything is now nice and smooth

Thank you so much Eric !

Philippe

Happy new year 2023 !
Tell me when you come to Normandy