I have designed a linear ramp generator with a NE 555 in a monostable mode and a transistor loading a capacitor with a constant current source.

The purpose of the circuit is to control a level crossing for a model train with a servomotor :

• First pulse : the servomotor opens the barrier during about 10 seconds
• The barrier remains open
• Second pulse : the servomotor closes the barrier during about 10 seconds

I have applied the ramp signal to the Control Voltage pin of the NE555.

The problem is the following :
At the end of the ramp, as it gets back to 0 V, the servomotor returns abruptly to its initial position !

So how to modify the circuit (Is the NE555 the right solution ?) :

• to maintain the position after the first pulse
• and allow at the second pulse to get back to the initial position ?

Thank you for your ideas !

 

Hi Philippe,
Please post the latest version of your circuit.
E

 

Hi Philippe,
Please post the latest version of your circuit.
E

Hi Eric,

Well this is actually the latest version.

This ciruit works for opening the barrier with the servomotor but
at the end of the ramp, as it gets back to 0 V, the servomotor returns abruptly to its initial position !

I was thinking about an additional circuit using an opamp to reverse the ramp to close the barrier but the above problem remains ...

Again best wishes for the new year for you and your family

Philippe

 

Hi Phil,
As you may know, a Servo has to be powered by a pulse at all times for it to hold its last position.
If you cut off the pulses, then any gravitational load on the servo system will move the servo to a resting position.

E

 

Bonjour Philexium,

I can only briefly look at this right now (at work) but am I correct in saying that you are controlling the duty cycle of the output signal by using the ramp?

If the motor is to stay in either position "A" or "B", and these two positions are the min and max positions possible for the motor, pour example, then the ramp must either remain at zero, or it must remain at V, to furnish the correct duty-cycle square wave to keep the motor in position.

Is that approximately what you are aiming to do?

If so, when you reset the ramp, the duty cycle therefore changes, which would mean your motor will indeed jump back to the original position.

Now how to solve...j'une question...is the motor only ever in either "A" position or "B" position, or do you sometimes stop it in between? And are the other observations corret as far as how it is supposed to work and overall circuit theory?

Paul
KI5VNH

 

Bonjour Philexium,

I can only briefly look at this right now (at work) but am I correct in saying that you are controlling the duty cycle of the output signal by using the ramp?

If the motor is to stay in either position "A" or "B", and these two positions are the min and max positions possible for the motor, pour example, then the ramp must either remain at zero, or it must remain at V, to furnish the correct duty-cycle square wave to keep the motor in position.

Is that approximately what you are aiming to do?

If so, when you reset the ramp, the duty cycle therefore changes, which would mean your motor will indeed jump back to the original position.

Now how to solve...j'une question...is the motor only ever in either "A" position or "B" position, or do you sometimes stop it in between? And are the other observations corret as far as how it is supposed to work and overall circuit theory?

Paul
KI5VNH

Hi Paul,

Nice to read you again !
Hope you have had a nice new year .

The purpose is to reproduce the operation of a level crossing for a model train.

Here are more explanations for the specifications :
Step 1 : the servomotor closes automatically the barrier of the level crossing during 10 seconds, to add a more realistic effect
Step 2 : the barrier remains closed during the model train is running
Step 3 : the servomotor opens the barrier during 10 seconds
Step 4 : the barrier remains open

What I have tested :
Yes the constant current source with the NE555 and the transistor loading a capacitor automatically resets at 0 V at the end of the ramp.
So I am wondering whether the choice of a NE555 is a good choice indeed !

Another point further to what Eric said above : my servomotor keeps its position when the supply is cut.

Look at the small drawing encosed : a picture is worth a thousand words
Thank you again for your time
Philippe

 

hi Phil,
What are the component values on this circuit clip.?
E

 

hi Phil,
What are the component values on this circuit clip.?
E
View attachment 284558

Here are the values :
VCC = 5V
R1 = 47 kΩ
R2 = 100 kΩ
RE = 10 kΩ
C = 220 µF

 

hi Phil,
Using your full circuit, I do not get the desired Pout, please check my circuit for any errors.
E

 

You could do an equal linear ramp up and down, plus hold, with an op amp integrator.

Ltspice simulation of example circuit below:
The output ramps up from 0V to the maximum in about 10 seconds when the input goes from 5V to 0V, and stays at about 3.6V (for the op amp shown) until the input goes back to 5V, where the output ramps back down.
D1 and D2 minimize integrator saturation windup which otherwise significantly delays the start of the ramp.

Would that work for you?

 

hi Phil,
Using your full circuit, I do not get the desired Pout, please check my circuit for any errors.
E
View attachment 284575

Eric

I have started to check again as I didn't see any error until now
I am preparing a document with the signals I get at every stage

But it's time for dinner now so talk to you tomorrow morning and thanks again for your help.
Have a nice evening

 

hi Phil,
Using your full circuit, I do not get the desired Pout, please check my circuit for any errors.
E
View attachment 284575

Good morning Eric !

I have double checked again this morning your LTSpice schematic and didn't see any error ...
Two remarks :
- The servomotor is turning 90 ° when R2 which is in fact a 22 k pot. is at mid course
- The C8 47 nF capa. is not always connected to 5V. I just give a quick contact to generate the pulse
So I have enclosed a new document including the signals at the output of each signal.
I really hope it will help you to identify where the problem is coming from.

Another point : have you seen the circuit to generate a linear ramp with the opamp from @crutschow ?

 

hi Phil,
The problem I have is that the Ramp is only 0.1Sec in duration.?
Using the component values in post #8
I am working on the problem.
E

Edit Ramp is now 1.8Sec

 

hi Phil,
The problem I have is that the Ramp is only 0.1Sec in duration.?
Using the component values in post #8
I am working on the problem.
E

Edit Ramp is now 1.8Sec

Yes with Re 10 k here on my breadboard the duration is 2 s so very close !
With Re 100 k I get 8,5 sec

I think the linear ramp with the NE 555 is a bad design

 

hi,
This is the first run.
What Pulse widths are you expecting on the Pout range.?

E
Note: the Ramp rise is not linear.

 

hi,
This is the first run.
What Pulse widths are you expecting on the Pout range.?

E
View attachment 284635

8 s. would be very close to the real level crossing operation

 

Hi Phil,
I did not make my query clear.
As the ramp rises, the Pulse ON period Pout should change, from what........... to what ...........
E

Added timing at 2sec shows the pulse width , when 22k is at 50%

 

I don't know the size or gauge of your model railway, but 6 -10 seconds for the barrier rise/fall is the normal range for a full-size railway (in the UK at least) according to this article. Wouldn't a ramp time scaled down somewhat be more realistic?

 

Hi Phil,
I did not make my query clear.
As the ramp rises, the Pulse ON period Pout should change, from what........... to what ...........
E

Added timing at 2sec shows the pulse width , when 22k is at 50%
View attachment 284638

OK sorry !

For opening the barrier 0,8 ms to 1,2
For closing 1,2 to 0,8 ms

It is approximate values as I have an old Tektro scope and it's difficult to measure better it as it has no memory !

 

I don't know the size or gauge of your model railway, but 6 -10 seconds for the barrier rise/fall is the normal range for a full-size railway (in the UK at least) according to this article. Wouldn't a ramp time scaled down somewhat be more realistic?

Hi Alec

The scale is HO, 1/87

Yes you are right a shorter time would be fine !
Best regards
Philippe