Modify duty at PWM signal

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Hi Alec_t,

I used the monostable circuit, I used this circuit:

https://www.utm.edu/staff/leeb/mono555.gif

Resistor is potentiometer 500K and capacitors are 15nF, then I can supply 20Hz 50% duty and adjust with the potentiometer. Sometimes I got strange behavieur at motor and then I saw that if I didn´t supply the PWM signal from the signal generator, the 555 output itself 50% 50Hz signal. When I supply 5V at input then the IC doesn´t output anything.

Thanks everybody.
 
The circuit you found is completely wrong, you should have used the circuit in the datasheet instead.
Pin 4 is RESET and should always be at the supply voltage, make it at ground to reset the 555. Pin 2 is triggered when its voltage is a low pulse to ground, not a positive pulse as shown wrongly in the sketch you found. Pin 2 must be high after triggering but before the monostable times out so maybe the trigger input should be capacitor-coupled like this:
 

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Hi Audioguru,

you are right, there is full of schematics in Internet and I choosed the wrong one!!!! I didn´t realize, I was thinking in putting a pull up resistor and a transistor in pin 2 to avoid that strange behavieur but if I connect pin 4 to ground it probably will work properly. Thanks.
 
alec_t said:
No, it won't. Connect pin 4 to the +ve rail.
Click to expand...

Sorry, that was what I mean, pin 4 to pin 8. I did it, now duty is not changing and also I have the output itself. Before it was working, I could change the duty correctly, the only problem was about the output when there was no trigger (no voltage, no ground).
 
OK, after doing the modification:
- Remove jumper between pin 2 and 4.
- Connect pin 4 to pin 8.
- Check all connections

I can´t modify the duty like this, I moved the 500K resistor from one side to the other just to see if the value should be different and duty always the same...

Any explanation? With 2 and 4 together it works!!!
 
Can you post a schematic showing exactly the circuit you now have?
 
Since now you have pin 2 floating with no DC path to logic high or logic low then we also need to see the schematic of what is driving it.
The monostable is not an oscillator so it does not have a duty cycle, instead it is a pulse duration stretcher. If you feed it a short duration pulse to ground then the potentiometer can adjust the output pulse to be a longer duration.
 
You need a negative-going trigger pulse on pin 2; not a positive-going pulse as shown.
 
Yes, the trigger is the 20Hz 50% duty square signal, as you recommended, so in low section of that 20Hz signal it should trigger the 555. Then I modify the duty with potentiometer.

The transistor I put in the schematic just to solve the problem when there is no logic input that the 555 creates an output. So there can be a pull up resistor only changing to low when a transistor switches to ground, it can be PNP or NPN, I left an NPN just as example.
 
You need to use just the falling edge of the 50% duty cycle input pulse to trigger the monostable, otherwise the monostable output will be 50% duty cycle too. In other words the trigger pulse duty cycle must be less than the required output pulse duty cycle.
Here's how you could do it :-
 

OK, I can try that one too but I don´t want to connect the 5V power to the output of my signal generator, should I put a transistor?

I don´t understand the problem yet with the other circuits, you have a positive edge and a negative edge at a square 20Hz signal so the IC should give an output only when it detects the falling edge and only until it gets again a positive edge... Do you mean the problem is coming from the duty input? For me is not important the 50%, I can change that value at input if I can modify the output to change a little the speed. This is just to check treadmill controllers when I´ve repaired them or just to see if they work properly.
 
Hi Alec,

as I need a minimum of 18% positive duty cycle output and then to increase the positive duty to increase the motor speed. I understand I should do it in two ways:

- Using a PNP transistor with 10% duty cycle high input, this means transistor goes low 90% at input of 555 so I can have a duty high from 90% modifying to duty lower with the potentiometer at output.
- Using a NPN transistor with 90% duty cycle high input, just in opposite way as before.

The strange thing is if I do this with the first circuit I choosed it works, but not with the correct one connecting 4 to Vdc. We will see if it works know with this last one.
 
If you keep the input high for longer than the designed monostable pulse width (~9mS) then the monostable output pulse stays high for longer than 9mS. Ok if that's what you want, but the design is intended to produce a ~9mS pulse when triggered by shorter duration trigger pulses.
2PAC Mafia said:
I need a minimum of 18% positive duty cycle output
Click to expand...
That's not what you originally specified . In that case make U2 a 500k pot to give a wider control range.
 
OK, I think I´ll keep the design where I connected the pin 4 to pin 2, with transistor to avoid not desired output. That one is working perfectly.

The rest was to have it properly connected when Audioguru told me that one was wrong but it seems it´s taking time from both and finally we don´t know why with 4 at plus doesn´t work and with 4 at 2 it works. When I have more time I´ll try this last design too just to test as I have another jobs repairing more things.

This is what originally I asked at first post:


Then I tried the astable but frequency was not stable if I modified the duty...

Thank you very much for your time.
 
Pin 4 on a 555 monostable is supposed to be at the positive supply voltage. The trigger pulse to pin 2 must be higher than 1/3rd of the supply voltage at rest then go low to near ground for a duration that is less than the desired output pulse duration.
Your input pulse was positive, not negative to ground and its low voltage duration was too long.

It would be simpler to use one 555 to make the frequency and another 555 to make the monostable then the adjustments would not affect each other. A 556 has two 555 circuits in it.
 
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