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PWM with control Voltage using 555 timer IC

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thecritic

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I want a PWModulator with 555 Timer IC. When I searched all I got was designs that needed you to turn Pots. I want control signals to do this. If I directly use pin 5 of 555, it will also vary the frequency, which I don't want.
 

ericgibbs

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I want a PWModulator with 555 Timer IC. When I searched all I got was designs that needed you to turn Pots. I want control signals to do this. If I directly use pin 5 of 555, it will also vary the frequency, which I don't want.

hi tc,
Look at page #13 of this pdf.
 

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thecritic

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The minimum High time (i.e pulse width) of the output is equal to the low time of the clock pulse, right?
 

ericgibbs

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The minimum High time (i.e pulse width) of the output is equal to the low time of the clock pulse, right?

The frequency of the pulse is set by the astable 555 driving the PWM monostable 555, so the PWM Vcontrol varies the pulse width out.
 

thecritic

Member
@4pyros Control Signals from other parts of the circuit (Magnetic Levitation System, the other thread).
I don't know why but I couldn't make that circuit (eric's link) work. In particular, even when I applied close to 0 V to the control pin, the output duty cylcle isn't 0. I want the output duty cycle to be 0% when I apply 2.5 Volts and then 100% when I apply 10 Volts to the control pin; and I want it to varry linearly.
 

4pyros

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@ thecritic; you should have kept it all in one thread!!!!!!!!!!
 

ericgibbs

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@4pyros Control Signals from other parts of the circuit (Magnetic Levitation System, the other thread).
I don't know why but I couldn't make that circuit (eric's link) work. In particular, even when I applied close to 0 V to the control pin, the output duty cylcle isn't 0. I want the output duty cycle to be 0% when I apply 2.5 Volts and then 100% when I apply 10 Volts to the control pin; and I want it to varry linearly.

hi,
As far I know the 0% and 100% ratio cannot be achieved when using a 555 PWM, it can be very close.

What ratios did you achieve on test.?
 

ericgibbs

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Last edited:

shortbus=

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hi,
Thanks for the pointer, I only hope the OP reads your post.!

EDIT:
Looked at the link, its a commonly used circuit but it does require more that just a 555 ie: a LM339 .

Bill is a member here also, Quietman, I think is his forum name.

When people started to suggest PIC's and such I thought to post this.
 

ronv

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Pwm

Here is a similar circuit that uses the other half of the 339. If you don't need the active pull up you can replace the totem pole output with a transistor. As is you need to decouple the positive supply just like when using the 555.
 

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