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how can i edit the 555-timer astable mode in order to get a larger span of duty cycles passing by values below 50%? D = tH/T= (R1 + R2)/(R1 + 2R2) this is the equation, by changing the values of R1 and R2 I get a maximum span of 50% how can I edit the 555 timer to make the span 60%. (duty cycle to vary between 40% and 100%)

Once I achieve duty cycle less than 50% can i add a notification Light Emitting Diode (LED) in the circuit that will be ON when the duty cycle is larger than 50% and it will go OFF when the duty cycle is below 50%. if yes how can I achieve that?

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Yes you can meddle with the duty cycle on a 555 and get somewhat limited results that are probably OK. This uses the diode across R2 modification: -

enter image description here

Or, you can get a proper chip that converts an analogue voltage to duty-cycle out like this: -

enter image description here

Link to device

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  • \$\begingroup\$ +1 for pointing to a V->PWM chip. I've never heard of such a thing before and yet that's what most people are really after when trying to fiddle with a 555. \$\endgroup\$ – horta Mar 24 '14 at 20:51
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"To achieve a duty cycle of less than 50% is to use a small diode (that is fast enough for the application) in parallel with R2"

http://en.wikipedia.org/wiki/555_timer_IC#Astable

During the charging of the capacitor (high time), a diode across R2 shorts R2. This will allow you to have a high time of Ln(R1)*C which is now independent of the low time. The low time still only depends on R2 as the diode doesn't affect the circuit when the capacitor is discharging.

To have an LED light depending on duty-cycle, see the circuit below. All component values are generic and should be adjusted according to the specs of the system.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Once I achieve duty cycle less than 50% can i add a notification Light Emitting Diode (LED) in the circuit that will be ON when the duty cycle is larger than 50% and it will go OFF when the duty cycle is below 50%. if yes how can I achieve that ? \$\endgroup\$ – Mechatronic_gar Mar 24 '14 at 15:19
  • \$\begingroup\$ The best way I can think of to do this would be to create a duplicate output wave using an inverter or buffer. Then take the second signal output and average it with a comparator. Then you would use a comparator and compare the average voltage to a voltage 50% of your rail-to-rail voltage. The Comparator would turn on above 50% and turn off below it. If you edit your original question with this, I'll see about creating a circuit for you to see what I mean. \$\endgroup\$ – horta Mar 24 '14 at 15:28
  • \$\begingroup\$ I would like to a see Circuit if possible \$\endgroup\$ – Mechatronic_gar Mar 24 '14 at 15:48
  • \$\begingroup\$ I understood the comparator part but can I do this in another way? since the part list I have doesn't contain a comparator! \$\endgroup\$ – Mechatronic_gar Mar 24 '14 at 16:51
  • \$\begingroup\$ There's not a simpler way that I can think of unless you use Andy aka's advice. If you use that other chip instead of a 555, then you can just have the led trigger at a certain input voltage that you want. \$\endgroup\$ – horta Mar 24 '14 at 20:49

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