# Are 555 PWM circuit resistor and capacitor values independent of the voltage applied to a 555?

I am new to electronics so my question sound dumb.

I am using this diagram to build a 555 timer IC PWM circuit:

The above circuit operates on 9V but I have 5V.

Do I need to change capacitor and resistor values also, or are these values independent of the voltage applied?

From the below picture, it doesn't looks like I need to change a capacitor and resistor values.

• um, what sense do the two diodes make there? This "schematic" (it's not a schematic, really) does not seem to be quite sensible! So, start with a better schematic. The Texas instruments NE555 datasheet has an example "9.2.2 pulse-width modulation", which makes very much more sense. Jul 26, 2021 at 11:42
• @MarcusMüller The diodes allow independent adjustment of the charge and discharge. Jul 26, 2021 at 11:58
• @HandyHowie but it's but one potentiometer, so these are inherently linked anyways Jul 26, 2021 at 11:59
• How accurately do you need to know? Do you see voltage shown in the equations for T? Well then... to a first approximation, no. But read the fine print in the datasheet ... if you need precision, you may notice a percent or so drift when changing voltage. But then, if you need precision, a 555 is the wrong tool for the job. Jul 26, 2021 at 12:06
• @MarcusMüller This answer explains the diodes - electronics.stackexchange.com/questions/563128/… Jul 26, 2021 at 12:12

## 1 Answer

The PWM CTRL signal works best from 0.6V to Vdd and is independent of supply voltage for duty cycle as the pulse ratio is determined by the dual thresholds of 1/3 & 2/3 V+ but low side is about 1 diode drop above ground due to Vce(sat) on reset.

This will work the same on 9V as 5V for f and d.c. but if you want full PWM range, I suggest a better way.

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