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how many ohm resistor would i need to resist 7 volts to 3.5 volts and 12 volts to 3.5 volts also....

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but for this project i will need a zener diode because im only using one fan and 2 switches, that reminds me, how would i use one switch to switch between 7 and 12 volts?

If I understand you correctly here, a SPDT or DPDT switch should serve your purpose (Single pole, double throw; Double pole Double throw)

If i try to translate your description : - you would like to be able to change the speed of a 12V fan using a switch Is this correct ? Though, i don't know what you want to do with the LED ... Questions : 1) Why 7V and not 6V or 4.5V or anyother value ? 2) What do you want to use the LED for ?

because in a computer they have 4 wires, 12v, 2 ground wires, and 7 volts, the LED to show what voltage is going through it, and i would like to change the speed of a 12v fan.

for example, a green LED to show 7v and yellow to show 12v

. Actually, it 12V and 5V (plus grounds). . Most computer fans can be controlled with a potentiometer or PWM module. Or, using the 12/5/G connector, you can wire a switch to send 0-5-7-12V to the fan. . From what I've read (but no experience), most 12V fans don't work well below 6-7V or so.

aww for a second i thought that was a "knight rider" cuircuit

opps circuit

If you're thinking of resistors think in terms of current.
If your total is e.g. 100mA, the 3.5V component will have a resistance (or equivalent) of 3.5/0.1 = 35 Ohm.
The rest of your system will have (12-3.5)/0.1 = 85 Ohm. (Totals 120 ohm, 100 mA @ 12 V)
And the power consumption of the 85 Ohm resistor would be (12-3.5)*0.1 = 850mW

L

(or did I get that wrong?)

well just for an LED and then im hooking computer fans to them

Yes, a voltage regulator would be more appropriate if you want to use it as a source of current. On the other hand, if you just need a voltage reference (without the need of a current) you could make a voltage divider with two resistors (see wikipedia) or use a zener diode ... And of course, if you want a correct and appropriate answer to your specific problem, i think you should give us some more informations about your montage.