Hello friends, I would like to know what would be the most suitable resistance to place the opto coupler that was powered with a voltage of 12v dc. the opto coupler is a pc817 and the resistance that I always use in my projects for this opto is 1K oh with a 5v source but in this case the source is 12v

Good . pc817 data is VF = 1.2Vdc IF = 5 mA, (you can use it up to 20mA) Then the calculation of the resistor is simple. R = (12V-1.2V) / 5mA = 2.16k, that may be the nearest commercial value 2.2k. P = (12-1.2) x 5mA = 54mW, power dissipated by the resistor, actually a little less since the current will be somewhat less than 5mA due to the increase in the resistor value .....

If you want to continue maintaining this relationship, you must use a resistance of 2K2 (2200 Ω) for 12V. 5V is at 1K as 12V is at X; rule of three simple (12 x 1000) / 5 = 2K4 but since it is not commercial value use 2K2 or 2K7 that they are. The value of this resistance depends on the voltage, the option to use and the type of signal that will pass. A signal from a simple ON / OFF button for eventual use is not the same as a high frequency PWM.