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The base of my transistor is activated with a positive signal 5v. I need that when this happens my output OUT receives GND, but I do not know if I should connect OUT on the collector and GND on the emitter or the opposite GND on the collector and OUT on the emitter. Usually my output is connected to a pull-up resistor.

schematic

simulate this circuit – Schematic created using CircuitLab

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enter image description here

Figure 1. A standard NPN switch.

Feeding a current into the base of the transistor turns it on and current can flow from the collector to the emitter.

The emitter is connected to GND and the load - or in your case the pullup - is connected to the collector.

Be aware that the signal gets inverted. When OUTPUT switches high the collector is pulled low.

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  • \$\begingroup\$ I did not understand where my "IN" is connected in your circuit. \$\endgroup\$ – Eduardo Cardoso Oct 27 '17 at 10:47
  • \$\begingroup\$ The micro-controller output is connected to the IN of your circuit. My R1 is the same as yours. Your R2 is optional and generally not required for a digital 'on-off' signal. See lednique.com/driving-leds-on-higher-voltage. \$\endgroup\$ – Transistor Oct 27 '17 at 10:49

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