I have a 12vdc photo cell sensor, when triggered a 3.3v is on the output.

How can I get a 12vdc out when the sensor is triggered?

 

You could use an external transistor. What's the voltage at the output when not triggered?

Also, please provide details of your sensor. It could be an open collector output.

 

"What's the voltage at the output when not triggered?" 0.0v
I do have it hooked up to an external transistor a 2N3904 but can't seem to get 12v

http://dypsensor.en.alibaba.com/product/765070552-218216361/_font_b_Mini_b_font_Human_font_b_Sensor_b_font_font_b_Detector_b_font_ModulePIR_font_b_Sensor_b_font_ModuleMini_font_b_IR_b_font_Pyroelectric_Infrared_PIR_Motion_Human_font_b_Sensor_b_font_font_b_Detector_b_font_ModuleSensors.html

 

The instructions say it outputs 3.3 volts.
This circuit will convert that to "0 volts to 12 volts" with a current capability of 50 milliamps. You can change the resistor values to get different amounts of current. You can add another transistor to get the voltages to be "12 volts to 0 volts".

 

Is this representing the connection to the 2N3904 from the Sensor?

 

What else would it represent? Some other transistor that I imagined connected to a sensor you are not talking about?