I want to turn on a 12 volt fan through a 110 wall light motion sensor.

Specs of the 12 V fan? Voltage and current?

Do you have a 12 V power supply?

This can be done with a relay with a 120 V coil and appropriately rated contacts.

You just need a 12 Volt DC? Power Supply that converts 120 Volts AC to 12 volts DC(If DC). Maybe keep the plug on the Power Supply and connect white from motion to a receptacle neutral, and the switched from motion to hot of receptacle.
The current of the fan is needed first.

Looked through a couple of your Post, Can you be more specefic on what you want, do you need a delay off. Bathroom? Lab?
Do you just need motion controlled fans of different currents and voltages? Good Luck

The fan draws 0.30a (computer fan). I do not have a 12 volt power supply.

Old Computer Power Supplies Have 12 Volts, or a Plug in Transformer over 500Ma(.5 Amps)@ 12 Volts DC.

I don't want to run two power supplys. I wanted to wire the 110 motion sensor and come out of it though what ever I need to and trun on the fan.

You just need 1 power supply or a 120 volt fan.
Refer to the above post.
With a 12 Volt Fan, you HAVE to have a power supply to convert the 120 out from the motion into the 12 volts of the fan.
I find these transformers for $2 at Yard Sales. 12 Volts DC 500Ma.
If you want the fan to rotate the other direction, reverse the 2 wires.

Stratmando, how about coming out of the 110 sensor to an AC outlet and pluging in the 12 volt transformer into that?

Exactly. Take a small 12 V power suply brick such as this one: Jameco Electronics Power Supplies & Wall Adapters: JAMECO RELIAPRO : DDU120100Z7974

Have the motion detector turn it on and connect the fan to the output.

BTW: Transformers step up or down AC voltages. A DC supply rectifies, filters and regulates. A line voltage regulated power supply is classified as unregulated. e.g. If the AC voltage drops, so does the DC output.

I rigged the motion sensor switch and to an outlet and plugged in the 12 volt/500a power supply, but there isn't enough draw from the supply for the switch to work. I did plug in a light and the switch works fine.
Thanks

So you have a light in parallel with the transformer. Wonder if a larger power supply would provide enough load to eleminate the light, the light gives off heat, competing with the fan to cool, Exhaust?

I plugged in the 12volt power supply and the fan keeps running it doesn't draw enough power for the switch to work properly. When I unplug the power supply and plug in a regular 110 lamp the switch work fine. All I'm trying to so is make the switch trun the fan on and off.
Thanks

How big of a 12 volt DC adapter power supply would I have to use to take the place of a 110 AC light? If I used a bigger adapter would that create more draw to make the light work?

Do you know the current rating of the 12 V fan?

You probably need 50-100 mA on the 120 V side to allow any solid state device to hold, so that's really about 6-12 watts. So, a night light can do. You can also use a resistor.

If you can try paralleling a 7W incadesent night light and the fan.