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Step 1: Video:

Watch the video for a bit of background if you like.

Step 2: Parts Needed

Project Box
PC Fan
PIR sensor [Amazon]
EZ Fan 2 board [Tindie]* or DIY transistor setup as outlined here
LiPo battery
Connector for LiPo, or could solder wires directly
Button/switch
Battery charger board [eBay]

*Original EZ fan used here, EZ Fan2 should work fine. I designed and sell both

Amazon link is affiliate, meaning I get some percentage of sales.

Step 3: Circuit Diagram

The first image shows a representative diagram of how things are hooked up with the EZ Fan2 board/transistor.

While I haven't tried it, it should also be possible to hook up a fan with a PWM input directly to the PIR sensor as shown in the second image. The sensor board has potentiometers that can be tuned for sensitivity, and for how long it stays on after triggered.

Step 4: Drill and Arrange Components in Enclosure

As seen in the video, the mechanics of this project are quite ad hoc, but work well. This concept can be applied to a variety of fan setups, so you'll need to arrange things according to your setup.

With this done, adjust the time/sensitivity to your liking, plug in and engage the button to use. When the battery is charged, it can work for a time without being tethered to the cable. Swipe your hand near the sensor, and it will suck fumes away from your face until it times out (which is reactivated via motion).