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Momentary Fan Switch

This project details how to make a momentary on/delay switch for anti-fog mask fans.

This simple circuit provides a little more control over the fan you use in your Airsoft or Paintball mask. Most fans provide a switch to turn the fan on or off. This little circuit will keep the fan on as long as you’re pushing the button, then continue running the fan for 10 or so seconds (adjustable) after you release the button. Handy, eh?



PARTS AND SCHEMATIC

As you can see from the schematic, only a few parts are needed – and the component values are not all that specific. You will need the following:

Stuff you probably already have:

  • A momentary switch (like a pushbutton)
  • A battery
  • A fan

Stuff you may not have (all available at Rat Shack or Digi-Key):

  • a 1k resistor (color code: brown black red)
  • a 2200uF capacitor
  • an NPN transistor (2N2222 should be good enough)

The 1k resistor can be a little off in value, but I wouldn’t go higher than 4.7k or lower than 470ohms. You should test your desired resistor with your chosen fan, capacitor, and transistor to make sure it will work out.

The 2200uF capacitor works for my setup and keeps the transistor turned on (and the fan running) for about 10 seconds after I let go of the button. Test with different values to get something you like.

The NPN transistor should be able to handle above the maximum current draw of your fan. If you get a (very common) 2N2222 transistor (which comes in a little metal can with three legs) it should handle most fans you will be using on a mask. The 2N2222A is _not_ the same, and only handles about half the current of the 2N2222. But if it’s all you can get, give it a try. The garden-variety 2N3904 is probably sufficient only for the smallest fans.

Here is a 2N2222 datasheet to help you figure out which pin is which.

HOW IT WORKS

The image at the top of the page shows the fan with the power supply (a rechargeable 9V battery) and a button with the control circuit.

Normally, you would just have the switch connect the fan to the battery so the switch turns the fan on and off. Or, you may have a button so that the fan runs for as long as you are pushing it.

In my design, instead of connecting the fan directly to the battery the button instead turns on a transistor (or more accurately, saturates it), which in turn allows current to flow through the fan – turning it on. The transistor acts as a switch in this case; a “switch” that is turned on by applying current to the base pin of the transistor.

So far, no different than a normal setup. But the twist is that pressing the button not only turns on the transistor, but it also allows power to flow into the capacitor. When the switch is released, the power in the capacitor takes over the job of keeping the transistor turned on until the capacitor is empty. How long this lasts depends on several things, but most of all it depends on the size (the uF) of the capacitor. Mine is 2200uF and keeps the fan running for about 10 seconds after I let go of the button.

This is not the most efficient design, but it is simple and able to be put together by most people with access to a soldering iron.

Note: Those of you who are techies will notice the lack of a clamping diode across the fan leads. I think (practically speaking) we can get away without it, but to each their own. To those who sort-of know what I’m talking about, you may want to add a diode across the fan’s leads: the cathode (the end with the band) should be attached to the + end of the fan, and the anode should be on the – end of the fan. Those of you who aren’t sure what this is all about can do without it.

Well, that’s about it. Here are a couple more images: