12v Remote Turn-On "Magic Box" for cooling fans

and other devices

WARNING - The information posted here is based on what little I know of electronics. I take absolutely NO repsonsibility for any damage that may be done by using this yourself. If you fry your equipment don't blame me!

What it does: This is a little box that allows the 12v control signal from your stereo to turn on other devices like cooling fans. It can also be used to control other items like screen raising and lowering, light control, drape control etc.

How it works: It's a very simple relay circuit. The relay has an electronic switch that switches the other half of the relay on or off. The 12v control from your reciever controls the switch - either on or off. The other half of the relay circuit is the 12v power supply (wall-wart, power brick etc). The relay switch is normally off - no power passes through. When it gets the 12v signal from your reciever it switches on and then passes the power through the relay - turning on the fans. Changing the relay to a 120vAC/12vDC model you could control 120v devices.

The finished assembly

More close ups of the finished box and the fan assembly

The finished wiring

How to wire up the circuit using either a box like relay or a reed relay (bottom pic)

Parts needed

I listed both Radio Shack and Parts Express product #'s. PE is 1/2-1/4 the price of RS but you do have to pay for shipping and wait for it.

Control Cable

• (2) male 1/8" mono plugs - like Radio Shack Cat# 274-286 or Parts Express 090-290 $.29@
• (1) panel mount female 1/8" mono plug - like Radio Shack Cat# 274-251 or Parts Express 090-296 $.48
• wire - almost any kind will do. I had red and black 22ga wire on hand so I used that.

  Or you could just buy a mono cable already made but you won't get the exact length you might want.

Fan Assembly

• 1 or 2 Silent PC cooling fans found here - www.pcpowerandcooling.com $7.50@
• (1) male 1/8" mono plug
• (1) female 1/8" mono plug
• computer "Y" cable (if using 2 fans)
• wire

Magic Box

• project box approx 2"x3"x1.5" - like Radio Shack Cat # 270-283 or Parts Express - 320-440 $.35 & 320-400 $2.10
• 12vdc SPST (single pole, single throw) relay - Like Radio Shack Cat# 275-233 $2.69 (the one I used isn't listed anymore but this reed relay works the same)
• wire (I used red and black to keep it simple)
• 12v DC power supply - 12v 200mA works well for 2 fans - like Radio Shack Cat# 273-1773 or Parts Express 120-387 $2.95
• female plug for the power supply. I can't tell you what kind as most power supplies have their own sizing. Parts Express 090-293 $.48 (if using 120-387)

Total Cost approx $30 for a 2 fan cooling assembly.

When assembling the cables make certain you keep the polarities the same. Most plugs are "tip positive". This means you need to keep the center of the plugs (the tip) positive w/ the female plugs and the power supply.

I'm hoping the pictures describe how to make this easily enough. If not please don't hesitate to email me.