I've been so busy lately that I haven't really posted much. I ended up getting something I was working on done a little early, but it was late and I was too tired for AH. Anyway, I ended up getting bored and decided to find some way to use my time. I had some spare parts in a big box of components (that seems to get bigger everyday
, but I couldn't figure out what to do with them. I finally thought of something and after about an hour I built a little circuit that some of you might actually find useful.
First some background, I leave my computer on most of the day and sometimes at night. This system is an Athlon 1300 Mhz @ 1333 Mhz, and as you can imagine it puts out quite a bit of heat when I'm working on something serious or playing games. For most of the time, however, it's basically doing nothing. It's got several case fans, and the combination of them means it can be pretty loud. (That's why I don't leave it on at night.
)
The simple little circuit I built is a temperature controlled fan driver. This little circuit just goes inline with a case fan and works just like a regular house thermostat in reverse. When the temperature in the case exceeds a settable level, the fan powers on until the temperature drops and then turns back off. The circuit is made up of a few very common and inexpensive components. (Definately under $20 US.) I've done some simple little tests and it seems to work very well.
I'm about to rebuild the circuit to make it as small as possible.
I have some pics of the circuit as originally prototyped on a general purpose wire wrapping board: (They are pretty big, I appologize. )
There are a few other pics I transfered over to my engineering work account. Look at
http://engr.oregonstate.edu/~bloom/P1010101.JPG http://engr.oregonstate.edu/~bloom/P1010102.JPG for some other views if you want. Don't get confused with the P1010099 and lower pics, that's something else. (Prototype infared 16 bit stereo transceiver for sending digital audio signals between devices.)
As you can see the circuit's pretty simple. That little device out front is actually the temperature probe. (For testing I put it way out front.) The picture is really blown up, for scale that "big" chip in the center is only about 1 inch by .25 inches.
To use the system you just hook up 7 - 18 Volts to the two little pins (you can't see them in this pic) behind the voltage regulator on the left. That means you can run this circuit right off your 12 V supply in the case. (Yellow wire on a 4 pin power IDE drive connector is 12V +, red is 5V +, blacks are ground. This is the type of connector that plugs into your CD drives, hard drive, front case fan, etc.) It's very simple to adjust, you just turn the little shiny potentiometer on the left until the fan starts, and then back it off a notch. If the temperature on the little probe in the front goes up the fan will then turn on until the temperature drops again. Using the little blue multiturn potentiometer on the right you can make the system very senstitive if you wish.
Here's a general run down of what you would need to build your own.: (If you are interested, contact me and I'll send you as much info as you need.)
1 LM7805 - Voltage Regulator 5V (This is the "thing" with the tab on the far left)
1 74HCT04 or 74LS04 chip - (The chip on the left.)
2 2.2 K Ohm resistors. (These are the dark brown resistors. One is right in front of the 7404 chip, one is behind it.)
2 1 K Ohm resistors (The lighter colored resistors on the right)
1 LM335A - Linear Temperature Sensor (the guy in the front and the "brains" of the operation.
)
1 LM336 - Precision 2.5V reference (little guy between the 2 chips)
1 LF356N Op Amp - (Smaller 8 pin chip on the right.)
1 STP16NF06L - Logic Level Mosfet Power Transistor (it looks just like the 7805, but it's the one on the right with the tab.)
1 10 K Ohm potentiometer (the silver one on the left)
1 10 K Ohm multiturn potentiometer (The blue thing with the screw on top on the right)
Assorted sockets and connectors. These are the sockets the chips plug into, and the header the little fan is plugged into. You can get either the smaller 3 pin connector like for a computer fan, or the larger 4 pin connector like used on hard drives.
(If any of you are interested I could give you exact part numbers from Mouser <
http://www.mouser.com> .)
Basic theory of operation is that the LM335 senses the temperature. At 25 Celsius it outputs 2.479 Volts, and it's output rises .01 Volts for every degree C the temperature increases. The LM336 provides a temperature stabilized voltage reference that can be adjusted anywhere between about 2.3 - 2.8 Volts. The LF356 op amp compares the outputs from both of these guys and when the voltage from the LM335 exceeds the voltage reference provided by the LM336, its output switches from low to high very quickly. The big chip on the left, the CD74HCT04 (74HCT04) looks at this voltage and is hooked up in such a way to provide very near 0 Volts output when the op amp's output drops below about 2 Volts, and goes to 5 Volts when the op amp's output exceeds about 3 Volts. (This chip is actually not necessary, but it just makes the fan be either on or off, and not just running slowly just as the fan is about to turn off.) The STP16N06L is a high power MOSFET transistor, that is controlled by the output from the 7404 chip. This transistor (if heatsinked) can handle up to 16 Amps at up to 60 Volts DC. (In other words, overkill for this little 12 Volts .25 Amp fan.
)
That means this circuit could control something else besides a fan if you like. Also, by flipping the op amp's inputs you can make something turn on when the temperature DROPS below a certain point, rather than RISES above a certain point. (For example, drive a relay with the transistor that turns on a heater when the temperature drops below freezing.)
The little potentiometers allow the outputs of the LM336 reference (controled by the metal potentiometer on the left) and the LM335 temp sensor (the blue potentiometer on the right) to be varied to basically choose any temperature threshold that the LM335 is capable of reading. (About -20 C up to 125 C, that's WAY below freezing up to significantly above boiling.) The 4 resistors just control the current through the LM335 and LM336 (the brown 2.2K ones) and the 1 K resistors just go on the outputs of the 7404 and LF356N op amp so they have some load to drive.
This circuit is very basic, and took me about an hour to design and build (designed it as I built it actually
). You could add a few more simple little components to expand its usefulness. (Example: Some capacitors and a diode would be a good idea for protection against "releasing the magic smoke" if you hook it up backwards.)
Anyway, I just thought I'd throw something a little bit different out. If you guys want me to continue posting this type of thing; I'd be happy to continue to post things like this that I come up with. If any of you want a schematic or some more information, contact me at
bloom@engr.orst.edu and I'll draw up something and send it to you.
I'm also considering using National's LM75 temperature sensor in a possible "upgrade" to this circuit. That chip is much more powerful, and digitally sends the temperature in Celsius. You could use this to not only control a fan(s), but display the temperature as well.
I hope you enjoyed reading this. (At least it gave me something to do tonight.
)
(Oh, Anandtech and Tom's Hardware both have articles up about AMD's new "Hammer" processors that were finally shown to the press at IDF the other day! They are set to replace the Athlon near the end of this year.)
