Lmandrakes Water Flow Sensor

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Something to let you know your pump is still pumping.–Otto Matheke

Flow Sensor

Those of us who indulge in water cooling have to contend with the possibility that a lack of water flow will kill our CPUs. This is usually the result of either pump failure or an inadvertent failure to turn the pump on. Now that “Bong” evaporative coolers have come into use, there is also the possibility of evaporation diminishing the water supply to the point where the pump no longer moves water. With AMD processors or peltier setups, the consequences of loss of coolant flow are rapid and catastrophic.

Visual flow indicators, which are usually water wheels behind a clear plastic face, are either available commercially or can be hacked from a dead fan. They possess two shortcomings; they have to be visible and you have to watch them. Putting them in plain view can be difficult and if you are in the midst of a dogfight or a fragfest, you won’t be looking at the flow indicator.

After seeing what another overclocker had done with a fan to make a low cost “water wheel” visual flow indicator and following some threads in which the possibility of reading the water wheel rpm through the motherboard fan headers, I began to think about using fan electronics to act as a flow sensor. I did not think that any watershed would turn fast enough to provide readable output to a fan header and started to think about a switch instead.

In coming up with a design, I had several goals – I wanted to avoid the use of any parts or materials that would be hard to find, the device had to be absolutely leakproof, and I wanted to avoid the use of tools or skills that most overclockers would not have. I think that I have achieved most of these goals and ended up with a usable and practical device.

The materials needed are fairly simple and will require a trip to a hardware store, your local Radio Shack, and wherever you can scrounge an old cheap 12v DC brushless fan.

Here is what you will need:


  • An old cheap 12v DC brushless fan that still runs
  • Some solid core copper wire
  • A plastic straw or a nylon binding post from a good hardware store
  • A 2 inch copper or PVC plumbing tee
  • Plumbers Goop, JB weld, or a good silicone caulk
  • Brass hose barbs or 17/32 brass tubing
  • A short length of 2 inch copper pipe if you use copper, PVC fittings to fit barbs to the tee if you use PVC
  • A cork that fits inside the tee as a stopper – good hardware stores have corks
  • Radio Shack 64-1895 Rare Earth magnets (one package)
  • Desoldering braid or a vacuum solder remover
  • Small heat shrink tubing
  • Electronic solder
  • Some small zip ties
  • PVC cement if you use PVC, solder and flux if you use copper pipe
  • 2 hose clamps
  • A straight pin, paper clip or a needle
  • A relay or some 12v diodes or diodes and suitable resistors
  • Some thin gauge light, flexible wire


  • Fine Point Sharpie (indelible marker)
  • Soldering iron
  • Torch, if you use copper
  • Dremel, tube cutter or something to cut tubing
  • Voltmeter (optional but very useful)
  • Needle nose pliers
  • Some flat toothpicks

The fan contains the key component. Brushless fans contain a device called a Hall Effect Sensor. Hall effect sensors react to the proximity of a magnetic field when a magnet passes close to the face of the device and turn an input voltage into some kind of signal. The inside rim of the hub of a brushless fan contains a strip magnet that passes by the hall sensor as the fan turns.

When one pole passes, the sensor latches in the “on” position and feeds power to an electromagnet that attracts part of the magnet and makes the fan spin. When the other pole passes, the sensor goes “off” and the fan circuitry sends power to another coil.

Since the hall effect sensor is a magnetically activated switch and the fan circuitry sends power to one of two places depending on the position of the magnet in relation to the sensor, we can use this to our advantage either to light some diodes or turn a suitable relay on or off depending on water flow.

Hall Sensor

Hall Sensor 1

The other component is either a copper or PVC plumbing tee. The straight section of the tee will be plumbed into the cooling system tubing. The side outlet is plugged with a cork that also serves as the mount for a hinged magnet holder.

When the pump is turned on, the magnet swings past the hall sensor and switches power to one of the coil feeds.

When the pump is turned off, gravity and water flow pull the magnet back past the hall sensor and power is routed to the other coil feed.

Since copper or PVC plastic are non-ferrous and will not interfere with a magnetic field, we can mount the sensor on the outside of the tee fitting.

The first step is to find an old, cheap DC brushless fan that still runs. I say old and cheap as older fans are more likely to have larger components that are easier to work with, cheaper and are generally easier to take apart.

Peel the decal off the hub on one side. You will find a snap ring on the shaft hold the fan to the hub. Pop the snap ring off with a small screwdriver and slide the fan off the hub. Flip back the decal side and free the red and black power feed from the hub so you can pull the internal circuit board off the hub and not break the wires or the hub.

Remove the bearings, springs, spacer etc. from the inside of the hub. If you are lucky, you can now slide the circuit board off the hub. Resist the temptation to pry on the edges of the circuit board with a screwdriver – you will probably just break the circuit board. Higher quality fans may have some arrangement to hold the circuit board on the central hub.

Some use a “plug” that goes inside the central hole to mushroom the end of the shaft; these can be driven out with stout tool like a pin punch, a ¼ inch drive ratchet extension or something else long or stout enough to wail on.



Fan Apart

Once you get the circuit board off, you need to find the hall sensor, the coil connections and the coil mounts. Look for hair thin wires coming off the coils to solder connections on the circuit board. There should be three coil connections for the two coil sets. One of them will be a common connection to both coil sets and the other two will each go to one coil set.

When you find them, mark them with a Sharpie or some other indelible marker, taking care to make a special mark at the “common” connection. After all the connections have been marked, cut the leads and remove the coil from board. Some coils are soldered on, some have plastic mounts.

Soldered ones have to be de-soldered and removed; plastic ones usually can be defeated by carefully slicing them off on the side of the board opposite the coil. Now that you have the coil off, you have to remove the hall sensor. Before you do, note the orientation of the sensor and remember which face is facing out.

Unsolder the sensor and carefully pull it from the board. Get three equal lengths of wire 2-4 inches long and some heat shrink tubing. Solder the wire leads to the sensor and the board so the sensor can hang free but the connections are the same. Remember to slide the heat shrink over the leads before you make the final attachments. Use the heat shrink to cover the leads of the hall sensor. Solder leads to your diode or relay to the solder pads where the coil leads went on the circuit board.

CONTINUED on page 2…

Otto Matheke

You will, of course, be tempted to play with the circuit at this point. Do not apply power to the circuit unless you have a load of some kind to replace the coil load. Failure to do so will just blow up the circuit board and you will have to pull another fan apart.

If you have a voltmeter and are using diodes, check the polarity before you hook up the diodes, as the common lead may be a positive lead and your LEDs will only work with correct polarity. If you are using relays, polarity won’t matter.

To test the device, wave the thin edge of the magnet past the face of the sensor. One LED should light when the magnet passes in one direction and another should light when it goes by again. Since fans usually use “latching” hall sensors, one circuit will stay on regardless of where the magnet is until it passes by again in another direction.

Now we need to mount a magnet inside the tee so water flow will push it back and forth past the hall sensor.

The cork plugs up the tee and provides a mount for the magnet holder. Take a straight pin and stick it in the middle of the narrow end of the cork. Push the cork into the side hole of the tee so the pin contacts the opposite inside wall of the tee. Continue to push the cork in until it is snug in the hole. Pull the cork out and measure the distance from the face of the narrow end of the cork to top of the pin. Subtract a smidge and we have the total distance allowed for the magnet holder and its hinge from the face of the cork.

To make the magnet holder, you take either a sturdy plastic straw or the female part of the binding post and cut it to length. Take your original distance minus the “smidge” and reduce it by one half of the diameter of the straw or the female part of the binding post. Cut it and flatten one end so you can stick the magnet in.

Shove the magnet in a little more than halfway so the narrow edge is sticking out. Using toothpick or some similar, pack Plumber’s Goop, JB weld or caulk into the tube so the magnet will be glued in place. Set it aside to dry.


Once dry, push down on the magnet end so the circular face of the magnet is flush against a table and mark the sides of the opposite end of the tube. Make your marks an equal distance from the end of the tube, one or two millimeters from the end. Practice on some scrap first and then heat a needle or straight pin and melt a hole in each location, just large enough to take your solid copper wire. Cut a piece of copper wire twice as long as your cork with about an inch left over.

Put the wire through the holes so the holder is in the middle of the wire and bend each end 90 degrees so the wire is going straight back, but leave a little room so the magnet holder swings freely. Punch two holes in the cork with a needle or straightened paper clip so the magnet holder will be centered on the small face of the cork when the wires go through.

Push the copper wires through the holes with needle nose pliers and once through, pull the ends so the magnet holder is aligned with the cork. Diddle with it until the magnet holder swings freely.


Cork Top

The cork and magnet holder can now be test fitted to the tee. Remember that the circular faces of the magnet have to face the holes on either end of the tee. Look through the ends and see if the magnet hits the inside wall of the tee. If it does, pull on the wires sticking out of the wide end of the cork until you have clearance. The little Radio Shack magnets are very powerful, so the clearance does not have to be tiny, but the closer to the wall, the better. Once you get the clearance right, cut the excess wire and fold over the top of the cork.

The tee needs to be plumbed into the tubing. I used copper and brass tubing. Short lengths of ½ copper pipe need to be soldered into the ends. If you use brass barbs, get some that will accept the ½ copper tubing an solder them into place. I have found that barbs waste space in tight places and that 17/32 brass tube – which can be found in good hardware stores, hobby shops or art supply stores – can be soldered inside the ½ copper and will hold ½ i.d. tubing securely if you use a hose clamp.

Copper T


Seal the cork into the tee so it won=t leak. Apply some Plumber=s Goop to the outside of the cork low enough to seal it but not low enough to get it on your magnet holder. Insert it into the tee while rotating it a bit to spread the adhesive around. Put a dab on the top of the cork to seal where the wires go through. Be sure to check the position and alignment of the magnet holder before it starts to set. Once everything is in position, you may want to lay a bead around the point where the rim of the tee fitting meets the cork. Let it dry.


The hall sensor now needs to be mounted on the outside of the tee. I held it in place with one hand and used a zip tie around the base of the tee to hold the leads in place. That helped me control its location until I fastened it down by taking two zip ties and linking them together so I could make an “X” pattern around the sensor as shown. The seams on the bottom of the tee helped me to align it. When using a smaller fan, where the hall sensor stays on the circuit board, I did the same thing by passing the zip ties through the hole in the middle of the circuit board.


You can now test the setup by powering up the circuit and rocking the tee back and forth so the magnet swings past the sensor. If all is well, one circuit activates when the magnet goes past in each direction. Depending on what you want the circuit to do, decide which way is water flow “on” and which is water flow “off” and mark the desired flow on the copper with your Sharpie.

If the test goes well, mount the circuit board to the tee. Be sure to insulate any face of the circuit board that may contact the copper tee. I just wrapped the board in electrical tape.

To install the flow switch in your cooling system, remember that it has to be oriented vertically so gravity will pull the magnet down when there is no flow.


What good does this project do?

You can use it to light a red LED when you have no flow and a green LED when you have flow and mount them in a drive bay cover. You could also power a piezo alarm when you have no flow.

If your pump does not turn on with your computer or is running all the time, you can wire in a relay in the leads to your computer’s power switch so the power switch will only work when you have water flow. If you, or your little brother or girlfriend or kid etc. cannot turn the computer on if the pump is not running, you have a margin of safety you didn’t have before.

Enough T-Birds have died already.

Otto Matheke


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