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How I Added a "Free" Rain Sensor in one Afternoon.

My Davis 7425 Weather Wizard III still hangs on the wall in the family room, as it has for the past decade, displaying the local weather info. I got my station in 2003 on sale at Fry's Electronics but even though this little station has been discontinued and is no longer available form Davis, you can still find it on line. The one thing that is irksome is the standard 7425 never came with a rain gauge... No additional software is needed, the number just sits at zero. The easy solution to problem #1 is just buying: Davis Instruments 7852 Rain Gauge for Davis Weather Wizard & Weather Monitor Stations. Nice, but the current price is $75 USD.

http://www.davisnet.com/weather/products/weather_product.asp?pnum=07852

it is a Tipping Bucket sensor. Basically, it is just a bucket with a hole in it, a balance with its arm connected to a couple of calibrated spoons that "tip" it when they fill with water dripping from the bucket, attached to the arm is a little magnet which moves like a pendulum past a reed switch that causes the switch to open a close. Simple. But, what mysterious protocol gets the sensor data to the station? Since I did not have one to take a part, I did a little more research in the spec sheet. They are actually very simple analog signals since it is intended to be wired to the base unit. Based on these specs from Davis Instrument, I came up with the schematics attached for all the sensors.

Armed with this information, we could make a tipping bucket sensor from scratch, there are instructions on line, but there are issues of calibration and time to build one. But I have found a quicker one afternoon wayto add this sensor.

There is a ready supply of inexpensive rain gauges now available as close as the local hardware store. The one described here came from the Home Depot. We used to measure the irrigation sprinkler watering on my newly sodded lawn. It has a reed switch and magnet...which attaches to a transmitter that wirelessly sends to the receiver station. This one failed after a few months, but I think the main failure mode of these devices is the complex transmitter circuits, not the switch and magnet assembly. It is no longer available at Home Depot but i found it on line:

16095 Acu-Rite 00899 Wireless Rain Gauge Price: $34.99 http://www.klockit.com/products/dept-64__sku-1609...

These types of products will likely fail and stop working in most of these after a month or so - and the plastic is probably not UV stabilized like the Davis 7852 so it may not be the best for harsh environments but it is readily available and ~oh yeah, I have a broken one!

Here is how I did mine...

Step 1: Tools and Supplies

Here are all things I ended up using for this project:

To Modify the Electronics:

  1. Small ( #0) Philips head screwdriver
  2. Cordless Drill motor
  3. Step bit
  4. Wire cutter/ Stripping tool
  5. Low wattage Soldering Iron
  6. Rosin Core Soldering Wire
  7. X-acto Knife and Blade
  8. Multi meter or continuity tester
  9. Hot melt glue gun and glue stick
  10. 40 foot Length of flat RJ-11 telephone cable. (Either 2 - or 4- conductor will work)

To Build the Platform Base:

  1. Plywood Base (appox 12" x 12")
  2. 1" x 2" length of wood
  3. Cordless Drill with #2 Philips Bit
  4. 3 or 4 #6 Wood Screws
  5. Enamel (or latex paint)
  6. Spirit Level
  7. Rock (Optional)

Step 2: Dis-Assembly

The Patient...

...arrives on the workbench! The tipping buckets attach through an interesting bit of injection molding genius. The theory here is similar to the water tight doors on the Titanic. There is a high wall of plastic water dam surrounding the electronics and tipping bucket assembly; with no gasket of any kind! The whole thing clamps to the bottom of the unit which is open to the wet world to let the water drain out from the buckets. But I guess if the water level tops the watertight doors, like the Titanic, you are sunk anyway. This scheme and missing gasket may be partially responsible for this unit's early demise as well.

Open the Case

Next, I removed the two small screws hold the electronics to the battery enclosure and the tipping buckets on top of the two plastic assemblies.

Sensor Switch Location

The spring on the left is the antenna and the reed switch ( the part we need) is the little glass tube in the center of the board on the two plastic standoffs. The original plan was to ditch the electronics and just wire to the switch. I changed the approach slightly when I saw the mounting for the reed switch was in fact, part of the circuit board. I guess I could have proceeded by removing the reed switch and hot gluing it to the top of the assembly, but in the interest of time and to keep the original alignment, and not introducing any axial rotation, to the switch, opted to leave the circuit board in place.

The Plan

The cable wires will be soldered each side of the reed switch. I soldered them to the bottom of the board, more on that later. Other sensors should be similar, just look for the glass tube...

Step 3: New Cable

Drill Holes
Passing the cable through the assembly took a little thought so as to minimally compromise the water rejection scheme. The cable simply requires two small holes in the base and one in the electronics compartment. I went through the battery compartment located where there is no interference to clamping the assembly together... It was a little hard to drill this right angle hole so I ended up chipping the plastic a bit, no harm though. I wanted to pass the cable through before I stripped out the wires as I wanted the holes to be tight as possible and did not know how much wire would need to be exposed. Is it wrong to be wiring a wireless sensor?

Prepare Cable
I used a pre-made cable and cut off one of the connectors. I cut off about 2 to 4 inches of the outer cable sheath to expose the inner wires. You can use the X-acto knife to do this by bending the cable where you want to remove and gently cut the outer covering only. Then simply pull the sheath off. You can use one of the larger holes in the stripper if you are careful to make the cut without nicking the insulation of the inner wires. Strip the insulation off of the 4 wires at about 1/2". The wires are very fine so be careful not to nick the wires. Twist the RED and BLACK Wires together and solder them. Repeat with the YELLOW and GREEN Wires as well. Trim the ends to 1/4". It looks like you can alternately use a 2 wire cable use one of the colors for each pair, but the distance to the sensor may be decreased due to the wire size. If you are color blind, use the GREY wires from Pins 1 and 2 and GREY wires from pins 3 and 4.

Assembly

The cable passes through the outer shell into the battery compartment and into the electronics compartment through a second hole in the bottom of the electronics compartment. With a reasonable service loop of wire so you can remove the sensor; hot melt does the sealing for a water tight cable entry. The cable tucks into the battery compartment. Once it is all ready put the two orignal screws back in. Hot melt glue seals the deal.

Step 4: Solder the Wires to Circuit Board

Solder to Circuit Board
The wiring looks straightforward, It should not matter which wire pair is used on which pin of the reed switch since this is a simple "dry contact" arrangement. (there is no power required to the sensor to make it work). I used the voltmeter in continuity mode to determine which relay lead was grounded.

I attached the grounded wired (RED / BLACK ) to the (-) side of the battery terminal ~ which worked OK. I decided not to trace but to attach to the other pair ( GREEN / YELLOW) directly to where the lead was soldered to the circuit board, technically these are the "High Side" pair. This side goes to the sensors electronics. What could go wrong - everything is turned off?

One Minor Issue...

When I re-assembled and connected the sensor to the station's input block, and powered it up. To test it, just simply rock the tippers slowly back and forth, the rain should now be counting up...if you listen carefully you can hear a slight click as the magnet passes by the switch, even if you don't hear it, as long as the rain count is incrementing you are all set. But... it did not initially work. The picture shows how it was originally wired, which in fact, will NOT without a slight modification to the circuit board. .Apparently, this end of the reed switch remains connected to the rest of the transmitter circuit there is a low impedance path to ground, low enough anyway, that the counts will not get detected by the Wizard!

Modification

Simply cut the lead the tube as it enters the circuit board and solder the wires to the free end of the lead. I also put a dab of hot melt on the joint and support as the sensor was only held to the board with one wire now. The battery was low on the camera so I did not get a pic of the modification.

Step 5: Install

Installation

I placed my unit on the patio cover. This turned out to be a great location as it is somewhat protected from the wind and sun, and I can get to it with a small step ladder to check and clean out any debris that may clog the drain hole.

Platform

The Idea of the platform is to provide a level base for the unit. I made a simple platform out of scraps of wood. To level I put the 1x2 under the plywood base and adjusted until level then screwed it in. I helped the roof was level in the other direction already. I painted the wood with some spray enamel, but as you can see it has since weathered off. I should repaint it.

Rock

Oh yeah, the “Weather Rock” was a late addition, dog approved. The bucket tipped over in a 40+ knot gust one night. I added this for a little more mass and I did not want to attach onto the roof to hold it in place. Watch out! This is optional and I would not recommend standing under it during high winds witout a helmet, the rock may roll... ( Maybe I should use a weather brick? ☺)

Step 6: Installation and Test

Wait for Rain

In Southern California it can be months between measurable rains. Fortunately, the first storm of the season the year I did this was only a few days after installation.

Here shown is the first recorded rain event. This modified unit works even better than I expected. The 0.09" was the exact amount reported from the official total taken less than 2 miles from this location, not the 0.02" per tip as the Acurite documentation stated which would have meant this would have been the indicated count for 0.18" So this means the bucket volumes are compatible!

Had they not been this is where the calibration would have happened, but luckily, these are the same volume per tip as the Davis' original. Calibration is a little complicated and time consuming.

Up and Running

This particular set up has been further verified with a Davis Vantage station that has been added to the family room since this was done, the count is within one tip of the other station located a couple hundred feet from this sensor.

Backup Plan

I found this other sensor that is wired. I think it will work directly with the same cable wired across its reed switch.

Any clue as to why there are two leads for each side of the reed switch?
<p>Good question. </p><p>My guess would be to decrease the electrical resistance between the digital input and the switch. But it is probaily overkill for a few tenths of an ohm as It seems to work just fine on two wires. But in this case it It sure helped to have both wires to solder to those wires strands were really fine.</p>

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Bio: I have been making gadgets ever since I can remember. One of my first projects I remember involved destroying my poor mother's prized tube ... More »
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