While the process should be similar for almost all RF thermometers, the specific model I'm hacking is a "Maverick RediChek Remote Wireless Smoker Thermometer Model ET-73". It can be purchased from my Amazon store here: http://astore.amazon.com/johspro-20/detail/B0000DIU49
This is functionally a fantastic remote thermometer. It has two temperature probes (one for food, one for smoker) with completely independent alarm settings. It's consistent downfall (as you may find from numerous user reviews) is range. Maverick claims 100' which I have verified to be true providing both the transmitter and receiver are outside and in line-of-sight of each other. As soon as you step in the house (or even behind a tree when far away) the signal is either blocked or range is drastically reduced. Other than that, most people seem to like it. This hack solves that problem.
Hopefully this will help some of you out. From start to finish this took me about 30 minutes. I would expect it to take 1 - 2 hrs if you're not familiar with the parts or basic electronics hacking.
Note: For those of you already familiar with "rev 1" of this Instructable, "rev 2" adds the following:
- Found and implemented ideal antenna length - 6.7" (based on whitepaper found here)
- Added antenna tube cover to protect exposed wire (and add to overall durability)
- General clean-up of the overall Instructable
Step 1: Gather Materials and Tools
a.) 6.7" of 22 gauge copper or steel wire
The antenna will require 6.7" of 22 gauge copper or steel wire which is commonly found at craft stores and used for flower arrangements. 22 gauge is perfect because it will bend and stay in place. You can coil it up for storage and point it out straight when in use.
Why such an exact length? Because antenna length is directly tied to the operating frequency (433.92Mhz) in which the device was designed for. There is a specific formula used to calculate the length which is what I used to derive 6.7". You still may get improved performance with a longer or even shorter antenna, but it won't be maximum efficiency.
Ideally the wire should be non-coated. If it is coated (as mine was) you will just have to sand the paint off one end for soldering.
If you need to buy something I would look for 22 gauge non-coated copper wire, something like this should work great:
b.) One Antenna Tube
In addition to making the project look very professional, the antenna tube protects your wire from breaking off, reduces strain on the PCB, and helps keeps water out (of the antenna hole). While any standard hobby antenna tube can be used, this Instructable is based on one from Dubro Racing, Model 2338 (Red, with cap). It can be seen and ordered here: http://astore.amazon.com/johspro-20/detail/B000BP4JC4
Finally, it is assumed that you already have a thermometer that you are willing to hack (which has obvious risks such as breaking the device if you're not careful). If you want to purchase a thermometer I highly recommend the same one I used (Maverick RediChk Model ET-73) here: http://astore.amazon.com/johspro-20/detail/B0000DIU49
2.) Tools Needed
a.) Soldering Iron (along with flux and solder)
b.) Sandpaper or some type of file (an emery board would even work)
c.) Wire cutters
d.) Drill (or Dremel Moto type tool) and small drill bit (1/8")
e.) Very small (jewelers sized) Philips screwdriver
Step 2: Remove Back of Transmitter
WARNING!!!! - Proceed at your own risk! There is always a possibility that you might break your thermometer. This will also void your warranty!
First of all, make sure you are working with the transmitter and not the receiver (see photos below for transmitter).
Remove the 6 screws on the back of the transmitter. You may not be able to take them out completely due to rubber washers on the inside of the case. Be careful not to lose the screws or rubber washers.
Step 3: Remove PCB
After the screws are loose, carefully separate the case. You will not have much room to work because of wires connecting the two halves.
Caution: The LCD display is held on by nothing other then pressure from the circuit board. It will fall out very easily after you remove the PCB so be careful not to drop it.
Remove the 4 screws holding the printed circuit board to the face plate (see screw locations in picture below).
The case halves can now be safely pulled apart and the PCB removed (again, be careful you don't drop that LCD when pulling apart. It will break very easily).
Note: You could skip this step if you feel that you have enough working room with the PCB mounted in the case. I personally wanted to remove it from the case so I could easily sand and solder it. If you want to sand and solder while in the case then skip this step.
Step 4: Find the Antenna
Let me digress for a moment...
Sometimes you search the internet for information about a product with very little luck. This was the case for for me. Nothing was available other than the product manual. When this happens it's worth while to search on the FCC ID of the product. Doing so may result in much greater detail about the product as it was filed with the FCC (circuit diagrams, block diagrams, internal photos, test reports, etc.). This is all public information but not typically indexed by search engines.
In this case I went to the FCC ID search page here:
I entered the FCC ID as can clearly be seen on the back of the transmitter (see pic below). By doing this I found the following circuit diagram which clearly confirms the arched bar to be the antenna:
In this case it's pretty clear just by looking at it but sometimes they are not this obvious. Either way it's always good to be absolutely sure before you start hacking away at it!
Note that I have another Instructable on this topic HERE (created for a contest in which I lost but was one of the finalists!)
Step 5: Modify Antenna
Now it's time to extend your antenna.
1.) Cut a piece of your 22awg wire to exactly 6.7". Through a bit of research I've determined that this is the ideal size for this thermometer which operates at 433mhz and a 1/4 wave length antenna (feel free to ask if you want more technical detail on this).
2.) Using your file or sandpaper, rough up about 1/4" of wire at one end. Don't skip this step even if you already have bare wire. It helps to remove oxidation which will make soldering easier.
3.) Using your file or sandpaper, carefully remove about 1/4" of the green coating from an area of the PCB antenna. Sand just enough so the copper becomes exposed but be very careful not to sand the copper right off.
The location you sand doesn't need to be exact but should be very close to what I have below so everything lines up correctly later. The wire will need to exit the case at a right angle from the PCB with minimal bends as further described below.
4.) Now it is time to solder the wire to the PCB as shown below. The antenna wire should leave the circuit board at 90 degrees like this __|__ (I straightened mine below as I was putting the housing back together). If you put a bend in the wire it may add impedance which can result in a drop in signal gain.
I assume you already know how to solder. Using flux helps in this case but it is not required. Just make sure you end up with a nice solid connection.
Step 6: Drill Hole and Thread Antenna
Now drill a 1/8" hole in the top of the case where the antenna will protrude out. Set the PCB in place where it is going to be (when reassembled) and eyeball where the hole needs to go. It doesn't need to be perfect but should be close to allow a 90 degree exit from the board and minimize the bending required for adjustment.
When done, carefully thread the antenna through the hole.
Step 7: Re-Assemble Transmitter
Time to put it all together!
1.) Secure the 6 back case screws with rubber washers (if not done already).
2.) Take up any remaining slack on the 22awg wire as you begin to draw the halves closer together. Carefully bend wire as needed and align so the on/off switch fits back in hole.
3.) Re-attach PCB using 4 screws (carefully align LED). You may need to give yourself a bit of slack on the 22awg wire to do this. Just push it back inside the housing a bit until you are done.
5.) Double check wires (make sure nothing broke...which happened to me more than once).
6.) Flip case over (so face is up) and insert TX push button (see pic below).
7.) CAREFULLY close the case, adjusting antenna wire as you are closing. Take your time with this step!
8.) Tighten all 6 screws on the back of case and we're ready to test!
Step 8: Testing
Now conduct a quick test to make sure you didn't break anything. With no probes attached you should see three dashes on the LCD. This confirms the LCD is correctly seated and you didn't break any of the power wires.
You should continue to conduct a full test with both probes to confirm connectivity on their internal wires.
If all is well, proceed with the next step...
Step 9: Attach Antenna Tube
Without the tube the wire is VERY easy to bend excessively. This weakens the solder joint and adds stress to the PCB which currently has no real strain relief. The tube also makes the project look very professional, as if you bought it this way!.
Stop! Don't continue with this step until you have confirmed that the thermometer is working (as noted in step 8). It will save you a ton of aggravation if you have to disassemble the unit again.
Final Assembly Steps:
- First make sure the antenna wire is as straight as possible. Gently bend to adjust if necessary.
- Remove antenna tube from package. You won't need the very small rubber piece that is used to secure small wire antennas with RC cars. You should have just the tube and cap.
- Cut the tube down to 7" in length
- Apply super glue to the antenna hole and last 1/4" of antenna tube as shown in the two photos below.
- With the glue side down, slide antenna tube down antenna. Push into hole. It should go in about 1/4".
- Apply small amount of super glue to outside top of antenna. Also apply a small drop to the inside of the antenna tube cap.
- Slide on the tube cap.
- Allow glue sufficient time to dry and you're done! (see next step for results)
Step 10: Final Results
Yeah! Were done!
If all goes well you just increased the range of your thermometer by at least 3 - 4 times. It now easily goes through walls (within reason) and you should receive a much stronger signal while in your house (pretty much anywhere). I walked around my entire house without loosing signal at all.
I hope you have enjoyed my first Instructable! Feedback for improvement is very much welcome. More to come in the near future!
Step 11: Additional Info / Possible Enhancements
Also subscribe if you want to stay posted on my future instructables as they are created!
I would love to hear from any armature, pro or semi-pro BBQers that have used this hack with success!
Thanks in advance for your vote!
Side Notes / Possible Enhancements
a.) It is also possible to modify the antenna on the receiver side for potentially even greater range. I personally didn't find that this was necessary. I'm getting fantastic range with just the transmitter modified.
Most mods for this particular thermometer I have seen to date are on the receiver rather than the transmitter. For me it made more sense to amplify the signal going out rather than increase the "listening" side for an already weak signal. Modify both sides and you'll probably get even better results but I honestly don't see any need for this unless you want to monitor your temps from VERY far way.
b.) Taking this a step further, you could also make the antenna detachable for easy storage.
c.) One other complaint about this thermometer is the location of the on/off switch inside the transmitter. You need to remove the battery back to turn it on or off. They do this to make it water resistant. You could easily relocate a switch to the outside of the case where it is more convenient. (I cover my thermometers when it starts to rain anyway - another good reason for this external antenna!). Here is one mod I found to ease access to the switch: http://www.youtube.com/watch?v=tBk5rUGQ4xU (thanks to the author for also linking back to my Instructable!)
Other Tips For Use
- Regardless of thermometer make/model, you should foil your probes! Wrap a very thick piece of foil about 4 - 5" long over the joint where your probe wire meets the metal probe tube. It should be several wraps and will be quite bulky (and ugly) when done. This area is the weakest part of the probe and it is very common for people to melt the probe with high temperature (and blame it on the thermometer manufacturer). I've melted probes myself in the past but have never lost a probe since I started using this technique.