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My TX4 Wireless Sensor Transmitter replaces the La Crosse TX4U wireless sensor which is no longer made.

 

I bought a La Crosse WS-9025U Weather Projection Station from Costco in October 2003 for $43.29 . It projects the time and outdoor temperature on the ceiling, which I like. It also receives WWVB to set the time. My WS-9025U spent its first two years in the mild climate of San Jose (California) and then 13 years in the Virginia City Highlands in Nevada where the temperatures can go from 0 degrees (F) in the Winter to 100 degrees (F) in the Summer.

 

Three years ago the outdoor sensor started eating batteries faster than it used to. Then I started getting an error message of OF.L for the outdoor temperature. That meant that the temperature/humidity sensor was bad. The transmitter part was still working but the data was bad.

 

La Crosse does not make the TX4U sensor anymore. For awhile they said to use the TX7U. Now their Web site doesn’t mention it at all. Besides, the TX7U Sensor cost too much ($28.65 on Amazon) and has gotten some bad reviews.

 

La Crosse doesn’t make the WS-9025U anymore either. Companies (including La Crosse) make new units that do projection but they get generally bad reviews. And they are ugly.

 

The sensing element in the TX4U is unknown so replacing it was not an option. So I decided to see if I could make my own TX4U replacement.

 

Most of the work on decoding the data protocol was already done by others. Most people who were interested in reverse engineering the La Crosse TX4U Sensor were interested because they wanted to be able to receive its signal, decode it, and use the temperature and humidity in their own projects. I found this explanation of the data essential to making my own: http://www.f6fbb.org/domo/sensors/tx3_th.php. The Webmaster is Jean-Paul ROUBELAT - F6FBB. Check out his home page: http://www.f6fbb.org/

 

Since I wanted to make the wireless sensor I had a small amount of work to do. Although my La Crosse TX4U went bad it was still transmitting, only it was transmitting bad data from the bad sensor. The La Crosse used a separate transmitter module so it was easy to find the pin with the data on it. I used my $10 logic analyzer USB pod. On eBay search for: 24MHz 8 CH USB Analyzer 8 Channel Logic Analyzer. It works really well with this open source software: https://sigrok.org/wiki/Downloads.


I am posting one of the screens shots of what I found the TX4U doing. When the TX4U sends data it sends two identical packets of 44 bits. I will call it a data frame. Power-Up starts with a pulse followed by two frames of data. The Power-Up pulse lasts 114 ms. There is much more but since I did it, you don’t have to.

 

The TX4U operates at 433 MHz (technically 433.92MHz) and uses OOK (On-Off Keying) which means that the transmitter is turned on-and-off to transmit data. That is opposed to protocols such as FSK (Frequency Shift Keying) where the carrier is always on during the transmission but the frequency is shifted to transmit data. OOK systems are less resistant to noise than FSK but the transmitters and receivers can be simpler and therefore cheaper. Because of the use of 433MHz for so many things you can buy cheap transmitters and receivers on eBay. Many are sold for use with the Arduino so you can have your own cheap (but slow) RF communications link.

 

The first transmitter I used was the STX882. The STX882 is rated to work down to -20 degrees C (-4 degrees F). It didn’t. It stopped working below -3.8 degrees C (28 degrees F). I tried several. No joy. A similar transmitter module (maybe the same but older version) is the FS1000A.

 

Since that didn’t work I am using the RFM68HW made by HopeRF (https://www.hoperf.com/). They make several versions of the RFM69. I chose the RFM69HW (433 MHz version). The pins are on 2.00mm centers (not 2.54mm = 0.1”) so you need 2.00mm headers. The RFM69 is a transceiver for sending and receiving digital data packets but can be configured for OOK. For complete info for the RFM69HW go here: https://www.hoperf.com/%20modules/rf_transceiver/RFM69HW.html

 

For the sensor I used the Bosch BME-280 which measures temperature, absolute air pressure, and humidity. (I have experience with the Bosch BMP-280 sensor which measures temperature and absolute air pressure.) I have had two problems with the BME-280.

 

1. Several times when I have bought the BME-280 on eBay the sellers have sent me the BMP-280. The way you tell the difference is that the BME-280 is square with the hole on the left while the BMP-280 is rectangular and has the hole on the right. They also have different device IDs which are programmed into the part and which you can read. I have written a more complete explanation in the article on my Web site. www.jmargolin.com/newprojects/mytx4/jm_mytx4.htm

 

2. I have recently checked to see what they cost (for this article in November 2021) and they have doubled or tripled in cost on eBay. Now they cost too much.

 

If you don’t need the humidity you can use the BMP-280. Then if the price of the BME-280 comes back to Earth you can use that one.

 

The skill level for stuffing the board is: Intermediate. Make sure you use a temperature-controlled soldering iron. I use a temperature of 340 degrees Celsius.

 

To compile the source code and download it into the microcontrollers I use Texas Instruments Code Composer Studio. It is free and is on this page: https://www.ti.com/tool/MSP-EXP430G2ET where you can get the versions for Windows, Linux and macOS.

 

The direct link to the current version for Windows is:

https://software-dl.ti.com/ccs/esd/CCSv10/CCS_10_4_0/exports/CCS10.4.0.00006_win64.zip .

 

The skill level to use Code Composer Studio is: Advanced. (Maybe Really Advanced).

 

Notes:

1. The antenna is a piece of #20 solid wire 6.50” long (? wavelength at 432.9 MHz.) Add 1/8” to solder it to the board.


2. My TX4 runs on a 18650 Lithium-Ion rechargeable battery. With the batteries I buy on eBay it lasts 6-8 months. To get longer battery life you could put two in parallel. Or you could use a small solar cell with a battery-charge IC.

 

 

3. To program the MSG430G2402 you connect the MyTX4 board to the Launchpad with three jumpers: GND, TEST, and /RES. To make it run standalone remove the three wires and use a header plug to connect the /RES and /Reset pins on the header (on the MyTX4 Board).

 

The MyTX4 board can be used for other things

 

A. Programmer with Zero Insertion Force (ZIF) socket.

 

I have some other projects that do not have the Launchpad port on them. When you plug a part into the Launchpad to program it you risk bending a pin. When you straighten the pin it almost always breaks off. You can stuff a Textool ZIF in the MyTX4 board. It is a tight fit but it does fit.

 

Stuff C6 bypass capacitor, three of the pins of J3, and two pins of J5. (If you stuff two pins on J5 you can solder it without burning your finger.)

 

It connects to the Laumchpad with four wires: GND, TEST, /RES, and 3V23.

 

 

B. Breakout Board For The RFM68HW

 

Stuff the RFM68HW on the board. Stuff the C6 bypass capacitor. Stuff two 1x10 headers instead of the socket for the MSP430G2402. This gives you access to the following RFM68HW pins: GND, VCC, RESET, the SPI port and DIO2. If you need more of the signals you can connect them with jumper wires (like #30 Wire Wrap wire) to the header in the MSP430G2402 pads.

 

I have posted more information (which you need) and more pictures on my Web site: www.jmargolin.com/newprojects/mytx4/jm_mytx4.htm

 

 

Happy MyTX4’ing

 

 

Jed Margolin

Virginia City Highlands

Nevada

11/15/2021


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