Design Con 2015

Tryout and teardown: Wireless temperature/humidity module

-September 30, 2013

Fall is here, and that means heating season will soon arrive. If you have an acoustic guitar, you know that humidity -- or lack thereof -- is the enemy.

That's why I keep three plastic film canisters with sponges in my case to keep the humidity up. But the sponges dry out. Wouldn't it be nice to know when the sponges need more water without having to open the case?

That's where a wireless temperature/humidity logger can help. The WiFi-502 temperature/humidity logger from Measurement Computing has 802.11b WiFi and USB connectivity. Having an evaluation unit on hand, I decided to use it.

The Wifi-502 comes with a micro-USB cable, which you can use to perform initial setup once you download and install the EasyLog software. Installing the software and connecting to my XP laptop was a breeze. The installation needed Microsoft .NET Framework 2.0, which downloaded and installed automatically.

With the software installed, I connected the logger using the supplied micro-USB cable. The system found the new hardware and connected to it. A setup screen then appeared that let me select temperature units, set high and low alarms (for both temperature and humidity), and set a sample rate and number of samples per transmission from the logger to the host computer. The default is 10 seconds between samples and six samples per transmission to the host. Temperature range is -20°C to +70°C.

Once you establish a wired connection, you can set up a wireless connection to your local network and disconnect the cable. Once the wireless connection is established, you can configure the module without the USB cable, which becomes a charge cable only. (The cable works with an iPhone charger.) Figure 1 shows the logger and the initial PC screen, which includes a live image of the logger's display. You can run multiple loggers with one network.

Figure 1. A networked temperature and humidity logger transmits data to a computer on the network. The difference in temperature between the logger and the computer occurred because the logger doesn't transmit in real time.

 

Figure 2 shows the operating screen. The last transmitted temperature and humidity, the WiFi signal strength, and the state of the battery charge are visible. Having battery charge visible is a huge help. Other icons let you mute the audible alarm (for temperature and humidity) and return to the settings screen. To view and save data, click on the View Graph icon.



Figure 2. The operating screen shows the latest readings and it provides links to graphing and alarms.

When you click on the View Graph icon, another window opens that lets you select a date range for viewing. A list of date ranges let you select which range to view. The graph (Figure 3) has a vertical line that moves with your mouse. Under the graph, you see the temperature and humidity at the selected time. You can save the data in a text file for later use. If you export the data, Excel will automatically open and you'll get two sheets, one with a graph and one with the raw data.


Figure 3. The graphing screen lets you find the temperature an humidity at any point in time.

 

Open it
Having shown that the WiFi-502 module works, I opened it using an Allen wrench. See Figure 4. The module uses a GainSpan GS1011MIPS WiFi interface, which provides 802.11b wireless connectivity (see Figure 5). The GainSpan board mounts to the motherboard and includes the etched copper antenna (looks like a square wave).


Figure 4. Inside the logger is the Wifi adapter, microcontroller, and connections to the battery and sensors.

A 14-lead flexible printed cable connects the motherboard to the display. The micro-USB receptacle is on the left. The microcontroller is an STM32L152R8T6 from STMicroelectronics.

Figure 5. The Wifi adapter module mounts to the logger's main board.

Note the connector in the lower left. It connects to a 4-lead ribbon cable that goes to a sensor attached to the back cover. That's the humidity sensor assembly. It's quite sensitive as the measured humidity rises quickly when you place the logger in your hand.

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