Learning the hard way that plastic TO-92 is affected by humidity

I've had my APRS temperature station running since 2012. It is based on a DS18S20 sensor that just sticks out of a window. However, after some time it started to give too high readings, and after having replaced it several times I found out that I needed to waterproof it better. It seems that the TO-92 plastic housing, shown here to the right, somehow was influenced by humidity.

I wasn't able to find waterproofed DS18S20 sensors, but I could find ones with DS18B20. I got one and connected it, but it only showed -1 C regardless of weather. The software in the Opentracker USB was not able to read it, although I have Arduino programs that can read both.

Series capacitors that failed according to the book

0.33 uF X2 capacitors which measured only
0.097, 0.1, and 0.118 uF.

Many devices now use a capacitor power supply thus saving the space that a mains transformer occupies. The principle is that a series capacitor from the mains supply is used to drop the voltage and reduce the current. Provided that the circuit is completely isolated from human touch, this is an economical way to provide DC power.

The image shows three such capacitors as I were measuring them. They came from three malfunctioning devices in my home: two wall-mounted thermostats for floor heating and a remote controlled mains switch.

Their power supplies were designed with a capacitor of 330 nF in series with a bridge rectifier which supplies the low voltage DC. This value is typical, it seems, for 230 Vac, 50 Hz circuits that are designed for about 20 mA. The value will be higher for an equivalent 115 Vac, 60 Hz circuit.

The malfunctioning happened because the value of the capacitor in my cases was reduced to 1/3 and less of the nominal value. These capacitors are all marked X2 and a voltage of 275 Vac.

The X2 means that they are safety capacitors which will not fail by short-circuiting as this would be a fire hazard in this circuit. They have self-healing properties and that means that they fail by "burning away" on their own foil, leading to a reduction in capacitance and eventually failure of the circuit as the power supply cannot supply the required current any more. They should never be replaced by anything but X2 capacitors with the same or higher voltage rating.

Go to the Wikipedia page Capacitive power supply for more description of this circuit.

By the way, the devices which these capacitor came from were 15 year old Microtemp MTN-1991 thermostats and a 20 years old Nobø System 500 RCE 512 remote receiver. They now all work again thanks to the fitting of new 0.33 uF capacitors. And all of them are safety capacitors of type X2 of course - no gambling with safety here.

Temperature compensation for an Arduino ultrasonic distance sensor

161.3 cm

27.0°  347.7 m/s

Ultrasonic distance sensors can find the range out to 2-4 meters and are popular in e.g. robotics. Here I look at how the accuracy can be improved by compensating for the variation of speed of sound with temperature. It actually varies quite a lot in air and around 0 C it is:

      c = 331.3 + 0.606 * T

where c is in m/s and T is in C. The formula is good to up to at least +/-30 C. There is also a dependence of humidity, but as it is so small it is neglected here.

The equation can be analyzed for sensitivity (a little bit of differentation, you know). The result is that a two-way range measurement creates an error of 1.8 mm/C/m. That means that with a 4 degree C error, the deviation will be 14.4 mm at a range of 2 meters. Not a lot, but more than the wavelength which is 9 mm at 40 kHz. Considering how easy it is to compensate for, then why not give it a try?

Twine "Internet of Things" Monitor

My children know what kind of gifts that please their father, so they gave me a Supermechanical Twine for Christmas. It is described in this way:

Want to monitor things and environments remotely without a nerd degree? Maybe you want to get a tweet when your laundry's done, an email when the basement floods, or a text message when you left the garage door open.

I am presently using it to monitor the temperature in the outer part of the basement, where there is a chance of freezing when the outside temperature drops below about -15 C (5 F). I can continuously read the status such as the temperature on any web browser on my computer or mobile phone. I can also set threshold values that trigger an email message.

The Twine is easy as a breeze to program, by using simple rules as shown below.

Half a year of APRS temperature monitoring

My APRS-based temperature monitor has now worked flawlessly for half a year. APRS stands for Automatic Packet Reporting System so it can send much more than temperature data, the chief usage is really for GPS position reports.

But I just needed a temperature monitor and here are the readings for December. As one can see, there were no days with temperature above freezing. At 800 m elevation in the mountains of Telemark in Norway, this is not unexpected for this time of year and makes for good skiing!

I use a Quanzheng TG-25AT handheld with a quarter-wave whip antenna on 144.800 MHz. Its signals reach the LD3GT digipeater at 1845 m above sea level. Although I don't have direct line of sight, the low power (1 Watt) setting is adequate as the distance is only 10 km. The APRS-beacon is an OpenTracker USB set up for transmission every 15 minutes. An external DS18S20 temperature sensor which measures the outside temperature is connected to the 1-Wire® bus of the OpenTracker USB.

Thanks to the infrastructure providers: The Tønsberg group of NRRL (LA1T) who operate the LD3GT digipeater, probably the one with the largest coverage in Southern Norway (Gaustadtoppen). Thanks also to the various operators who receive packets from LD3GT and pass them on to the internet, and thanks to aprs.fi for processing and displaying the data on their excellent web site!