Teardown: Mi Thermometer/humidity sensor

Many people know Xiaomi for their smartphones. They are becoming more and more popular also in Europe. However, in their home market in China they sell also smarthome components. As many of these systems they use a central hub that connects to and controls sensors and actuators. One sensor that I personally found very interesting is the Mi temperature/humidity sensor. With it’s diameter of only 36mm, you can install it almost everywhere.

One thing, you will notice is that the build quality is very good. The case is plastics, but still feels very well made.

Is this sensor locked to the Xiaomi smart hub or can you use it also in DIY environments? Let’s open it and see how it works.


The device is powered by a small CR2032 cell. Let’s hope this one lasts at least a year. But let’s check the main board which will tell us more about how this thing works:


It also looks very well-designed and assembled. The main processor is a JN5169 from NXP. It is a Zigbee controller. Do you remember Zigbee? While today most new designs use Bluetooth LE, Zigbee is still there. And it works quite well, so why not use it. The only thing that makes it a bit harder to use is the fact that no PC comes with an integrated Zigbee interface. That means you need to have an additional Zigbee controller connected to your PC, Raspberry Pi or Arduino to read data from this sensor. Is it easy to integrate? We don’t know yet, but we will look into this in the future.

Teardown: Broadlink RM Mini 3

You might have read already, that I don’t like the Broadlink products as they do not offer any open APIs. But maybe, they are at least hackable?

Let’s check the RM Mini 3 Infrared sender/receiver.

Taking of the cap shows us the infrared LEDs and a single IR receiver:


This is what I would have expected. Are there any surprises on the main board?


Not really. The main processor is an Marvel 88MC200. This is a simple Cortex M3 based micro-controller that is clocked at 200 MHz. There is no need for a more powerful processor in this device. As it doesn’t have a WiFi controller integrated, there is a second controller on the board: the 88W8801 is a Marvel WiFi chip.

While in theory it might be possible to upload a different software onto the system, it will be quite complicated. You don’t want to fiddle around multiple days to re-use a 20$ device? The easiest way to re-use it is removing the whole controller, replace it with an ESP8266 and put your own firmware onto it. However, for this you don’t need to buy this device first. You can just directly connect some IR LEDs to an ESP8266 chip. This will be cheaper than the RM Mini 3.

Teardown: 433MHz window sensor

We’ve had a look at at 433Mhz window sensor before, but are they all the same? Let’s check another one:


Ist this one different? It definitely is a bit larger. Why?


It runs on AAA batteries. That makes replacing the batteries easier and most likely cheaper than the special 12V battery we saw in the other sensor. If you want to use it as a security device, it is also nice to see that there is a switch connected to the case. If somebody tries to open the device, it will also send a message.


On the back there aren’t any surprises. Just the transmitter chip with a few external components and an LED.

The protocol used for the 433MHz RF transmissions is trivial. There is no encryption. This means it is extremely simple to record the signal from the sensor and play it back again. It depends on your use case, if this is a problem.

As the module tested before, this one also only sends a signal when the window opens, not when it closes. Therefore it can’t be used to detect if a window is open or close.

Measuring air quality

When people think about comfort in their apartment, most people think first about temperature. But another important factor is air quality. I don’t have to tell you that smells from cooking aren’t always the best. There are much more particles in the air that impact the quality. These particles are generally named “Particulate matter”. The EPA defines PM as follows:

“Particulate matter, or PM, is the term for particles found in the air, including dust, dirt, soot, smoke, and liquid droplets. Particles can be suspended in the air for long periods of time. Some particles are large or dark enough to be seen as soot or smoke. Others are so small that individually they can only be detected with an electron microscope. ”

Today, most measurements you find are PM2.5 measurements. This measures particles smaller than 2.5µm. Roughly one out of every three people in the United States is at a higher risk of experiencing PM2.5 related health effects.

Do you know the PM2.5 values in your living environment? Most likely not, but it is relatively easy and cheap to do do these measurements today. While calibrated sensors is still very expensive, for home-use uncalibrated sensors do a good job to give you a rough idea about possible problems.

These sensors work as follows:


A light sends light into an air flow. Small particles will reflect the light. A receiver than measures these reflections. Seems very simple – right? It really is. The cheapest sensors are available for less than $20 on Chinese shopping sites. They use LEDs as a light source. Laser-based devices are a bit more expensive ($50 and up), but they can detect smaller particles.

Let’s have a look about a laser-based sensor like this:


The fan creates an continuos air flow through the device. Looking into the device you see the air channel and the detector.


The detector looks like this:

What else is inside?

This is the simple PCB. It is based on a small microcontroller and some external components.

The good thing about these (more expensive) sensors is that they already do the hard work for you. They have a serial output that already sends the PM1.0, PM2.5 and PM10 values.

You can use a simple USB-to-Serial converter and read the data directly from your PC. How to connect this sensor to the adapter is shown here.


What’s inside a cheap Chinese LED light?

LED lights have become cheaper and cheaper in the last years. But they are still a bit more expensive than Halogen lamps. To save money, some people might have a look at Chinese shopping sites. I wanted to know what you get if you pay less than 5$ for an LED lamp.

My delivery arrived like this:


Yes, it fell apart already during shipping. It consists of some PCBs with LEDs soldered together by hand and a cheap plastic cap. You don’t really want to use something like this on mains voltages – right?

However, this gives us the chance to look what’s inside of this thing.


Ok, this is really the simplest circuit you can think of. You see a suppressor capacitor (the red one), a bridge rectifier (the small black part with the 4 pins), a small 4.7uF electrolytic capacitor and a few resistors.

The two 51Ohm resistors are connected in parallel and they limit the current flowing through the LEDs.

Better designs use switching mode power supplies with a fixed current output. However, you won’t get this if you buy the cheapest stuff on the internet.

Especially the fact that this ding falls apart on the slightest touch and every part of the circuit runs at mains voltages makes them extremely dangerous. After the the rectification you can expect almost 300V DC on the board. This is even more dangerous than 230V AC.

Do not use something like this!