Elektor_Mag_-_January-February_2021

lektor January & February 2021 9

attractive, it also makes it easier to recognise objects in the image. The

range of the false colour gradient is also extended somewhat beyond

the minimum and maximum temperature values, acting like a magni-

fying glass over the temperature readings (Figure 2).

The AMG88xx thermal sensor

To capture thermal images, Panasonic has developed a powerful

thermal MEMS (Figure 3) available in two variants covering different

temperature ranges. The very tiny package includes the optics, the

thermoelectric transducers, analogue-to-digital conversion and signal

conditioning [3]. The AMG8853 covers the range from 0°C to 80°C

while the AMG8854 covers –20°C to +100°C, with maximum error

specifications of ±2.5 K and ±3 K respectively. Both are accessed

over an I^2 C bus. The absolute accuracy is not outstanding, but the

device is nevertheless perfectly good for qualitative assessment of

relative values.

With a couple of lines of software it is possible to calibrate the readings

from the device that increase the accuracy of the results and reduce

noise. The sensor is available in a space-saving SMD package that is

designed for reflow soldering. For our application we use it mounted on

a small, home-made breakout board that also includes a few passive

components (in easily hand-solderable 0805 packages) required for

decoupling the power supply and pulling up the I^2 C bus lines. Pads on

the edge of the board carry the +5 V and GND power supply pins as

well as the three signal lines required (INT and the I^2 C bus). This allows

a straight or right-angled pin header to be fitted. Figure 4 shows the

circuit diagram of the breakout board and, for interest’s sake, a little

As well as instruments that measure temperature by direct contact
there are also contactless sensors that measure infrared radiation
from objects, thus determining the average temperature in the field of
view. This is done by using the pyroelectric effect where the electrical
potential of electrodes in a polarised crystal change when exposed
to thermal radiation. This effect can be exploited using electronics [1].

The project
MTheCam takes measurements up to five times per second simul-
taneously from 64 points arranged in eight rows of eight columns.
Similar to an ordinary camera, these are arranged to have a 60° field
of view. Each point can detect a temperature between 0°C and 80°C
(or alternatively from –20°C to +100°C). The individual readings are
displayed using a gradient of colour values, resulting in an image with
very low resolution. This image can be served as a web page over
a wireless network so that it can be displayed on a smartphone, for
example. The gaudy but chunky image is certainly not reminiscent
of HDTV but it does clearly display hot and cold spots in contrast-
ing colours that distinguish them from surrounding objects. As well
as this false-colour image, the temperature of each pixel is shown in
degrees Celsius, allowing for a more precise analysis (see Figure 1).

Furthermore, the 64 readings can also be requested in JSON format,
making it easy to share the information with other applications.

A little bit of mathematics lets us give the illusion of a higher resolu-
tion than provided the 8-by-8 pixel matrix. Bicubic interpolation [2] is
used to create a ‘fake’ 32-by-32 pixel image that not only looks more

Figure 1: Screenshot showing the eight-by-eight
grid of pixels.

Figure 2: Screenshot showing values

interpolated in a 32-by-32 grid.

Figure 3: The Panasonic AMG88xx sensor.