So cool you can even measure it!
incredibly simple thermometer plugs on any free serial port. Does
not make use of any programmable components as microcontrollers.
It gives temperature readings accurate to 0.5°C with no
calibration. It's cheap, so I've put one on any PC I use. And it
is so nice to have the temperature shown on the Windows taskbar,
that a million friends asked me to build one!
Build yourself an accurate thermometer
I have no time to build a million pcTHERMs, I give you the plans
and the software to build one on your own.
This project is easy enough for beginners, the only difficulties
possibly arising from serial port hardware incompatibility from PC
to PC. In the single-sensor version, you need only the sensor IC,
a voltage regulator and and handful of diodes and resistors. Build
it, and learn the secrets of IIC bus, how to implement IIC bus
using only two resistors and a couple of zeners, how to drive it
on a serial port using Visual Basic .Components involved are
available on the worldwide RS-components catalogue.
displays both indoor and outside temperature on the Windows
taskbar (see figure)
plugs in any free PC com port
range -20 ... +125°C (-4 ... 257°F)
basic accuracy and resolution 0.5°C
Centigrade (°C) of Farenheit (°F) scale
data logging on easily readable text file
sampling rate 1, 5, 30 or 60 seconds
one or two temperature sensors (upgradeable up to 8)
com port powered, no external power supply required
easy to build, no exotic nor programmable parts inside
full source code available for free (educational and
non-commercial uses only)
2 x 1N4148 diodes
2 x 5.1 volt 1/4W zeners
2 x 4700 ohm resistors
2 x 100 nFcapacitors
2 x 47uF 16V el.capacitors
2 x DS1621 digital temperature sensors (Dallas Semiconductor)
1 x LM2936-z5 low-droput 5V regulator(National Semiconductor)
1 x DB9 female plug.
How it works
circuit is derived from the Claudio Lanconelli's PONYPROG
programmer (I recommend you to visit the Claudio Homepage at
http://www.cs.unibo.it/~lanconel ). The key component
is the Dallas Semiconductor's DS1621 temperature sensor. This tiny
8 pin IC needs only +5 volts to measure the temperature and to
send it out via its IIC bus output. Since many IIC bus devices can
be connected in parallel, three address inputs (A0, A1, A2) are
provided to select one out 8 addresses the device will respond on.
This way, up to 8 sensors can be connected in parallel. I have set
the internal temperature sensor to address 0 and the external one
to address 1. If you plan to use only one sensor connect it as
Interfacing the IIC bus to the RS232 com port is a matter
of adapting levels. IIC works on 0..5V signals, RS232 uses -12V ..
+12V. The trick here is that, altough specified for -12V..+12V,
almost all PC com port I know work equally well with 0..5V
signals. This eliminates the need to raise the IIC output to RS232
levels, and the SDA data line connects directly to the PC CTS
line. On the opposite way, the RS232 signals can damage the IIC
inputs, so I placed voltage limiters (R1, DZ2, R2, DZ1) on
the SCL clock input and SDA data input. (note that SDA is
bidirectional: receives from the DTR line and transmits to
the CTS line).
Since the circuit draws very little current, there is no
need to add an external power supply. The +12V from the RS232
lines are conveyed to the regulator by diodes D1, D2, filtered by
C1 and regulated to +5V by the LM2936-Z5. Don't replace it with an
ordinary 78L05 regulator unless you want to add an external 9V
battery: the LM2936 is capable to regulate even with input
voltages near to 6V, as is the case of many serial ports.
the pcTherm window expanded
the software in Visual Basic 5 (Yes, no assembler or C++ this
time!). I've done it the straight way, with no optimizations that
would make it less readable. Even with this limitations, the IIC
runs at a respectable 1,5 kHz even on a slow P90 in interpreted
mode. The very first time you run the program, you will be warned
that the setup file does not exists (it will be automatically
created at the end of the session) and defaults will be used. When
you start the program, it runs minimized on the taskbar, providing
a "temperature icon". This is my preferred way to use it, just
like the "clock" icon provided with Windows.
Clicking on the icon opens up the setup window, allowing you to
select one of 4 com port, the number of sensors attached, the
interval between successive measurements, the measuring unit and
if you want to log temeperatures on the file "pc_therm.txt". If
you select the wrong port, you can get both false measurements or
a "unable do read" messages.
Visual Basic Sources (VB5, 6kB) for those who
have VB installed
Executables, reduced size (13 kB)
pc_thermometer.exe only, does not include the VB runtime and
system DDL's, copy to a folder and run pc_thermometer.exe. Works
on many machines with others VB5 programs installed
Executables, full size (1,5 MB)
setup.exe utility and everything but the sources
Be sure to check the
FAQ page for further information about the thermometer.