Keep your PC cool this winter

An accurate PC thermometer you need all year round

Award-winning designer Alberto Ricci Bitti designed this simple microcontroller-free DS1621 PC thermometer that requires no calibration. It's so cheap and simple because all you need is the sensor IC, a voltage regulator and a handful of diodes and resistors. It can be plugged into any free serial port and the temperature is shown on the Windows taskbar. Lucky for us, Alberto's friends all asked for a PC thermometer of their very own, so he decided to release the build instructions into the wild. A cheap and simple PC thermometer? Now that's hot. Or should we say cool?


PC thermometer

So cool you can measure it!

This 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's 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

Since 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.

View circuit schematic

Making your PC-therm, SMT style

Building PCtherm is easy. I'll detail the version using surface-mount parts. Those not familiar soldering oh-so-tiny SMT parts will be happy to know that a PCB layout for through-hole parts is also available.

First step is to collect all of the parts except the PCB.

And this is the enlarged assembly view of the SMT board (the small PCB is the optional outdoor sensor).

View SMT board

As soon as I collect all of the parts, I make a real-size printout of the PCB in order to verify dimensions of all parts against it. Should a part be too big or small to match its PCB pads, I can adjust PCB layout or try to find a compatible part before the hard work starts.

Once all parts are verified, I prepare the PCB. As the SMT layout is single-sided, you can easily etch one yourself. It takes less than an hour and no special materials with the method described here.

The PCB must be spotlessly clean (no trace of oxidation of fingerprints) in order to etch and solder well. Clean it with a mild abrasive until shiny (kitchen scrubs, steel wool, and even a pen eraser work).

Remember to use the mirrored track layout for toner-tranfer! I love SMT prototypes because there aren't too many hole to bore before soldering begins.

Soldering requires a fine-tip iron, sharp tweezers and a steady hand. I tape the board to the desk to keep it in place while soldering. Actually, I tape it to the parts layout printout so its easier to check during soldering.

In order to avoid inadvertent swaps, keep parts in their original packaging until needed. I suggest you to start soldering smaller parts (resistors, diodes...) and to end with the cumbersome ones (electrolytic), as tall components can make difficult to access and solder small part's pins.

Don't apply too much solder, and be careful not to overheat parts (especially diodes and ICs). If necessary, let the part cool down before reworking. Most parts are polarised so be careful not to reverse them. Diode's cathode (K) is marked by a black ring, and electrolytic capacitors negative pin is designated by a black strip. If you prefer to use tantalum capacitors, remember that their marking is reversed, with the black strip designating the positive leg!

Those inexperienced with hand soldering SMT parts might be concerned soldering sensor ICs.
I clean soldering iron tip before each solder joint, and use smallest diameter soldering alloy in order to apply as little solder as possible. I start applying a very small quantity of solder on the pad designed for pin 1 ONLY.

I place the IC over the pads, and when all pads are perfectly aligned I clean the tip and put it on pin 1 until it gets soldered. I verify that the IC is still correctly positioned (all pins centred and touching their respective pads). If it moved, I heat up pin 1 and restart, otherwise I continue soldering remaining pins, always cleaning soldering iron's tip before each joint and applying very little solder. Last step is soldering again pin 1 as its initial joint was made with very little solder.

The LM2936Z5 voltage regulator needs special preparation for soldering. I had a through-hole part at hand, but I wanted it to solder on the SMT side of the board. The picture should make it clear how to bend and cut its pins for this purpose.

The PCB is designed to fit between the pins of the serial port connector. This is the last part to solder. Don't forget to bridge its pin 7 and 8 on the opposite side of the PCB.

I usually clean residues of solder flux with a solvent like acetone, letting the circuit to dry completely before powering it. Once the board is tested and working, I apply a coat of spray clear varnish to protect the copper from oxidation.

Last step is to download and install the software (zip file).

Tech facts:

• 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 (good for Excel)
• 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
• no calibration required
• full source code available for free (educational and non-commercial uses only)

Quantity RS Part # Part description
1 294-0994 Digital temperature sensor
1 533-5604 Ultra low dropout voltage regulator*
1 652-7494 Small signal diode
1 544-9753 5.1 Volts Zener diode 0.5 W
1 537-0180 Electrolytic capacitor
1 434-8104 Low voltage ceramic capacitor
1 223-2350 0.25W resistor
1 480-3976 9-pin female connector

* Not an SMD product but a TO-92 Package



  1. Alberto Ricci Bitti

    Maxim suggests replacing the DS1621 with the DS1631, which includes the lead free variant. By the way, the DS1631 can be used as well in the PCtherm circuit.

  2. I / We have used the DS1621 for a considerable time now, but I believe that they have been / are being, withdrawn from the European market as they will NOT be offered in lead free variant. This is true of several of the Dallas (Maxim) i/c’s which can make standalone temperature measurements etc — very annoying.

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