I found very nice e-shop called DealExtreme. I ordered from there a year ago several goods and was satisfied with their services. I recommend this shop for everyone! It is important to notice, that some goods are extreamely cheap not only in price, but also in quality, nethertheless, the price for quality is mostly exellent. It is also very important, that there are almost for every item client reviews, photos and videos. Also forums for items are very useful.

Cheap thermometer

At the moment I have most interested in this cheap simple digital thermometer price 2.95 USD (about 2.24 EUR).
Cheap thermometer


Measuring range -50..+70C
Accuracy 1C
Resolution -50..-20C: 1C; -19.9..+70C: 0.1C
Measuring period about 1.5 seconds (can be changed using soldering skills). I would suggest to lengthen the measuring time to keep the battery running longer.
power: 1.5V LR44, 2 in parallel, works also with one.
Case is suitable in mounting in a panel. The hole in panel should be around 27x45mm, the dimensions of thermometer panel are around: 28x48mm. Suitable panel thickness: 1.5..4.5mm.

Using in car

The first reason I want this thermometer, is to use it in my car, which came without outside thermometer. One of the problems installing this thermometer will be the power supply from car battery. I am not intrested in using batteries in thermometer when I can use power from car battery.
The voltage of 12V car battery is somewhere 14.4V.
The current consumption of this thermometer is somewhere 6uA in standby and around 10 times more in measuring cycle.
As I am not very sure about the measuring cycle max current and those currents are really low, I take that max current would be 1mA.
To get 1.5V @ 1mA from 14.4V I need a resistor (R=U/I) R=(14.4V-1.5V)/0.001A=12900Ohm. Around 13kOhm. If you would let it be only resistor in power supply chain, it would give you voltage drop in this resistor in case of sleeping period (@6uA) (U=IxR) U=0.000006Ax12900Ohm=0.0774V and resulting voltage on thermometer to be 14.4V-0.0774V :). Of course, this is not possible, as thermometer would burn out or would use whole 1mA. Who knows.
Anyway, I would not go for that. You would just need to use a second resistor in parallel with the thermometer, or even better, a RED LED (Light Emitting Diode). It has to be RED, as red diode voltage drop is around 1.5V. A cheap one is good enough. White led for example may give a 3V voltage drop which is probably too much.

So in short:
Put around 13kOhm (somewhere 16..8 should also be ok) resistor in series with a red LED so that + goes through resistor to LED anode and LED cathode goes to ground. Be sure to connect LED with right polarity (anode to the resistor and cathode to the ground), as LED are very sensitive to reverse voltage and may burn down!
If LED lights up you can connect thermometer in parallel to it, connecting + to anode and - to ground of course.
Thats it!

I am planning to connect the thermometer in ignition supply, so it would work only when engine is running. I also plan to put a light to illuminate the display so it would be possible to see the temperature at night.
I will give more information when I have done it myself. This will probably take some weeks (it is 27th of January 2011 at the moment).

4th of February 2011: I finished with this project. The thermometer is in the car, with backlight, getting supply from car battery. Here is how I did it:
First I took my old phone (which just stopped working in one good day) and soldered out 4 keyboard backlight LED's. The phone was from time when screens were monochrome and backlight green. So I got 4 green LED's. By testing I found out that 10mA is about suitable light and voltage drop would be in this case somewhere 2V. Connecting those in series I get 8V. So I needed a series resistor to get a voltage drop about 5V. I took 510Ohm. I made a frame to connect LED's on top and bottom below the LCD screen. I had to remove the mirror floid from LCD back and I also took away the front polarizer foil and turned it inside out to get black background and white numbers. In case of both foils I removed glue from glass by rubbing it with finger. I but also a white sheet of paper to the PCB to reflect light (I did not risk to put there the metal foil because it could short circuit something) and another under LCD to get better light distribution. It did not turnedf out very well, as light near LED's is much brighter, but numbers are readable.

Power supply:
I did not use red LED ad I suggested, instead I took 2V from a green backlight diode and divided it with resistors to around 1.5V. If I remember correctly, I put a 910Ohm resistor in series and 2.2kOhm in parallel with thermometer. More than some kOhm in series put the numbers blinking during the measuring period. thermometer LED's came from mobile
Picture of mobile from where LED's ware taken for thermometer lights.

thermometer lights up!
The frame for thermometer lights, lights on.

thermometer lights frame in place
Thermometer lights frame in place

The thermometer sensor:
Although I planned to extend the cable and took the sensor to outside, I did not go there. I put the sensor to the floor. We'll see how it works out. The sensor is on the metal baseof the car which should be with quite the same themperature as outside air. There are a 1.5cm felt, a 2cm polystyrene, again a 1.5cm felt and a finishing material on top of the sensor, so the inside temperature should not influence very much.
8th of February 2011: Yeah, it did not work this way. The thermometer shows 10 degrees warmer than outside. I have to put the sensor outside.

Of cource there are other thermometers with reasonable price as well:

Baby thermometer

Baby thermometer price 2.50 USD (1.87 EUR)
Cheap baby thermometer

Cheap ear thermometer

Cheap ear thermometer price 14.00 USD (10.31 EUR)
Cheap ear thermometer

Aquarium thermometer

aquarium thermometer price 4,72 USD (3.53 EUR)
aquarium thermometer

In/Out thermometer - watch

In/Outthermometer with watch price 4,27 USD (3.19 EUR)
Cheap In/Out thermometer with watch

I ordered today (28.12.2010) some of those thermometers. I also ordered some with input/output sensors as seen below in this page. Those thermometers can be used in many places. There are always to few of them :).
In DealExtreme the usual steps of package delivery are as follows:
  • Step 1 (1-5 business days): Products are collected from our suppliers.
  • Step 2 (1-2 days): Order waits to get packaged.
  • Step 3 (1 day): Order is packaged and shipped* to carrier (e.g. Hong Kong Post, EMS, UPS etc).
  • Step 4 (3-14 days): Order is enroute via EMS or Hong Kong Post.
    In same time, it is said, that during new year season december to february, the delivery may last much longer.
    The status at the moment:
    04.01.2011: Step 2. The goods should be collected and are waiting for packaging.
    06.01.2011: The shipment tracking number appeared.
    06.01.2011: The order is again in the status of "waiting for supplier". This must be meaning, that there are some goods leftp out of package and will be shipped later.
    06.01.2011: I just received a letter whichs says, that package has been shipped and estimated delivery time is 7-10 days. This confirms, that the goods will be shipped in several packages.
    07.01.2011: The second part of goods are in packaging status.
    10.01.2011: The second package got tracking number. Klicking on first package tracking number brings me to Hong-Kong Post web page where is said, that the package is prepeared to send to Estonia on 10th of january.
    11.01.2011: The second package got status "shipped" Hong-Kong Post tracking service says that the first package will be sent to Estonia on 12th of January.
    14.01.2011: Hong-Kong Post tracking service says about the second package: "Posting Estonia 2011-01-13".
    17.01.2011: Estonian Post package tracking web page says that package is in Estonia: "17.01.2011 18:08:00 Vastuvõetud Sorteerimiskeskus (PI-8 postimüük)" meaning Received in sorting center.
    18.01.2011: 18.01.2011 2:34:00 Estonian Post says: "Lähetatud Sorteerimiskeskus (PI-8 postiüük)" meaning Sent from sorting center
    18.01.2011: 08:13:00 The doorbell: postman brought the package to my home.
    25.01.2011: Estonian Post package tracking web page says that package is in Estonia: "25.01.2011 16:18:00 Vastuvõetud Sorteerimiskeskus (PI-1 sisemaised kirjad)" meaning Received in sorting center.
    26.01.2011: 26.01.2011 2:31:00 Estonian Post says: "Lähetatud Sorteerimiskeskus (PI-1 sisemaised kirjad)" meaning Sent from sorting center
    26.01.2011: 09:05:00 Postman brought the package to my home.

    Thermometers seem to be pretty good.... It takes some time to find the usage of each and one of them :). Then I can come later back to say how they perform in action.