Introduction: Cheap and Easy Digital Thermometer

I had to monitor my aquarium’s temperature. For the purpose, the idea came to my mind why not use digital table clock that can also display the environment’s temperature. Most of you will have such a clock at home I am sure.

Step 1: What You Will Need:

A table digital clock with temperature display
A male stereo pin and a female jack too (hope you will not have to buy these)
1 foot or so length of shielded cable (as used in headphones)
Heat Shrink tubes
Mercury thermometer (for comparison purpose only, no calibration reqd)
Solder wire
Soldering iron

Step 2: Open the Clock

Unscrew the clock and find out the sensor. The picture below may help you to find it. Or you can touch the component you suspect to be the sensor and see the temperature reading rising on the display. Remove the batteries once you got the clue.

Step 3: De-solder the Sensor

Yeah!!!! You’re right, this is the sensor. De-solder it and keep it aside.

Step 4: Solder the Female Jack

To make the sensor sense outside world, solder the female jack to the points the sensor has been detached from. Two inside wires will do this. And now solder the shield to the nearest negative point (i.e. battery negative). Make a hole on the back plate of the clock to fit the jack.

Step 5: Make the Probe

Using a foot long piece of shielded wire, now solder the sensor detached in step 2 at one end of the wire (the shield has no place to be soldered to at this end) and the stereo pin at other. The heat shrinks come into use at this time. Use these to separate the two soldered pins of the sensor and another larger dia heat shrink to cover the whole end so as the tip (or the body of the sensor) remains outside the heat shrink. This step, as I think, will not be a big challenge for a person having little bit knowledge of electronics.

Step 6: Finish

You are ready now. Plug in the stereo pin into the jack. Compare the reading on the display to a mercury thermometer. One good use is to place your new state-of-the-art digital thermometer on the lid of the aquarium, attach the probe to the outside glass of the aquarium using sticky clear tape and keep the temperature changes under monitoring.
Note: Since the glass is a poor conductor of heat, expect the inside water temperature to be a degree over as displayed on the clock. You may re-use your mercury thermometer to find the exact difference.