Home > Group Projects > Colour Thermo Project
UPDATED:.21:34 05 March 2020


A compact easy to read thermometer using colours.

A common problem with thermometers is that they can be difficult to ready quickly. Even digital LCD displays with a backlight can be tricky. This thermometer uses 5 coloured LEDs to display the temperature to give a fast indication of whether it is cold, mild, cozy or hot!

The circuit runs on just 3v but with 4.5v supply the LEDs are bright enough to make a temperature mood light.

There are many situations where a quick visual check of the temperature would be useful.

Just checking a colour is much easier than reading off from a scale.

With 4.5v supply you can make an attractive mood light for the lounge or bedroom.

The measurement scale is shown on 5 coloured LEDs starting at Blue for 10 degrees C up to Red for 30 degrees C.

For more resolution, when the temperature is more than halfway between two LEDs, then they both come on.

For example: If Yellow and Orange come on, the temperature is between 22.5 and 25 degrees C.

Flashing Red LED means 35 degrees C or more.

Flashing Blue LED means less than 5 degrees C.


The circuit is based around a PIC12F675 microcontroller which has an ADC on board making the circuit very simple indeed. Just a resistor for the potential divider with the sensor (TH1) and another to regulate the current to the LEDs.

Since each LED has a different brightness (measured in lumens), the IC uses PWM to control the indivdual brightness of each one.

The temperature is sampled around twice a second which is more than fast enough.


The PCB measures 28mm x 52mm and can be orientated vrtically with the red LED at the top, or horizontally with the red LED on the right.

A good tip when assembling the PCB is to feed the wires from the battery holder up through the large PCB hole and back into the board, as shown below:

The LEDs need to be set at the correct height to ensure they protrude through the case, while leaving enough space for the components on the PCB underneath the cover. This can be achieved by using a guide placed in between the wires of each LED as shown. Make the spacer about 7mm high.

Find an image that you would like to use, in this case a dog from Fotosearch.

Resize it so that the PCB template fits comfortably within it.

Create an outline of the image, using certain features as separate pieces to give a relief effect.

Remember to leave enough space for the 2xAA batteriy holder. It is about the same size as the PCB and will sit above the board in this design.

Next break the design up into individual parts and make templates to stick on to 4-5mm plywood or MDF.

The back plate is made from the main body but without the head section and a hole added to hang it on a wall if desired.

Cut out the parts and sand them smooth.

Use the PCB template to mark out the 5mm holes for the LEDs. Use a scriber or sharp nail. to mark the exact centers of each hole. Then drill the holes with a 5mm wood drill bit.

Remember to drill the wood on top of another piece of scrap wood to ensure the wood does not split when the drill breaks through.

Glue together some scrap pieces to form thicker wood to make the sides. These encase the battery holder and allow a fixing for the back plate.
Now glue the parts together.

A lid can be made from a small piece of wood and small screw, to sit on top of the battery holder.

The final model actually looked better standing on a surface. .