# Heat Warning for Kids

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## Introduction: Heat Warning for Kids

Hot house hold items like cooking stoves, radiators or oven dishes can cause injuries to unsuspecting children. This heat indicator will warn children when they should not touch an item that is still hot. The indicator uses heat to generate an electrical current that triggers an LED as a warning sign.

### Teacher Notes

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## Step 1: The Product

Parts:

• 2 x Peltier element
• Heat sink
• Electrical wires
• LED
• Metal plate
• 4 x bolts & nuts

Tools:

• Soldering iron
• Drill

## Step 2: How to Build It

Step 1: Use the soldering iron to connect the + side from one Peltier to the - side of the other Peltier.

Step 2: Connect the LED to the Peltier elements using the soldering iron. Make sure that the + and - are connected to the right sides.

Step 3: Place the elements underneath the heath sink and use the soldering iron to check if the setup works.

Step 4: Use the bolds to attach the heat sink to the metal plate, make sure the Peltier elements are placed in between so everything is held together.

Step 5: For a quick test you can use a lighter to heat up the setup from underneath to see if the LED starts burning.

## Step 3: How It Works

The practical application (how does it work?)

A thermoelectric generator is a solid state device that uses heat to generate electrical energy. A Peltier element is a kind of thermoelectric generator. Peltier elements generate electricity through the Peltier-Seebeck-effect. This effect is a combination of the Peltier and the Seebeck effect, which are each other’s opposite.

The Seebeck-effect is the direct conversion from a temperature differential to an electrical voltage. This conversion starts when there is a temperature difference between the two sides of the element. Inside the element there are small ordered groups of blocks of a p-doped- and a n-doped semiconductor. See fig. 1.

The doped semiconductors contain either an excess of electrons (n-doped), or so called ‘holes’ (p-doped). Because of the temperature difference there is a diffusion of the electrons/holes towards the colder side. See fig. 2.

This diffusion causes a differential in electron density, or, as we call it, voltage.

The semiconductors are all connected in pairs of two (both a n-doped and a p-doped) by a conductor. The voltage can thus be summed up. The amount of blocks determines the total voltage of an element.

Current strengths and weaknesses

+ Small, flat, lightweight

+ solid state, thus no maintenance

- Efficiency of the Peltier element is very low (<10%).

- The temperature difference needs to be high for a usable voltage.

Compared to alternative technologies

Steam engines have a far higher efficiency, but are noisy and bigger. The needed maintenance is far larger. There is a danger because of high pressure.

Sterling motors have the same problems, except for the high pressure, as sterling motors work on lower pressures.

Elaborate on future prospects of the technology

Semiconductors (especially the P-doped) are still being developed and innovation is being pushed from several markets, such as Nano technology and Nano chip manufacturers.

When development is a step further and efficiency increases, the elements can be used, not as a by-component, but as the main function of a product.

Recourses

http://www.mn.uio.no/fysikk/english/research/projects/bate/thermoelectricity/

https://nl.wikipedia.org/wiki/Peltier-effect#/media/File:Peltierelement.png

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## Discussions

What a cool idea! This would save many burns from happening!