electronics

Christmas lights .... TY grad lights

http://www.ciphersbyritter.com/RADELECT/LITES/LEDLITES.HTM

http://wild-bohemian.com/electronics/led_lights.htm

http://www.allaboutcircuits.com

http://www.absorblearning.com/electronics/demo/units/AEProj_01.html#Initialspecification

For all Projects we need to plan,

1 What do we want the project to do

2 What dimensions (size & mass) should the project be

3 What should be the units operating voltage

4 How do we make the project more effective

5 How do we increase the projects products lifespan

LEDs

1 LEDs are diodes, they only work in 1 direction and so must be put in the circuit correctly, or will fail

2 The Max Current LEDs can take is 10 ~ 20 mA, circuits must be worked out before attempting

The brightness of LEDs is often measured in millicandelas, mcd. The candela is the SI unit for luminous intensity. A standard 5 mm red LED provides a light output of 80 mcd when the current flowing is 10 mA. A typical ultrabright LED provides 1000 mcd. More expensive types provide 4500 mcd – this is a very bright light.

To work out the resisitor required we need to know a few values

The Forward Voltage required V_f.

The Max Voltage V_max.

The max current I_max.

Between the forward voltage and the max voltage pick a value you want the voltage to enter the LED.

The remainder of this voltage must drop across the resistor (if you have a number of LEDs in Series the size of the drops will be added to each other)

So if we choose 2V drop across the LED then we will need a 7 V drop across a resistor, before it gets to the diode. The operating current will also be specified by the LED, usually stated alongside the brightness value, a couple of mA ... so the resistance can be found by

R = 7 / 0.02 = 350 .... okay so we need a resistor about 350 ohms, good

Be sure to quoate the Volts in Volts and the Current in Amps, ie 20mA = 0.02 A

Use the V to be the number of volts you need the Potential Difference to drop by.

The Current must be less than the maximum current quoted. usually they specify an optimum current, use it.

So How do we find a 350 ohm resistor ?

Resistors Magnitudes

We need a Resistor with a Brown band, a green band and a red band.

Use A Breadboard to construct a circuit !!

Variation of the resistance of a metallic conductor with temperature

We just wound insulated copper wire around a cylinder (plastic pen-outer) about 100 times.Because it is insulated it wont short, and the plastic keeps it "tidy".

Remove 2cm of insulation from each end and attach croc clips.We leave them in the glycerol all year around, and cover them with clingfilm to keep them clean!

It works well. You will get a very small change in R (about 0.5ohms over 100degrees)

But I tell them that we want it like that otherwise the resistance of our house wiring would change from summer to winter, with all kinds of unwanted effects.

It costs next to nothing and every student can have/make their own one.

Start with electricity, static

The like and unlike tests with

perspex, polythene and glass.

Plastic shopping bag test.