Diode…

A voltage divider is constructed from a pair of resistors connected in series. In the circuit to the left, a fixed resistor and a light dependent resistor are connected in series. A light dependent resistor has a low resistance in the light and an extremely high resistance in the dark. The LED only needs a small current to make it glow which is why I have not chosen a lamp or motor which would require much more current.

In the dark, the resistance of the LDR is very high so most of the voltage is distributed over the fixed resistor causing the current to flow through the LED, which lights up. In the light the resistance of the LDR decreases so the current takes the path through the LDR and bypassing the LED.

.

Diodes are silicon based devices that only allow current to flow in one direction. They can be used to filter out the audio from a radio signal (which is how a crystal radio works) or they can be used to turn alternating current into direct current.

Light emitting diodes emit light at very low currents. Modern high brightness diodes are replacing traditional filament lamps in most situations. We use LED lighting in Macmillan for lighting plays because they can be made to change colour. Most of the classrooms now have LED lighting which produces a lot less heat and uses less electricity. You may have noticed that some of the new LED lighting outside a slightly harsher colour than traditional lighting. This is because the spectrum of colours it produces is not as close to sunlight as filament lighting. You can read more about colour here – ROYGBIV.

When you draw a diode in a circuit, make sure that you have the current flowing in the direction of the arrow. The vertical bar shows the direction from which the current is blocked. An LED can be made to light with a 3V watch battery. They are usually used with protective resistors because exposure to too high a voltage can damage them The DT department uses ‘high bright’ LEDs that you can happily connect to a 9 volt pp3 battery without any apparent ill effect.

LEDs produce light of a specific wavelength, so there is no such things as a white LED. For an LED to produce white light it takes one of several different methods. One is the use of a blue LED and some yellow phoshpor, which produces yellow light when excited by having blue light shone through it. The resulting mixture of blue and yellow lights are perceived by our eyes as white. There are other combinations of phosphors that can be used with increasing levels of complexity such as green, blue and red phosphor excited by a UV LED. There is also a white LED that uses quantum dots! Tiny dots (only 20 to 50 atoms across) of selenium and cadmium which emit white light when a blue LED is used to excite them. The technique we use in the theatre to produce white light with LED is to use arrays of red, green & red LEDs that are mixed to make white. They can also be mixed in other different proportions to produce a huge range of shades. There is another method of making white LEDs called homo-epitaxial ZnSe that is similar to the first example I gave using a blue LED to excite the zinc selenide which emits an orangey light.

Questions…

1. The resistors are connected in series in the circuit above, what is the other way in which components can be connected?
2. In which direction does the current flow in a circuit diagram?
3. What unit is used to measure electric current?
4. What device is used to measure current?
5. Current is the result of what two other properties in a circuit?
6. Name a metal and a non-metal element mentioned in the text above.
7. How does the resistance of an LDR vary with varying light intensity?