Electronics Made Easy - a Complete Introduction to Electronics - Martin Denny [12]
Rectangular shaped LED's can be formed into a figure eight configuration where seven LED's are connected in either common cathode or common anode mode, see figure 2.
To produce numbers, segments are powered in the following sequence:
where 0 º 0v and 1 º +Vs
Liquid crystal displays are also available. These displays do not emit light, but generate black lettering on a silvered background which reflects incident light. Some displays have additional backlighting as the example below. The main advantage is extremely low power consumption.
Photoconductors
If radiation falls upon a semiconductor its conductivity increases. The conductivity of a semiconductor is proportional to the concentration of charge carriers present, radiant energy supplied causes covalent bonds to be broken and additional hole-electron pairs are created. The increased current carriers reduce the resistance of the material, such devices are called photoresistors or photoconductors.
Phototransistors are commonly used with LED's in fibre optic circuits or opto isolators, where a pulsed signal is fed to the LED and converted into a light pulse. This is directed to a photo-transistor within the same package or via a fibre optic cable. Figure 3 shows an optical isolator.
DIGITAL ELECTRONICS
LOGIC
Logic circuits are used extensively in modern electronics and replace complicated relay logic previously used. Gate packages are cheap and require few external components. As previously discussed in the integrated circuits section, there are two systems in general use, TTL and CMOS. The two systems operate at different voltage and frequency ranges but the functions are comparable.
Definition of Logic Functions:
AND: When inputs A and B are high, ie logic 1, output Z is high. This can be described as the exclusive state as other combinations produce a low output, ie 0v logic 0.
OR: When inputs A and/or B are high then output Z is high. The exclusive state only occurs when A and B are low ie logic 0, and Z is low.
XOR: Defined as exclusive OR, inputs A and B are either both high or both low output Z is low. When either A or B is high, Z is high.
NAND: (Not AND) The output is inverted, ie to produce a low output when A and B are high. This also enables the gate to be used as an inverting buffer ie A and B connected together.
NOR: (Not OR) The output is inverted so when A and B are both low the output Z is high. Again connecting A and B will produce an inverting buffer.
The symbols of the logic gates together with their truth tables are shown below:
Most logic designs will require inverting buffers at one stage of the design as the gates must be manipulated to operate at their exclusive state. A good design will minimise gate packages therefore a combination of packages may be used in a design.
The example above shows gates with two inputs. Packages are available with up to eight inputs but the operation remains the same ie the exclusive state remains the same. Inverting buffers and non inverting buffers are also available in packages of 6. Figure 1 shows a typical CMOS and TTL package.
Boolean Algebra
Terms A, B, and Z can be expressed in the form of Boolean Algebra as follows:
where ¯ denotes “not 1”
The gate functions shown on the previous page can be expressed as follows:
Only the values of Z = logic 1 are shown in the equation therefore all other combinations of A and B are equal to logic 0. A truth table can therefore be easily constructed from a Boolean Expression.
The gate combinations shown below can be expressed in the form of a truth table. All the combinations of A, B, C and D are shown on the left hand side of the truth table, the various output values are shown on the right hand side. For four inputs the number of combinations possible is 16, the binary code formed is expressed by numbers 0 to 15 in the left hand column.
Truth Table
To optimise the number of packages it is sometimes more practical to use NAND and NOR logic. Figure 2 shows a circuit using AND gates and inverting buffers, and the equivalent