DIGITAL ELECTRONICS - LOGIC CIRCUITS - 2

V. Ryan © 2003 - 2009

 LOGIC circuits are normally composed of gates. A combination of gates make up a circuit and some digital circuits can be extremely complex. It is the logic gates that produce pulses of electrical current (1s and 0s). At school level, digital logic circuits are relatively simple. Below are simple drawings that help explain the two most popular logic gates - the AND gate and the OR gate. The simplified AND gate shown above has two inputs, switch A and switch B. The bulb Q will only light if both switches are closed. This will allow current to flow through the bulb, illuminating the filament. The simplified OR gate shown above has two inputs, switch A and switch B. The bulb Q will light if either switch A or B are closed. This will allow current to flow through the bulb, illuminating the filament. When the bulb lights this represents a 1 as current is running through the filament. If current is not running through the filament the bulb will not light and this represents a 0 (zero). THE ROLE OF TRANSISTORS Transistors are vital for digital circuits to work. These components are used as very fast switches in digital logic circuits. Transistors are normally so small that hundreds of thousands fit on one processing chip on a computer motherboard. The types of transistors used in school projects are normally large enough to fit on the end of a small finger. However, the way they switched on and off is the same (click here for transistor information sheets) . When a transistor is switched on it produces a 1 and when it is switched off it produces a 0. Transistors in the circuit of a computer microprocessor can switch on and off thousands of times per second. Without the invention of the transistor, computer processing power would be very limited and slow. Two basic examples of simple transistor driven logic (AND / OR) circuits are shown below. This is an AND gate circuit and it can be made quite easily. The example shown is built from a modular electronics kit. Both switches A and B must be pressed together for the bulb to light. If you construct this circuit, you may need to alter the value of the resistors. This will depend on the type of transistors used and whether to bulb or an LED is used. This is an OR gate circuit. Either switch A or B must be pressed for the bulb to light. The switches do not have to be pressed together. QUESTIONS: 1. Explain how an AND gate works. Use a circuit diagram to help explain your answer. 2. Explain how an OR gate works. Use a circuit diagram to help explain your answer. 3. Build a simple logic circuit using a breadboard and available components. You may wish to build one of the circuits shown above. CLICK HERE FOR NEXT LOGIC PAGE CLICK HERE FOR ELECTRONICS INDEX PAGE

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