Sharp Rubber Duck

Automation Resources

Resources for Growing Quality Medical Marijuana

Integrated Circuits

Prior to the invention of the Transistor electronic circuits tended to be very complex arrangements of valves, wires, screws, connecting strips and various other bits of hardware. It is not difficult to understand why this type of wiring was often referred to as "rats nest" wiring. The transistor allowed miniturisation of circuitry with all the components soldered onto a "Printed Circuit Board" (PCB) made up of thin copper tracks on and insulating base such as fibre glass. The copper tracks interconnected all the components and replaced the old-fashioned 'hard-wiring'. Components were soldered directly onto the PCB. It was possible to miniaturise very complex circuits down to the size of a small PCB that would easily fit in your hand.

Individual components soldered on to a Printed Circuit Board (PCB).

A 74LS244 Integrated Circuit

The next great technological leap was the development of a process that allowed components to be manufactured directly on to a substrate of crystalline Silicon. All of the components are "integrated' onto one tiny base. (Hence the name: "integrated circuit", or "IC".)

A transistor on a 10 cent piece.

This CPU has the equivalent of over 10 million transistors!!

Early ICs contained the equivalent of just a few components. Modern devices have millions of components crammed onto a base that would easily balance on the end of one finger!

A Typical Integrated Circuit - The ULN2803

The output of the ULN2803 is "inverted". This means that a HIGH at the input becomes a LOW at the corresponding output line. eg If the printer port line connected to pin 1 goes HIGH, pin 18 on the ULN2803 will go LOW (switch off).

The ULN2803 is described as a "8-line driver". This means that it contains the circuitry to control eight individual output lines, each acting independently of the others. The IC can be thought of a an 8-line 'black box'. There is no need to know what its internal design is (although one representative 'line' is shown below for interest.) The 'schematic diagram' (above) is all we need to understand.

The interfaces described on this site make extensive use of the ULN2803. It is inexpensive (around $2.50 each) and very easy to use.

Connecting the ULN2803 to the Printer Port

The ULN2803 is connected between each of the eight 'output' lines of the printer port and the device it controls. The output 'device' can be as simple as a LED, a motor, or a relay which in turn controls a much larger device.

In the diagram above a LED and Battery are connected to the output line. A similar arrangement is used with part of the "Simplified I/O Board shown below. The "battery" in this case is a common nine volt type.

The "common" line is connected to the positive rail via a 15 volt Zener diode to prevent damage to the IC due to "back emf" when loads such as motors and relays switch ON and OFF. This application of zener diodes will be discussed in the MODULE about "Relays".

The 74LS244

The second IC used in the interfaces described on this site is the 74LS244 "buffer" IC. This IC can be thought of as the 'opposite' of the ULN2803 in that it is used for input, rather than output. Its operation is quite similar, but it is not able to handle the large currents that the ULN2803 can handle. The other feature of the 74LS244 is that it is able to 'clean-up' inferior input signals and make them more acceptable to the computer (hence its name: "buffer"). As with the ULN2803 all you need to know is that signal goes in to the 74LS244 IC, is cleaned-up (buffered) and then it goes out to the computer.

The resistors used in the circuit make sure that the IC is 'stable' by holding its input lines HIGH (at 5 volts). When an input switch is closed the input to the corresponding line of the 74LS244 is switched LOW (zero volts, or "ground") and the computer detects the change (assuming you write the code to do so).

The LM7805

The LM7805 is a "Voltage Regulator". Its function is to provide a stable 5 volt supply for the 74LS244. The LM7805 has only three 'legs', but it is NOT a transistor. Inside the regulator is a complex circuit consisting of transistors, resistors diodes and capacitors designed to provide stable voltage output.

The input voltage may be up to 30 volts and it must be DC. The IC is designed to shut down if it gets too hot, but it is not protected against damage caused by connecting the input voltage the wrong way around. A 1N4004 diode is used in the circuit to protect against connecting the battery the wrong way around.

A small capacitor is connected across the output of the regulator and "ground" to enhance its stability by 'shorting' out any unwanted high-frequency 'spikes'.

<- Menu

Integrated Circuits - QUIZ

<- Menu Next ->

Add to Your Social Bookmarks: Del.icio.us Digg Furl Reddit Simpy Spurl Y! MyWeb - Netscape StumbleUpon