Integrated Circuits

Integrated Circuits are usually called ICs or chips. They are complex circuits which have been etched onto tiny chips of semiconductor (silicon).

The silicon chip is usually packaged in a plastic holder with pins spaced on a 0.1" (2.54mm) grid which will fit the holes on stripboard and breadboards. Very fine wires inside the package link the chip to the pins.

Surface-mount device (SMD) ICs are designed for machine assembly. They have very short closely-spaced pins and are not suitable for educational or hobby circuits.

The pins are numbered anti-clockwise around the IC (chip) starting near the notch or dot. The diagrams show the numbering for 8-pin and 14-pin ICs, but the principle is the same for all sizes.

Rapid Electronics: ICs (all types)

Many ICs are static sensitive and can be damaged when you touch them because your body may have become charged with static electricity, from your clothes for example. Static sensitive ICs will be supplied in antistatic packaging with a warning label and they should be left in this packaging until you are ready to use them.

It is usually adequate to earth your hands by touching a metal water pipe or window frame before handling the IC but for the more sensitive (and expensive!) ICs special equipment is available, including earthed wrist straps and earthed work surfaces. You can make an earthed work surface with a sheet of aluminium kitchen foil and using a crocodile clip to connect the foil to a metal water pipe or window frame with a 10kΩ resistor in series.

The outputs of ICs must never be directly connected together. However, diodes can be used to combine two or more digital (high/low) outputs from an IC such as a counter. This can be a useful way of producing simple logic functions without using logic gates!

The diagram shows two ways of combining outputs using diodes. The diodes must be capable of passing the output current. 1N4148 signal diodes are suitable for low current devices such as LEDs.

For example the outputs Q0 - Q9 of a 4017 1-of-10 counter go high in turn. Using diodes to combine the 2nd (Q1) and 4th (Q3) outputs as shown in the bottom diagram will make the LED flash twice followed by a longer gap. The diodes are performing the function of an OR gate.

Logic ICs process digital signals and there are many devices, including logic gates, flip-flops, shift registers, counters and display drivers.

Logic ICs can be split into two groups: the 4000 series, and the 74 series which consists of various families such as the 74HC, 74HCT and 74LS.

For most new projects the 74HC family is the best choice. The tables show the supply voltage and maximum output current for each family. The 74LS and 74HCT families require a 5V supply so they are not convenient for battery operation.

Logic IC inputs have high impedances and unused inputs must be connected to 0V or +Vs to avoid erratic behaviour due to inputs switching state in response to stray electrical noise. 74LS ICs are unusual because their inputs 'float' high when unconnected.

The number of logic IC inputs which can be driven by one output of the same family is called the fan out. Usually 50 (10 for 74LS), it is unlikely to matter in simple circuits.

For more details of the logic IC families, including pin arrangements for many ICs, please see these pages:

• 4000 series
• 74 series (74HC, 74HCT & 74LS)

It is best to build a circuit using just one logic family, but if necessary the different families may be mixed providing the power supply is suitable for all of them. For example mixing 4000 and 74HC requires the power supply to be in the range 3 to 6V. A circuit which includes 74LS or 74HCT ICs must have a 5V supply.

A 74LS output cannot reliably drive a 4000 or 74HC input unless a 'pull-up' resistor of 2.2kΩ is connected between the +5V supply and the input to correct the slightly different logic voltage ranges used.

Note that a 4000 series output can drive only one 74LS input.