Design & Technology
There are many different types of electrical and electronic components, including resistors, capacitors and diodes. Each of these has a specific use in a circuit.
Electronic components can be divided into two groups: discrete electronic components and integrated circuits (ICs).
Discrete (meaning separate) electronic components can be selected individually and put together to make a circuit. Examples of discrete components include resistorsresistor: An electrical component that restricts the flow of electrical current. Fixed-value resistors do not change their resistance, but with variable resistors it is possible to vary the resistance., capacitors [capacitor: Circuit component which stores and discharges electrical current. They are made from two parallel metal plates separated by an insulator (called a dielectric). ], diodes and transistors [transistor: Components which do not conduct electricity unless they are turned on by a (different) electrical current. This means they can be used as switches, amplifiers and in other ways. ].
Discrete components can also be used as components in circuits that include an integrated circuit. For example, a 555 astable [astable: An astable circuit gives a pulsed digital output. For example, it could be used to make an LED flash. ] integrated circuit requires two discrete resistors and a discrete capacitor to make it work.
These are miniature circuits etched onto a piece of silicon, often called a chip. These chips are fitted inside a protective plastic package and are manufactured in large quantities.
The circuits inside the package are arranged in different configurations depending upon the type of chip and its function. The most common type of configuration is called the dual-in-line or DIL package, which has two rows of connecting 'legs', one on each side.
You don't need to understand how the circuit inside a silicon chip works. It's best to think of ICs simply in terms of their function: eg as timers, counters, logic gateslogic gate: Digital electronic devices that allow an electronic system to make a decision based on a number of inputs. or operational amplifiers [operational amplifier: A device which takes a relatively weak signal as an input and produces a much stronger signal as an output. ] (op-amps).
Normally they are one of the process blocks in an electronic system, as in the image here:
When using ICs you need to know:
A circuit diagram that includes one or more ICs should show the pin numbers and how the pins are connected to the rest of the circuit.
Resistors restrict or limit the flow of current in a circuit. The ability of a material or component to resist current flow is measured in ohms [ohm: Units of electrical resistance (R). 1 volt will force a current of 1 amp through a resistance of 1 ohm. ]. There are three main types of resistor:
These are the most common type of resistor. They have three important uses:
Resistors can be used to protect other components (such as an LED [LED: Stands for Light-Emitting Diode. LEDs glow when current passes through them. ]) from damage by too much current.
In a potential divider, a fixed resistor is used to split voltage between different parts of the circuit. Potential dividers (or voltage dividers) are used, for example, with LDRs in circuits which detect changes in light.
A fixed resistor can be used in series with a capacitor to control a time delay.
There are two types of variable resistor:
The main difference between the two types of variable resistor is their size. Pre-set potentiometers tend to be smaller and are usually adjusted with a screwdriver. A variable resistor has a long spindle with an operating knob attached.
Thermistors change resistance as temperatures change. Most thermistors have a negative temperature coefficient, meaning their resistance falls as temperature increases. Thermistors are used in temperature-sensing circuits.
Light-dependent resistors (LDRs) change resistance as light levels change. The light levels are detected by a photo-sensitive plate on the resistor. Most LDRs have a negative light coefficient, meaning that their resistance falls as the amount of light falling on them increases. LDRs are used in light-detection circuits.
The ohm is the unit of resistance. Larger values are measured in kilo-ohms (1000 ohms) and mega-ohms (1,000,000 ohms). Resistors are marked, using a code specified in British Standard 1852, as follows:
Resistance value is shown by a series of coloured bands, read from left to right.
Potential dividers divide up the voltage within a circuit, so that parts of a circuit only receive the voltage they require. Potential dividers usually consist of two or more resistors arranged in series across a power supply.
Potential dividers form an important part of sensor circuits. For example, an LDR [LDR: A Light Dependent Resistor, or LDR, is a type of resistor which is affected by changes in light levels. A cadmium sulphide layer causes a decrease in resistance in the light and increase in the dark. ] or thermistorthermistor: An electrical device whose resistance decreases as its temperature increases. can be used in place of one of the resistors, with the output voltage signal being directed to an op-amp [operational amplifier: A device which takes a relatively weak signal as an input and produces a much stronger signal as an output. ] or IC.
Instead of using a fixed resistor, a variable resistor allows the output voltage to be adjusted. A low-value fixed resistor should be placed in series with the variable resistor to prevent the full power of the circuit from being routed down the output voltage path if the variable resistor is accidentally moved to a low resistance.
Potential dividers are important in both transistor-switching circuits and op-amp comparator circuits [Comparator circuits: Circuit with a comparator - a component which compares two voltages and gives either a high or low output depending on the result of the comparison. ]. The diagram shows a darkness-sensor circuit with a transistor [transistor: Components which do not conduct electricity unless they are turned on by a (different) electrical current. This means they can be used as switches, amplifiers and in other ways. ] used as a switch. When the LDR senses a drop in light, the LED is switched on.
When the resistance of the LDR is small, its share of the voltage supply is small too; this means that the output voltage (Vout) from the voltage divider is small and the transistor is switched off.
In the dark the large resistance of the LDR takes a large share of the voltage supply so Vout is large and the transistor and LED both switch on.
A capacitor is a discrete component that can store an electrical charge. The larger the capacitance the more charge it can store.
The unit of measurement of capacitance is the farad. Often you will see capacitors of much less than a farad. These will be measured in microfarads (one millionth of a farad or 1/1,000,000) or picofarads (one million-millionth of a farad or 1/1,000,000,000,000).
There are two types of capacitor:
These generally have larger capacitance values. Polarised capacitors have a positive pole and a negative pole, so they must be connected to a circuit the correct way round.
Polarised capacitors may be either axially mounted (on their side, connected at each end) or radially mounted (upright with both connections at the bottom).
These are usually much smaller than the polarised type, and have smaller capacitance values. These might range from a few picofarads to a few microfarads. They don't have positive or negative poles so they can be connected to a circuit either way round.
Capacitors are used to smooth rectified alternating-current voltages into steady direct-current voltages. They can also be used to filter out fluctuations in a signal.
Capacitors are often used in series with resistors to achieve a time delay. The time it takes for the capacitor to become charged is related to the size of the capacitor and the value of the regulating resistor.
A diode is a discrete component that allows current to flow in one direction only. It is a polarised component with two leads, called the cathode and the anode. The cathode is normally marked with a silver or coloured band or the symbol '-'.
If the anode is connected to a higher voltage than the cathode, current will flow from anode to cathode. This is called forward bias.
If the diode is put in the circuit back to front, so that the voltage at the cathode is higher than the voltage at the anode, the diode will not conduct electricity. This is called reverse bias.
Diodes are normally used to prevent damage to other polarised components in circuits: eg a diode can protect against current flowing the wrong way if the battery is put in back to front.
An important use of diodes is to prevent circuit damage due to back electromotive force (known as EMF). This is a momentary change in the direction of the flow of electricity when components such as motors, solenoids or relays are switched off. The diode is connected in parallel to the component, which generates the back EMF in reverse bias to the 'normal' direction of flow of electricity in the circuit. A diode used in this way is called a clamping diode.
A light-emitting diode (LED) is a special kind of diode that glows when electricity passes through it. Most LEDs are made from a semi-conducting material called gallium arsenide phosphide.
LEDs can be bought in a range of colours. They can also be bought in forms that will switch between two colours (bi-colour), three colours (tri-colour) or emit infra-red light.
In common with all diodes, the LED will only allow current to pass in one direction. The cathode is normally indicated by a flat side on the casing and the anode is normally indicated by a slightly longer leg. The current required to power an LED is usually around 20 mA.
A seven-segment LED is a special type of LED display used in digital clocks, video recorders and microwave ovens.
Batteries are the most common source of power used for electrical circuits in schools. They come in a variety of sizes, which can be combined in series battery holders. This allows the voltage produced to be equal to the sum of the batteries used: eg 3 x 1.5 V AA batteries would produce a voltage of 4.5V.
When choosing a battery, the following should be considered:
As an alternative to batteries, external power supplies can be used with low-voltage regulators (such as L7805 and 78L05). Their use is often limited by cost, the need to be attached to the external power supply and the power requirements of the circuit.
The standard symbols for the key components used in electronic circuits are shown in the table below.
|Cell||Light-emitting diode (LED)||Push-to-make switch||Diode||Bell||Voltage rails|
|Battery||Flashing LED||Push-to-break switch||Thyristor||Microphone||Earth|
|Resistor||Bi-colour LED||Single-pole single-throw switch||NPN transistor||Buzzer||555 timer IC|
|Variable resistor||Tri-colour LED||Single-pole double-throw switch||Field effect transistor (FET)||Loud-speaker||Operational amplifier (op-amp)|
|Potent-iometer||Photo transistor||Capacitor||AND gate||Lamp||Voltage regulator|
|Thermistor||Opto-isolator||Electrolytic capacitor||OR gate||Motor||Crossing of conductors|
|Light-dependent resistor (LDR)||Ammeter||Piezo crystal oscillator||NOT gate||Voltmeter||Joined conductors|