Basic Electricity and Electronic Devices
LED Light Emitting Diodes are great for projects because they provide visual entertainment. Leeds use a special material which emits light when current flows through it. Unlike light bulbs, Leeds never burn out unless their current limit is passed. A current of 0. 02 Amps (20 ma) to 0. 04 Amps (40 ma) is a good range for Leeds. They have a positive leg and a negative leg Just like regular diodes. To find the positive side of an LED, look for a line in the metal inside the LED. It may be difficult to see the line. This line Is closest to the positive side of the LED.
Another way of finding the positive side Is to find a flat spot on the edge of the LED, This flat spot Is on the active side. When current Is flowing through an LED the voltage on the positive leg Is about 1. 4 volts higher than the voltage on the negative side. Remember that there is no resistance to limit the current so a resistor must be used in series with the LED to avoid destroying it. Resistors Resistors are components that have a predetermined resistance. Resistance determines how much current will flow through a component. Resistors are used to control voltages and currents.
A very high resistance allows very little current to flow. Air has very high resistance. Current almost never flows through air. Sparks and lightning are brief displays of current flow through air. The light is created as the current burns parts of the air. ) A low resistance allows a large amount of current to flow. Metals have very low resistance. That Is why wires are made of metal. They allow current to flow from one point to another point without any resistance. Wires are usually covered with rubber or plastic. This keeps the wires from coming In contact with other wires and creating short circuits.
High voltage power lines are covered with thick layers of plastic to make them safe, but they become very ungenerous when the line breaks and the wire is exposed and is no longer separated from other things by insulation. Resistance is given in units of ohms. (Ohms are named after Ohm Ohms who played with electricity as a young boy in Germany. ) Common resistor values are from 100 ohms to 100,000 ohms. Each resistor is marked with colored stripes to indicate it’s resistance. To learn how to calculate the value of a resistor by looking at the stripes on the resistor, go to Resistor Values which includes more Information about resistors.
Variable Resistors Variable resistors are also common components. They have a dial or a knob that intros are variable resistors. When you change the volume you are changing the resistance which changes the current. Making the resistance higher will let less current flow so the volume goes down. Making the resistance lower will let more current flow so the volume goes up. The value of a variable resistor is given as it’s highest resistance value. For example, a 500 ohm variable resistor can have a resistance of anywhere between O ohms and 500 ohms.
A variable resistor may also be called a potentiometer (pot for short). Switches Switches are devices that create a short circuit or an open circuit depending on the position of the switch. For a light switch, ON means short circuit (current flows through the switch, lights light up and people dance. ) When the switch is OFF, that means there is an open circuit (no current flows, lights go out and people settle down. This effect on people is used by some teachers to gain control of loud classes. ) When the switch is ON it looks and acts like a wire. When the switch is OFF there is no connection.
Using a Bread Board To build our projects, we will use a breadboard like the one shown below. The bread board has many strips of metal (copper usually) which run underneath the board. The metal strips are laid out as shown below. These strips connect the holes on the top of the board. This makes it easy to connect components together to build circuits. To use the bread board, the legs of components are placed in the holes. The holes are made so that they will hold the component in place. Each hole is connected to one of the metal strips running underneath the hole. Each strip forms a node.
A node is a point in a circuit where two components are connected. Connections between different components are formed by putting their legs in a common node. On the bread board, a node is the row of holes that are connected by the strip of metal underneath. The long top and bottom row of holes are usually used for power supply connections. The row with the blue strip beside it is used for the negative voltage (usually ground) and the row with the red strip beside it is used for the positive voltage. The circuit is built by placing components and connecting them together with Jumper wires.
Then when a path is formed from the positive supply node to the negative supply node through wires and components, we can turn on the power and current flows through the path and the circuit comes alive. A series connection of 2 resistors on a breadboard looks like the stricture below on the left and a parallel connection of 2 resistors looks like the picture below on the right. For chips with many legs (CICS), place them in the middle of the board (across the middle dividing line) so that half of the legs are on one side of the middle line and half are on the other side. A completed circuit might look like the following.
This circuit uses two small breadboards. 1. 6 Transistors and Leeds closer at a component that was introduced in Section 1. 2. The LED An LED is the device shown above. Besides red, they can also be yellow, green and blue. The letters LED stand for Light Emitting Diode. If you are unfamiliar with diodes, take a moment to review the components in Basic Components, Section 1. 2. The important thing to remember about diodes (including Leeds) is that current can only flow in one direction. To make an LED work, you need a voltage supply and a resistor. If you try to use an LED without a resistor, you will probably burn out the LED.
The LED has very little resistance so large amounts of current will try to flow through it unless you limit the current with a resistor. If you try to use an LED without a power supply, you will be highly disappointed. So first of all we will make our LED light up y setting up the circuit below. Step 1 . ) First you have to find the positive leg of the LED. The easiest way to do this is to look for the leg that is longer. Step 2. ) Once you know which side is positive, put the LED on your breadboard so the positive leg is in one row and the negative leg is in another row. In the picture below the rows are vertical. ) Step 3. ) Place one leg of a 2. K ohm resistor (does not matter which leg) in the same row as the negative leg of the LED. Then place the other leg of the resistor in an empty row. Step 4. ) Unplug the power supply adapter from the power supply. Next, put the ground (black wire) end of the power supply adapter in the sideways row with the blue stripe beside it. Then put the positive (red wire) end of the power supply adapter in the sideways row with the red stripe beside it. Step 5. Use a short Jumper wire (use red since it will be connected to the positive voltage) to go from the positive power row (the one with the red stripe beside it) to the positive leg of the LED (not in the same hole, but in the same row). Use another short Jumper wire (use black) to go from the ground row to the resistor (the leg that is not connected to the LED). Refer to the picture below if necessary. The breadboard should look like the picture shown below. Now plug the power supply into the wall and then plug the other end into the power supply adapter and the LED should light up.
Current is flowing from the positive leg of the LED through the LED to the negative leg. Try turning the LED around. It should not light up. No current can flow from the negative leg of the LED to the positive leg. People often think that the resistor must come first in the path from positive to negative, to limit the amount of current flowing through the LED. But, the current is emitted by the resistor no matter where the resistor is. Even when you first turn on the power, the current will be limited to a certain amount, and can be found using ohm’s law.
Revisiting Ohm’s Law Ohm’s Law can be used with resistors to find the current flowing through a circuit. The law is I = VT/R (where I = current, VT = voltage across resistor, and R = resistance). For the circuit above we can only use Ohm’s law for the resistor so we must use the fact that when the LED is on, there is a 1. 4 voltage drop across it. This means that if the positive leg is connected to 12 volts, the negative leg will be at 10. 6 volts. Now we know the voltage on both sides of the resistor and can use Ohm’s law to calculate the current. The current is (10. 6 – O)/ 2200 = 0. 048 Amperes = 4. 8 ma is flowing through the LED and the resistor. If we want to change the current flowing through the LED (changing the brightness) we can change the resistor. A smaller resistor will let more current flow and a larger resistor will let less current flow. Be careful when using smaller resistors because they will get hot. Next, we want to be able to turn the LED on and off without changing the circuit. To do this we will learn to use another electronic component, the transistor. . 6. 1 The Transistor Transistors are basic components in all of today’s electronics.
They are Just simple switches that we can use to turn things on and off. Even though they are simple, they are the most important electrical component. For example, transistors are almost the only components used to build a Pentium processor. A single Pentium chip has about 3. 5 million transistors. The ones in the Pentium are smaller than the ones we will use but they work the same way. Transistors that we will use in projects look like this: The transistor has three legs, the Collector (C), Base (B), and Emitter (E). Sometimes hey are labeled on the flat side of the transistor.
Transistors always have one round side and one flat side. If the round side is facing you, the Collector leg is on the left, the Base leg is in the middle, and the Emitter leg is on the right. Transistor Symbol The following symbol is used in circuit drawings (schematics) to represent a transistor. Basic Circuit The Base (B) is the On/Off switch for the transistor. If a current is flowing to the Base, there will be a path from the Collector (C) to the Emitter (E) where current can flow (The Switch is On. ) If there is no current flowing to the Base, then no current can flow room the Collector to the Emitter. The Switch is Off. ) Below is the basic circuit we will use for all of our transistors. To build this circuit we only need to add the transistor and another resistor to the circuit we built above for the LED. Unplug the power supply from the power supply adapter before making any changes on the breadboard. To put the transistor in the breadboard, separate the legs slightly and place it on the breadboard so each leg is in a different row. The collector leg should be in the same row as the leg of the resistor that is connected to ground (with the black Jumper wire). Next move the umpire wire going from ground to the 2. K ohm resistor to the Emitter of the transistor. Next place one leg of the kick ohm resistor in the row with the Base of the transistor and the other leg in an empty row and your breadboard should look like the picture below. Now put one end of a yellow Jumper wire in the positive row (beside the red line) and the other end in the row with the leg of the kick ohm resistor (the end not connected to the Base). Reconnect the power supply and the transistor will come on and the LED will light up. Now move the one end of the yellow Jumper wire from the positive row o the ground row (beside the blue line).
As soon as you remove the yellow Jumper wire from the positive power supply, there is no current flowing to the base. This makes the transistor turn off and current can not flow through the LED. As we will see later, there is very little current flowing through the kick resistor. This is very important because it means we can control a large current in one part of the circuit to Ohm’s Law We want to use Ohm’s law to find the current in the path from the Input to the Base of the transistor and the current flowing through the LED.
To do this we need to use two basic facts about the transistor. If the transistor is on, then the Base voltage is 0. 6 volts higher than the Emitter voltage. 2. ) If the transistor is on, the Collector voltage is 0. 2 volts higher than the Emitter voltage. So when the kick resistor is connected to VIVID, the circuit will look like this: So the current flowing through the kick resistor is (12 – 0. 6) / 100000 = 0. 000114 A = 0. 114 ma. The current flowing through the 2. K ohm resistor is (10. 6 – 0. 2) / 2200= 0. 0047 A = 4. 7 ma.
If we want more current flowing through the LED, we can use a smaller resistor (instead of 2200) and we will get more current through the LED thou changing the amount of current that comes from the Input line. This means we can control things that use a lot of power (like electric motors) with cheap, low power circuits. Soon you will learn how to use a microelectronic (a simple computer). Even though the microelectronic can not supply enough current to turn lights and motors on and off, the microelectronic can turn transistors on and off and the transistors can control lots of current for lights and motors.
For Ohm’s law, also remember that when the transistor is off, no current flows through the transistor. 1. 6. 2 Introduction to Digital Devices – The Inverter In digital devices there are only two values, usually referred to as O and 1. 1 means there is a voltage (usually 5 volts) and O means the voltage is O volts. An inverter (also called a NOT gate) is a basic digital device found in all modern electronics. So for an inverter, as the name suggests, it’s output is the opposite of the input (Output is NOT the Input). If the input is O then the output is 1 and if the input is 1 then the output is O.
We can summarize the operation of this device in a table. Input Output To help us practice with transistors we will build an inverter. Actually we have already built an inverter. The transistor circuit we Just built is an inverter circuit. To help see the inverter working, we will build a circuit with two inverters. The circuit we will use is shown below. Second Inverter First Inverter (already built) To build the circuit, use the transistor circuit we Just built as the first inverter. The first inverter input is the end of the kick ohm resistor connected to the yellow jumper wire.
Build another circuit identical to the first (the basic transistor circuit from Section 1. 6. 1) except leave out the yellow Jumper wire connected to the kick ohm resistor (the inverter input). This circuit is the second inverter. Connect the output of the first inverter to the input of the second inverter by putting one end of a jumper wire in the same row of holes as the 2. K ohm resistor and the Collector of the transistor (the output of the first inverter) and putting the other end in the same row of holes as the leg of the kick ohm resistor of the second inverter (the input to the second inverter).
Here is how to check if you built it correctly. Connect the first inverter should come on and the LED in the second inverter should stay off. Then connect the first inverter input to OVA (the ground row). (You are turning off the switch of the first inverter. The first LED should go off and the second LED should come on. If this does not happen, check to make sure no metal parts are touching. Check to make sure all the parts are connected correctly. The input can either be connected to IV or OVA. When the Inverter Input is IV, the transistor in the first inverter will turn on and the LED will come on and the Inverter Output voltage will be 0. V. The first Inverter Output is connected to the input of the second inverter. The 0. IV at the input of the second inverter is small enough that the second transistor is turned off. The circuit voltages are shown in the diagram below. When the Inverter Input is connected to OVA, the transistor in the first inverter is turned off and the LED will get very dim. There is a small amount of current still flowing through the LED to the second inverter. The voltage at the first Inverter Output will go up, forcing the second inverter transistor to come on. When the second inverter transistor comes on, the second inverter LED will come on.
To find the voltage at the output of the first inverter (10. IV), use Ohm’s law. There is no current flowing through the transistor in the first inverter so the path of the current is through the first LED, through the 2. K resistor, through the kick resistor, through the second transistor to ground. The voltage at the negative side of the first LED is fixed at 10. IV by the LED. The voltage at the second transistor base is fixed at 0. IV by the transistor. Then given those two voltages, you should be able to find the voltage at the point in the middle (10. IV) using Ohm’s law. Hint: First find the current and then work through Form 1 of ohm’s law to find the voltage at the point between the 2. K resistor and the kick resistor. ) Switch the input back and forth from Veto IV and you can see that when the first stage is on, the second stage is off. This demonstrates the inverting action of the Inverter. 1. 7 Oscillators, Pulse Generators, Clocks… Capacitors and the 555 Timer ICC Introduction As electronic designs get bigger, it becomes difficult to build the complete circuit. So we will use overbuilt circuits that come in packages like the one shown above. This overbuilt circuit is called an ‘C.
ICC stands for Integrated Circuit. An ICC has many transistors inside it that are connected together to form a circuit. Metal pins are connected to the circuit and the circuit is stuck into a piece of plastic or ceramic so that the metal pins are sticking out of the side. These pins allow you to connect other devices to the circuit inside. We can buy simple CICS that have several inverter circuits like the one we built in the LED and Transistor section or we can buy complex CICS like a Pentium Processor. The Pulse – More than Just an on/off switch So far the circuits we have built have been stable, meaning that the output voltage stays the same.
If you change the input voltage, the output voltage changes and once it changes it will stay at the same voltage level. The 555 integrated circuit (C) is designed so that when the input changes, the output goes from O volts to PVC (where c is the voltage of the power supply). Then the output stays at PVC for a certain length of time and then it goes back to O volts. This is a pulse. A graph of the output voltage is shown below. The pulse is nice but it only happens one time. If you want something that does something interesting forever rather than Just once, you need an oscillator.
An oscillator puts out an endless series of pulses. The output constantly goes from O volts to PVC and back to O volts again. Almost all digital circuits have some type of oscillator. This stream of output pulses is often called a clock. You can count the umber of pulses to tell how much time has gone by. We will see how the 555 timer can be used to generate this clock. A graph of a clock signal is shown below. 1. 7. 1 The Capacitor If you already understand capacitors you can skip this part. The picture above on the left shows two typical capacitors. Capacitors usually have two legs.
One leg is the positive leg and the other is the negative leg. The positive leg is the one that is longer. The picture on the right is the symbol used for capacitors in circuit drawings (schematics). When you put one in a circuit, you must make sure the positive leg and the negative leg go in the right place. Capacitors do not always have a positive leg and a negative leg. The smallest capacitors in this kit do not. It does not matter which way you put them in a circuit. A capacitor is similar too rechargeable battery in the way it works. The difference is that a capacitor can only hold a small fraction of the energy that a battery can. Except for really big capacitors like the ones found in old TV’s. These can hold a lot of charge. Even if a TV has been disconnected from the wall for a long time, these capacitors can still make lots of sparks and hurt people. ) As with a rechargeable battery, it takes a while for the capacitor to charge. So if we have a 12 volt supply and start charging the capacitor, it will start with O volts and go from O volts to 12 volts. Below is a graph of the voltage in the capacitor while it is charging. The same idea is true when the capacitor is discharging.
If the capacitor has been charged to 12 volts and then we connect both legs to ground, the capacitor will start discharging but it will take some time for the voltage to go to O volts. Below is a graph of what the voltage is in the capacitor while it is discharging. We can control the speed of the capacitor’s charging and discharging using resistors. Capacitors are given values based on how much electricity they can store. Larger capacitors can store more energy and take more time to charge and discharge. The values are given in Farads but a Farad is a really large unit of measure for common capacitors.
In this kit we have 2 fop capacitors, 2 pouf capacitors and 2 buff capacitors. If means picador and if means microfarad. A picador is 0. 000000000001 Farads. So the fop capacitor has a value of 33 picadors or 0. 000000000033 Farads. A microfarad is 0. 000001 Farads. So the pouf capacitor is using the value of the capacitor you have to use the Farad value rather than the isobaric or microfarad value. Capacitors are also rated by the maximum voltage they can take. This value is always written on the larger can shaped capacitors. For example, the buff capacitors in this kit have a maximum voltage rating of 25 volts.
If you apply more than 25 volts to them they will die. We don’t have to worry about that with this kit because our power supply can only put out 12 volts. 1. 7. 2 The 555 Timer Creating a Pulse The 555 is made out of simple transistors that are about the same as on / off switches. They do not have any sense of time. When you apply a voltage they turn on ND when you take away the voltage they turn off. So by itself, the 555 can not create a pulse. The way the pulse is created is by using some components in a circuit attached to the 555 (see the circuit below). This circuit is made of a capacitor and a resistor.
We can flip a switch and start charging the capacitor. The resistor is used to control how fast the capacitor charges. The bigger the resistance, the longer it takes to charge the capacitor. The voltage in the capacitor can then be used as an input to another switch. Since the voltage starts at O, nothing happens to the second switch. But eventually the capacitor will charge up to some point where the second switch comes on. The way the 555 timer works is that when you flip the first switch, the Output pin goes to PVC (the positive power supply voltage) and starts charging the capacitor.
When the capacitor voltage gets to 2/3 PVC (that is PVC * 2/3) the second switch turns on which makes the output go to O volts. The pinot for the 555 timer is shown below Deep Details Pin 2 (Trigger) is the ‘on’ switch for the pulse. The line over the word Trigger tells us that the voltage levels are the opposite of what you would normally expect. To turn he switch on you apply O volts to pin 2. The technical term for this opposite behavior is ‘Active Low’. It is common to see this ‘Active Low’ behavior for ICC inputs because of the inverting nature of transistor circuits like we saw in the LED and Transistor Tutorial.
Pin 6 is the off switch for the pulse. We connect the positive side of the capacitor to this pin and the negative side of the capacitor to ground. When Pin 2 (Trigger) is at PVC, the 555 holds Pin 7 at O volts (Note the inverted voltage). When Pin 2 goes to O volts, the 555 stops holding Pin 7 at O volts. Then the capacitor starts charging. The capacitor is charged through a resistor connected to PVC. The current starts flowing into the capacitor, and the voltage in the capacitor starts to increase. Pin 3 is the output (where the actual pulse comes out).
The voltage on this pin starts at O volts. When O volts is applied to the trigger (Pin 2), the 555 puts out PVC on Pin 3 and holds it at PVC until Pin 6 reaches 2/3 office (that is PVC * 2/3). Then the 555 pulls the voltage at Pin 3 to ground and you have created a pulse. (Again notice the inverting action. ) The voltage on Pin 7 is also pulled to ground, connecting the capacitor to ground and discharging it. Seeing the pulse To see the pulse we will use an LED connected to the 555 output, Pin 3. When the output is O volts the LED will be off. When the output is PVC the LED will be on.
Place the 555 across the middle line of the breadboard so that 4 pins are on one side and 4 pins are on the other side. (You may need to bend the pins in a little so they will go in the holes. ) Leave the power disconnected until you finish building the circuit. The diagram above shows how the pins on the 555 are numbered. You can find pin 1 by looking for the half circle in the end of the chip. Sometimes instead off Alfa circle, there will be a dot or shallow hole by pin 1. Before you start building the circuit, use Jumper wires to connect the red and blue power rows to the red and blue power rows on the other side of the board.
Then you will be able to easily reach PVC and Ground lines from both sides of the board. (If the wires are too short, use two wires Joined together in a row of holes for the positive power (PVC) and two wires joined together in a different row of holes for the ground. ) Connect Pin 1 to ground. Connect Pin 8 to PVC. Connect Pin 4 to PVC. Connect the positive leg of the LED to a 330 ohm resistor and connect the negative ND of the LED to ground. Connect the other leg of the 330 ohm resistor to the output, Pin 3. Connect Pin 7 to PVC with a ask resistor (RA = ASK). Connect Pin 7 to Pin 6 with a Jumper wire.
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