science for school
science for school science in school science school science is cool electronics for kids science for kids cool science electronics school transistor fun
home

electronics projects
project led
project transistor
Full quick circuit guide
__________

electronics kits

contact

book list
Component sourcing

 
 

lecture notes

These are my lecture notes. They are not intended as full information, but should be used in conjunction with background explanation. These circuits and explanations are copyright Nick Bushell 2006-2007. Please refer to the authour if you use them. Thank you.

 


Contents

 

Simple Circuit
Circuit with a switch
LED Circuit with a switch
We conduct electricity
Cardboard Circuit board
Water conducts electricity
Resistors
Switch with two LED circuits
Potentiometer – a variable resistor with dual LEDs
Switch with two circuits
Simple LED Circuit
Switches in combinations
Switches in parallel
Special house light switch combination
Magnetic switch
Resistors in series
Resistors in parallel
Potentiometer – a variable resistor
Capacitors
Capacitors in series
Capacitors in parallel
Light Dependant Resistor
Transistor
Transistor with LDR
Transistor react to light
Two Transistor flasher
Integrated Circuits


 

Simple Circuit

 

What makes a circuit?
A circuit must have:
A source of electricity
A closed loop must exist to allow the electricity to flow around.
A break in the loop, breaks the circuit, and the electricity will not flow.

 

Electricity is the flow of electrons. Electrons are:
Very small
Have a negative charge
Spin round and round very fast
Move very fast
So you can not see electricity, but you can see how it affects things.

Circuit with a switch

 

Switches are very handy. They make it safe and easy to control electricity.


LED Circuit with a switch

 

 

Switches are very handy. They make it safe and easy to control electricity.


We conduct electricity

We conduct electricity.
Why do we conduct electricity?
We are made up of about 95% water. Water conducts electricity, so we do because we are mostly water.
Some bits of us, like our nails are not very watery, so those bits do not conduct very well.


Cardboard Circuit board

 

Pencils are made of a conductive material. It is called carbon.
We can make a complicated shape, like a real circuit board track, that will conduct  electricity.

Carbon is used in real electronic components called resistors.

 

 



Water conducts electricity

 

Water conducts electricity, but salty water conducts electricity even better.

 

 



Resistors

 

Resistors are used in electronic circuits to control and limit how much electricity is flowing. Instead of using the blue LED, if we use the red LED, we MUST put a resistor in the circuit – otherwise the red LED will break!

They are very important to make real circuits that operate efficiently and accurately. Without resistors, electronic circuits could not be made to work. They would explode as too much electricity would flow!


 

Switch with two LED circuits

This circuit swaps which LED is on as you toggle the switch.

Toggling circuits are very common in electronic circuits.



Potentiometer – a variable resistor with dual LEDs

 

In this circuit, as you turn the pot, one LED gets brighter as the other gets darker.

 


Switch with two circuits

 

This circuit swaps which Bulb is on as you toggle the switch.

Toggling circuits are very common in electronic circuits.


 

Simple LED Circuit

 

LEDs only conduct electricity one way.

You have to wire them correctly in the circuit for them to light up.

The long lead must be connected to the + side of the battery. The short lead connects to the – side.

LEDs can break easily if they are connected the wrong way round.


 

Switches in combinations

 

Switches can be joined up in different combinations.
The circuit here is called  a series combination.
Both switches have to be closed for the LED to come on.

 

 


Switches in parallel

 

These switches are in a parallel circuit.
In this circuit, either switch will turn the LED on.

 


 

Special house light switch combination

Switches are often wired like this in home and offices.
It allows the person at either switch to turn the light on or off.
Can you work out how the circuit does this?


Magnetic switch

 

People have made switches that can be turned on and off using a magnetic field.

The switch uses two different types of metal. One of them is sensitive to magnetic fields, and the other one isn’t. The two metals are sandwiched together. When the magnetic field comes close to the sandwich, the straight sandwich becomes bent, and bends onto  another non-bending piece of metal, and makes the circuit.

 

 


 
Resistors in series

 

Putting two resistors in series as shown in the circuit here, will halve the amount of current flowing through the circuit, compared to a single resistor of the same value.



Resistors in parallel

Putting two resistors in parallel as shown in the circuit here, will double the amount of current flowing through the circuit, compared to a single resistor of the same value.


Potentiometer – a variable resistor

A potentiometer is a resistor that can be varied. It is often called a variable resistor.
In this circuit the LED gets brighter or dimmer as you turn the shaft on the potentiometer from one side to the other.
It is also called a pot for short..

 



Capacitors

Capacitors are used in electronic circuits to store small amounts of electricity.

The circuit above shows how a capacitor can store electricity then use this stored electricity to flash an LED once.

The circuit below uses a capacitor in a different way. It will also flash the LED once, and this time it does the flash when the battery is first connected. To make the LED flash again we need to let the capacitor first lose its charge. This can be done by putting a croc lead directly clipped across the capacitor so that the electric charge in the capacitor is discharged in itself and the wire.

Capacitors are not as easy to see quickly what they do or what they do in different combinations. The combination of capacitors and resistors results in even more uses and features used in electronic circuits from radios to handphones to computers to toys..

 



Capacitors in series

Putting two capacitors in series as shown in the circuit here, will halve the time the electricity flows for, compared to a single capacitor of the same value.



Capacitors in parallel

Putting two capacitors in parallel as shown in the circuit here, will double the time the electricity flows for, compared to a single capacitor of the same value.


 

Light Dependant Resistor

 

 

Using very special chemicals, scientists have made resistors that change when light is shone on them.


 

Transistor

The simplest way to think of transistors is as very small switches. Instead of using a mechanical movement like in a normal switch, transistors use a little bit of electricity. You can switch them on just using the electricity that can flow through your body.

Notice how much brighter the LED is when you use a transistor in the circuit compared to just your fingers – see earlier experiment with an LED and fingers.

With this circuit, you can use your fingers to switch on any LED, not just sensitive ones.

Transistors come in many shapes, sizes and types. The one shown here is of the earliest types invented. There are three wires on it. The base controls whether any electricity flows between the collector and the emitter. By outing a little electricity through from the base to the emitter, a lot of current will flow between the collector and the emitter.

There are thousands or even millions of transistors in every day equipment like computers and handphones. They are the working components in microchips or Integrated circuits.

Transistor with LDR

LDR circuit

This circuit will cause the LED to come on when the ambient light drops. The resistance of the LDR in bright light will be low, and will therefore keep the transistor turned off. In dark, the LDR resistance will rise, and the resistor connected using the Green leads will be able to turn the transistor on. However, the value of the resistor will need to be chosen to work with the resistnace of the LDR. LDRs vary in resistance values in dark and light depending on the part number of the LDR. If you have trouble making the circuit work, try different resistor values. A variable resistor can help correctly “bias” the transistor.
The resistor should probably be in the 10K ohm range.


Transistors react to light

 

Transistors will react to light if you let them. That is why they are always sealed in cans or packages.

 

Two Transistor Flasher

This circuit turns on each LED for a short while then turns on the other one. You can change the speed of the flashing by chnaging the resistor values and capacitor values.

transistor flash

Integrated Circuits

 

 

Integrated Circuits, or ICs, or Chips are lots of transistors and other components all joined together on a single piece of silicon. They are called integrated, because they have all the circuit components joined or integrated together on the chip of silicon.

And because there are many different circuits that you could design, there are many different types and designs of chips. In fact, the number is growing daily, and by now there are millions of designs.

Each design needs its on data sheet on what the circuit is inside the chip. It also needs to describe how to connect to it.

When you look at a chip, you do not how to connect to it unless you know which one it is, and you have read and understood the data sheet. Some data sheets are over 1000 pages long.

So although these devices all have a common name – chip – there are many many different types, doing lots of different functions.

The phrase - computer chip – means only a very general thing. To electronic engineers, they need to know its model number and use its data sheet to know how to design circuits around it.

The first Integrated Circuit chip we will use is called a 555 timer. We can use this chip to easily make a flashing LED. Flashing LEDs are common in lots of things from toys to phones to cars to planes.

So, how do we make something flash? We need two things – something to time how long the LED is off, and something to time how long it is on.

The 555 timer has all this circuitry inside it, integrating lots of transistors inside it to make the timers. Below is the circuit inside the chip. This is a very simple chip compared to almost all other ones these days. But it is very complicated compared to all the other circuits we have made earlier. All these components take up a space less than 1mm x 1mm. The package we connect to is much bigger to protect the chip, and make it easy to connect and solder to.

So to connect to the chip, we do not have to know all the details inside. The manufacturer of the chip gives us information on how to connect to it with examples and values.

Above is the information from the manufacturer on how to make an oscillator. We can use this circuit to make an LED flasher.

Below is a conversion of the circuit above to one we can wire using croc leads. You have to be very careful when connecting the leads to the chip, as the chip is really designed to be soldered onto a circuit board.

Make sure none of the croc leads touch each other. You can bend the legs of the chip a bit to make it easier to connect. Connect the battery last to make it safe as you wire up the rest of the circuit. Take your time..


Connect the white croc leads to the LED, capacitor and the chip. Note which leg goes where.

Connect the green croc leads to the resistors  and the chip.


Connect the two yellow leads. From the capacitor to the chip. From the resistors to the chip.

Connect the black croc leads to the battery, chip and capacitor.


Connect the red leads to the battery +, the resistors and the chip. Connect the battery last to make it safe as you wire up the rest of the circuit.

The LED will start flashing if you have connected everything correctly.

If the circuit does not work, check every lead is connected correctly.

It is very common for circuits not to work the first time  - the reason is nearly always because they have not been connected up correctly. Occasionally there is a broken component, but this is very rare.

So check your circuit carefully.

 


 

 


|site designed by www.grumpyoldgeek.net|