Electrical Circuits / Electronics
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Transcript of Electrical Circuits / Electronics
Gary Plimer 2013Electrical Circuits / Electronics
Electricity is one of the most important forms of energy available to man. It affects everyone’s lives in many ways. If you take time to think about your everyday life you will realise that our lives are full of devices that depend upon electricity.
Some important terms:
Electric current
Electric current is the name given to the flow of negatively charged particles called electrons.
Current is measured in amperes, usually referred to as ‘amps’ (A). Current is the rate of flow of electrical charges (called electrons) through a circuit.
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Electrical CircuitsVoltage
In most circuits a battery or voltage supply is used to drive the electrons through the components. Voltage is measured in volts (V).
R+
_V C o nve ntio na l
C urre nt
ResistanceAll materials conduct electricity. The materials that conduct electricity well are called conductors and those that are poor conductors are called insulators. Metals are good conductors while rubber and glass are good insulators.Resistance is therefore a measure of how much voltage is required to let a current flow. Resistance is measured in ohms ().
Gary Plimer 2013Batteries & Voltage Supplies
Single battery or cell
Multiple batteries or cells
6 volts Voltage supply-ve +ve
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Components - Resistors
Fixed Resistor Symbol
Resistors are basic components in electrical and electronic circuits. They limit the amount of current flowing in circuits or parts of circuits. Resistors are roughly cylindrical and have coloured stripes. They also have connection wires sticking out of each end.
The stripes indicate the value of the resistors. The colours represent numerical values according to a special code.Although the symbol for ohms is ‘’ it is often shown as a capital R; that is, 270 ohms can be expressed as either 270 or 270 R.
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Resistor Colour Code
First and second colour band Digit Multiplier
Black 0 x 1
Brown 1 x 10
Red 2 x 100
Orange 3 x 1000 or 1 K
Yellow 4 x 10 000 or 10 K
Green 5 x 100 000 or 100 K
Blue 6 x 1 000 000 or 1 M
Violet 7 Silver means divide by 100
Grey 8 Gold means divide by 10
White 9
Tolerances: brown 1% red 2% gold 5% silver 10% none 20%
Gary Plimer 2013Resistor Value Calculation
4 Band Resistor Colour Code Layout
4th bandtolerance
3rd bandmultiplier
1st band1st digit
2nd band2nd digit
If the colours on the resistor are:1st band red2nd band violet3rd band brown4th band gold
Then its value is: 2(red) 7(Violet) x 10(Brown) with a 5% tolerance (Gold) i.e. 270ohms 5% tolerance.
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Pupil Assignment
Calculate the value of the following resistors:
1) blue – violet – brown – silver
2) orange – white – brown – gold
3) brown – black – red – gold
4) brown – black – green – brown
What colours would the following resistors have?
1) 270 R
2) 1K5
3) 33 K
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Diodes
Current can pass this way only
Anode Cathode
Symbol for Diode
Diodes are devices that allow current to flow in one direction only.
Current will flow through the diode only when the anode (positive side) is connected to the positive side of the circuit and the cathode (negative side) is connected to the negative side of the circuit.
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Light Emitting Diode
A light-emitting diode is a special diode that gives out light when current is flowing through it. LEDs are used as indicators to tell when a circuit (or part of a circuit) is working. You can tell the cathode of an LED as it is the short leg and there is a ‘flat’ on the plastic casing.
-ve
LED’s use less energy than bulbs, hence the reason we see their use in torches now.
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Switches
Toggle Slide
Key Tilt
Rocker
Reed
Switches are useful input devices (or transducers) that have metal contacts inside them to allow current to pass when then they are touching. There are several ways in which the contacts in mechanical switches can be operated. The main types are push-button, toggle, key, slide, magnetic (reed) and tilt. These switches are ‘digital’ input devices as they can only be on or off.
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Switches
Switches are useful input devices (transducers).
There are several ways in which the contacts in mechanical switches can be operated. Such as push button, key, slide, toggle, magnetic (reed) and tilt.
These switches are digital input devices as they can only be on or off.
The contacts on a switch can be NO or NC (normally open, normally closed)
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Switch ContactsTypes of switch contacts:
SPST (Single Pole Single Throw)
SPDT (Single Pole Double Throw)
Double-pole double-throw switch (DPDT)Double-pole single-throw switch (DPST)
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Switch Contact Use
SPST
SPDT
DPST
DPDT
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Pupil Activity
We have now seen a number of common electronic components. Lets now try and combine some of these into a working circuit.
390Ror390
Switch
LED
6V
ICopy the circuit into your workbook
simulate the circuit using. Add voltmeters / Ammeters and measure the voltage drop over each component.
How would you write up a test plan and results for this circuit?
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Series Circuits
When components are connected end to end, as in the last activity, we say they are connected in series.
This leads to an important law, Kirchoff’s 2nd Law
The sum of voltages dropped across each component (V1, V2 ) is equal to the total voltage supply in the circuit.
18 V
6 V 6 V 6 V
VT = V1 + V2 + V3 + …
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Measuring Voltage Drops
V
Note how voltage is measured over the components
Make sure you take a note of the symbol for VOLTMETER
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Pupil Activity (Voltage Drops)
Task:
Measure the voltage drop over the 2 bulbs. Enter your findings into a table.
Bulb No. Voltage (v)
1
2
9V
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Pupil Activity (Voltage Drops)
Task:
Measure the voltage drop over the 2 bulbs and resistor. Enter your findings into a table.
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Prototype Board
M ETALLIC STRIPC O NN EC TO R
Prototype Board is used to test circuits prior to manufacturing the circuit in large numbers.
Build a series circuit using 2 resistors of different values as shown by your teacher.
Using the multimeter, check the voltage drop over each resistor.
Do the results confirm Kirchoff’s law?
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Circuit Simulation
As in Pneumatics, it is possible to simulate electrical circuits. In this case we will use a program called Crocodile Technology. Your teacher will demonstrate the use of Croc Clips to simulate the circuit shown below..
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Measuring Current
6V
A
Current is measured through components or parts of circuits, as shown in the circuit diagram opposite.
Note that it is necessary to ‘break’ the circuit and connect the meter in series with the components.
Take a note of the symbol for an Ammeter
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Current measurement
Using circuit simulation, measure the current flowing through all three components in the LED circuit.
In a series circuit the current flowing through all components is the same. Try placing the meter at different parts of the circuit to prove this. In parallel circuits the same current does not always flow through each component you will find out about this later.
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Measuring Resistance
COMmA10A V
R1
R2
Connect two resistors in series on a prototype circuit board and measure the overall resistance.
You should find that
Rtotal = R1 + R2
And the general rule for finding the sum of any amount of resistors in series is
Rtotal = R1 + R2 + R3 + Rn
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OHMS LAW
R = V
I
Ohms law can be used to calculate theoretical Voltage drops, Current and Resistance in circuits.
V
I R
Using the triangle shown opposite, we can rearrange the formula to obtain V or I.
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Ohms Law in Practice
La mp
C urrent 0.06 a mp s
6 vo lts
R =V
I
R = 6
0.06
R = 100
The task is to calculate the resistance of the lamp.
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Worked Example
c
S
For the series circuit shown, calculate:
a) The total resistance (RT)b) The circuit current (IC)c) The potential difference (DROP) across both
resistors (V1 and V2)
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Worked Example
R = R + R
= 6 + 18
R = 24
T 1 2
T
V = I R
I = V
R
12
24I = 0.5 A
S C T
CS
T
C
V = I R
0 . 5 1 8
V = 9 V
2 C 2
2
a) b)
V = V + V
3 + 9
V = 12 V
T 1 2
T
c)
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Pupil Problems
12V
For the circuit shown below calculate:
a) The total resistance of the circuit
b) The circuit current
c) The voltage drops over the resistors
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Pupil Problems
6V
For the circuit shown below calculate:
a) The total resistanceb) The circuit currentc) The voltage drop across each resistor.d) Use Kirchoff’s second law to verify your answers to
(c).
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Pupil Problems
24V
For the circuit shown below calculate:
a) The total resistance of the circuit
b) The circuit current.
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Pupil Problems
A circuit has three resistors in series. Their values are 15 R,
24 R and 60 R. Calculate the total resistance of the circuit.
Two resistors are connected in series. Their values are 25 R and 75 R. If the voltage drop across the 25 R resistor is 4 volts, determine the circuit current and the supply voltage
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Series Circuits
One of the problems with series circuits is if a component fails, then the whole circuit fails. Consider a set of bulbs connected in series.
If one of these bulbs fail, then current cannot flow through the circuit, hence the remaining bulbs will fail to light also.
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Parallel Circuits
Parallel circuits are circuits where there is more than one path for electricity to flow along or that have more than one ‘branch’. Each branch receives the supply voltage, which means that you can run a number of devices from one supply voltage. A good example of a simple parallel circuit is a set of Christmas-tree lights where all the bulbs require a 230 volt supply.
240 volts
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Parallel Circuits Activity
12 volts
Parallel circuits can be arranged in many ways, but are normally set out so that you can easily see the parallel ‘branches’. A simple parallel car-alarm circuit is shown below with the switches wired up in parallel.
Simulate the circuit shown below, then describe its operation in your note book.
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Resistors in Parallel
COMmA10A VR1 R2
Connect two resistors in parallel on a prototype circuit board and measure the overall resistance
1
R =
1
R +
1
RT 1 2
The formula to calculate the theoretical value of resistors in parallel is shown below.
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Worked Example
12 volts
R2
R1
The resistance values are R1 = 270 R, R2 = 100 R and for the buzzer 240 R.
Calculate the resistance of the parallel branch and the total circuit resistance.
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Pupil Activity (Parallel Circuits)
Task:
Build the circuit, Measure the voltage over each of the bulbs. Enter your findings into a table.
Gary Plimer 2013Current in Parallel Circuits
I I
I
I
T T
2
1
There are two important points to remember about resistors in parallel.
1) The voltage drop across each resistor is the same.
2) The sum of the currents through each resistor is equal
to the current flowing from the voltage source.
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Worked Example
12 volts
R2
R1
The resistance values are R1 = 270 R, R2 = 100 R and for the buzzer 240 R.
Your teacher will work through this problem on the white board.
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Pupil Problems
9V
For the circuit shown below calculate:
(a) The total resistance of the circuit
(b) The branches and circuit current.
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Pupil Problems
110V
For the circuit shown below calculate:
(a) the total resistance of the circuit(b) the circuit current(c) the current flowing though R1 (10 R)(d) the current flowing through R2 (24 R).
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Pupil Problems
240 V
For the circuit shown below calculate:(a) the total resistance of the circuit(b) the circuit current(c) the current flowing through R1 (660 R).(d) the current flowing through R2 (470 R).
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Pupil Problems
A 6 R resistor and a 75 R resistor are connected in parallel across a voltage supply of 12 V. Calculate the circuit current.
A 440 R resistor is connected in parallel with a 330 R resistor. The current through the 440 R resistor is 300 mA. Find the current through the 330 R resistor
Gary Plimer 2013Combined Series & Parallel
Consider the combined series and parallel circuit shown in the figure below.
You can see that R2 and R3 are connected in parallel and that R1 is connected in series with the parallel combination.
Gary Plimer 2013Combined Series & Parallel
Some points to remember when you are dealing with combined series and parallel circuits are:
The voltage drop across R2 is the same as the voltage drop across R3 The current through R2 added to the current through R3 is the same as the current through R1 The voltage drop across R1 added to the voltage drop across R2 (which is the same as across R3) would equal the supply voltage Vs.
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Worked Example 2
24R
48R
10R
12V
For the combined series and parallel circuit shown, calculate:
The total circuit resistance (RT) The circuit current (IC) The voltage drop across resistor R1 (VR1) The current through resistor R2 (I2).
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Pupil Problems
7.5 V
For the circuit shown calculate:
The resistance of the parallel combination
The total circuit resistance.
The branch currents
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Pupil Problems
24 V
For the circuit shown calculate:
The total resistance
The circuit current
The branch current
The voltage drop across each resistor.
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Pupil Problems
110 V
For the circuit shown calculate:
The total resistance of the circuit The circuit current The current through each resistor The voltage drop across each resistor.
Gary Plimer 2013Voltage Dividers Activity
COMmA10A V
R1
R2
VS
0 V
Volts
Build a voltage divider circuit using any 2 values of resistor.
Using the multimeter measure the voltage drop over R2.
This voltage is known as Vo or the output voltage from the divider.
Gary Plimer 2013Voltage Dividers Activity
V = R
R + R VO
2
1 2S
Measure the resistance of the 2 resistors from the last activity.
Enter the values into the formula below and calculate Vo.
Simulate the circuit using croc clips and measure Vo.
Hopefully! The value of Vo should be the same in all three cases, (within reason).
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Worked Example
0 volts
R = 40kV
V = 12 volts
2
2
S
R = 80k 1
V = V R
R + R
V = 12 40
40 + 80V = 4 volts
2 S2
1 2
2
2
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Pupil Problems
0 volts
R = 810RV
V = 12 volts
2
2
S
R = 270R 1
Calculate Vo in the following exercises
0 volts
R = 10KV
V = 12 volts
2
2
S
R = 390R 1
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Pupil Problems
0 volts
R = 47KV
V = 6 volts
2
2
S
R = 10K 1
0 volts
R = 2.2KV
V = 9 volts
2
2
S
R = 10K 1
Calculate Vo in the following exercises
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Power in Circuits
Electrical power is measured in watts (W).
Electrical power can be converted into other forms of power using electric circuits. For example the power used in overcoming electrical resistance can be converted into heat – this is the principle of an electric fire.
The power in an electric circuit depends both on the amount of current (I) flowing and the voltage (V) applied.
The formula for power in electric circuits is:
Power = Voltage x Current (watts)
P = V x I (W)OR V2/R
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Data Charts
Thermistor types
You must be able to extract data from a graph.
There are 2 types you will meet, Light Dependant Resistor and a Thermistor.
Your teacher will work through the use of the chart.
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Pupil Activity
0 volts
V
V = 9 volts
O
S
R = 10K 1
-t
1) Copy the circuit shown below into your note book.
2) Using the Yenka software, construct the voltage divider circuit.
3) Using a multimeter measure Vo.
4) Warm the thermistor up with the slide and re measure Vo.
5) Describe the operation of the voltage divider.
6) Reverse the position of the thermistor and resistor. Repeat 3,4 & 5.
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Pupil Activity
0 volts
V
V = 9 volts
O
S
ORP12
10K
1) Copy the circuit shown below
into your note book.
2) Using the Yenka, construct the voltage divider circuit.
3) Using a voltmeter measure Vo.
Change the LDR with the slide and re measure Vo.
4) Describe the operation of the
voltage divider.
5) Reverse the position of the LDR and resistor. Repeat
3,4 & 5. Describe what is
happening.
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Pupil Activity
A potentiometer configured as a variable resistor can be used in a circuit as a voltage or current control device. They are often used in voltage divider circuits to adjust the sensitivity of the input.
Build a voltage divider using a potentiometer. Check its operation by measuring Vo from the voltage divider.
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Potentiometers
Some more examples of potentiometers.
Gary Plimer 2013Voltage Divider Sensitivity
0 volts
V
V = 9 volts
O
S
ORP12
47K
With an analogue sensor it is normally desirable to adjust the sensitivity of the circuit. Rather than using a fixed resistor we can replace it with a variable resistor (or potentiometer).
This allows us to fine tune the sensitivity of the voltage divider.
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Pupil Problems
9 V 5 V
0 V 0 V
Calculate the voltages that would appear across each of the resistors marked ‘X’ in the circuits below.
6v
0v
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Pupil Problems
12 V 16 V 12 V
0 V 0 V 0 V
In each of the following voltage divider circuits determine the unknown quantity.
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Pupil Problems
What would happen to the voltage across the thermistor as the temperature increased?
What would happen to the voltage across the resistor in the circuit as the temperature increased?
0 volts
V
V = 9 volts
O
S
R = 10K 1
-t
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Voltage Dividers
We have seen that Voltage Dividers, divide the voltage depending on the value of resistors used. In addition, if we include a variable resistor, we can alter the sensitivity of the voltage divider.
If we include a thermistor, we can measure changes in temperature.
If we include a LDR, we can measure changes in light levels.
If we include a potentiometer, we can measure changes in position.
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Transistors
The transistor is a semiconductor device. This means that it is sometimes a good conductor of electricity and sometimes a poor one. A transistor is made up of three layers of semiconductor materials that are either ‘n type’ or ‘p type’.There are two types of bipolar transistor available: pnp or npn. Transistors come in many variations and sizes. However, they all are reliable, efficient, small and relatively cheap.
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Transistors
Collector
Emitter
Base
NPN Bipolar Transistor
A transistor is an electronic switch
Transistors amplify current which enables them to
drive heavy loads such as motors
A voltage of 0.7V will switch on a NPN transistor
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Transistors Activity
1k
Build the following transistor circuit using Yenka.
Adjust the voltage reaching the transistor base by altering the value the potentiometer.
At what voltage does the transistor switch on?
Measure the current flowing to the base.
Now measure the current flowing in the collector leg.
What is the transistor doing?
5V (B)
10K
5V (A)
Buzzer
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Relays
Although relays are often considered to be output devices, they are really output switches from electric or electronic circuits.
When an electric current flows into the relay coil, the coil becomes an electromagnet. This electromagnet attracts the armature and moves the contacts. This movement provides the switching, just as the contacts in any other switch do.
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Relays
The relay is a very useful device because it is the vital link between microelectronics and high-energy systems that require substantial amounts of current. The relay is perhaps the most commonly used switch for driving devices that demand large currents.
Gary Plimer 2013Relays – Protection Diode
As seen earlier, relays have a coil that is energised and de-energised as the relay switches on and off. During this process of switching, the coil can generate a large reverse voltage (called a back e.m.f.). This reverse voltage can cause considerable damage to components, especially transistors.
The transistors and other sensitive components can be protected by the inclusion of a diode that provides a path for the current caused by the reverse voltage to escape.
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DPDT Relay
TO SEN SO RC IR C U IT
0V
+V
As electric motors normally draw larger currents, relays are ideal devices for such circuits. By using DTDP switching, relays can control the direction of rotation of motors.
Simulate a sensing circuit using an LDR in a voltage divider
Add a transistor driving circuit and a DPDT relay
Connect the relay up so as the motor drives clockwise and anticlockwise depending on the amount of light hitting the LDR
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Motor Reversal Circuit
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Capacitors
Capacitors are electronic components that store electricity for short periods of time within electronic circuits or networks.
RADIALCAPACITATOR
AXIALCAPACITATOR
ELECTROLYTIC
Electrolytic capacitors are polarity conscious. This means that they must be connected ‘the right way round’. The negative lead must be connected to zero volts with the positive terminal towards the higher voltage side of the circuit.
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Pupil Activity
+
100uF
10K
9V
0V
Simulate the following circuit
Allow the capacitor to charge up
Connect the end of the LED to 0V
The LED should light up for a short period of time